ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-13233-2017Atmospheric pollution over the eastern Mediterranean during summer – a reviewDayanUrimsudayan@mscc.huji.ac.ilRicaudPhilippehttps://orcid.org/0000-0002-7133-1734ZbindenRéginaDulacFrançoisDepartment of Geography, The Hebrew University of Jerusalem, Jerusalem, IsraelCNRM, Météo-France, CNRS UMR3589, Toulouse, FranceLaboratoire des Sciences du Climat et de l'Environnement (IPSL-LSCE), CEA-CNRS-UVSQ, Univ. Paris-Saclay, Gif-sur-Yvette, FranceUri Dayan (msudayan@mscc.huji.ac.il)8November20171721132331326329January20171October201727September201730March2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/13233/2017/acp-17-13233-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/13233/2017/acp-17-13233-2017.pdf
The eastern Mediterranean (EM) is one of the regions in the world where
elevated concentrations of primary and secondary gaseous air pollutants have
been reported frequently, mainly in summer. This review discusses published
studies of the atmospheric dispersion and transport conditions characterizing
this region during the summer, followed by a description of some essential
studies dealing with the corresponding concentrations of air pollutants such
as ozone, carbon monoxide, total reactive nitrogen, methane, and sulfate
aerosols observed there.
The interlaced relationship between the downward motion of the subsiding air
aloft induced by global circulation systems affecting the EM and the depth of
the Persian Trough, a low-pressure trough that extends from the Asian monsoon
at the surface controlling the spatiotemporal distribution of the mixed
boundary layer during summer, is discussed. The strength of the wind flow
within the mixed layer and its depth affect much the amount of pollutants
transported and determine the potential of the atmosphere to disperse
contaminants off their origins in the EM. The reduced mixed layer and the
accompanying weak westerlies, characterizing the summer in this region, led
to reduced ventilation rates, preventing an effective dilution of the
contaminants. Several studies pointing at specific local (e.g., ventilation
rates) and regional peculiarities (long-range transport) enhancing the
build-up of air pollutant concentrations are presented.
Tropospheric ozone (O3) concentrations observed in the summer over
the EM are among the highest over the Northern Hemisphere. The three
essential processes controlling its formation (i.e., long-range transport of
polluted air masses, dynamic subsidence at mid-tropospheric levels, and
stratosphere-to-troposphere exchange) are reviewed. Airborne campaigns and
satellite-borne initiatives have indicated that the concentration values of
reactive nitrogen identified as precursors in the formation of O3
over the EM were found to be 2 to 10 times higher than in the hemispheric
background troposphere. Several factors favor sulfate particulate abundance
over the EM. Models, aircraft measurements, and satellite-derived data have
clearly shown that sulfate has a maximum during spring and summer over the
EM. The carbon monoxide (CO) seasonal cycle, as obtained from global
background monitoring sites in the EM, is mostly controlled by the
tropospheric concentration of the hydroxyl radical (OH) and therefore
demonstrates high concentrations over winter months and the lowest concentrations during
summer when photochemistry is active. Modeling studies have shown that the
diurnal variations in CO concentration during the summer result from
long-range CO transport from European anthropogenic sources, contributing 60
to 80 % of the boundary-layer CO over the EM. The values retrieved from
satellite data enable us to derive the spatial distribution of methane
(CH4), identifying August as the month with the highest levels over
the EM. The outcomes of a recent extensive examination of the distribution of
methane over the tropospheric Mediterranean Basin, as part of the Chemistry-Aerosol
Mediterranean Experiment (ChArMEx) program, using model
simulations and satellite measurements, are coherent with other previous
studies. Moreover, this methane study provides some insight into the role of
the Asian monsoon anticyclone in controlling the variability of CH4
pollutant within mid-to-upper tropospheric levels above the EM in summer.
Introduction
The relationship between atmospheric air pollutant
concentrations and large-scale atmospheric circulation systems has been
examined over the past decades (e.g., Davis and Kalkstein, 1990; Dayan et al.,
2008). This strong relationship and its issuing dispersion condition at
several scales, and climatically related variables such as air pollutants, is
presented in this work as part of the Chemistry-Aerosol Mediterranean
Experiment (ChArMEx; http://charmex.lsce.ipsl.fr).
However, a first major drawback in attributing air pollutant concentrations
to variations in large-scale atmospheric circulation arises from the fact
that changes in removal processes and upwind emissions are not necessarily
concurrent with variations in circulation. Some efforts were undertaken,
mainly through coupled chemistry–climate models, to treat and analyze at the
same time the changes in general circulation and atmospheric chemistry (Hein
et al., 2001; Dastoor and Larocque, 2004). Moreover, secondary pollutants
such as tropospheric ozone (O3) result basically from photochemical
reactions among precursors and, as such, are controlled by air mass
characteristics such as temperature, humidity, and cloud cover/solar
radiation. Accordingly, changes in trace gases' concentrations are modified
with respect to exposure of the differing air masses driven by changes in
atmospheric circulation.
A second substantial shortcoming in trying to associate changes in pollutant
concentration to variation in circulation patterns is their different life
span and distribution. For example, durable greenhouse gases (GHGs) such as
methane (CH4) and carbon dioxide (CO2) are characterized by
long lifetimes of years as compared to nitrogen oxides and aerosols which
are most relevant for short spatial and temporal scales (Andreae, 2001;
Voulgarakis et al., 2010). Radiative forcing of aerosols is of much higher
spatial variability than GHG forcings due to the relatively short aerosol
lifetime (daily–weekly scale) compared to that of GHGs (monthly–yearly
scale).
Both natural and man-made factors converge over the EM, favoring the
accumulation of pollutant concentration during summer. This region is in the
crossroad of both large-scale convective motions: Hadley and Walker cells
leading to subsidence. This process results in a reduced mixing depth, which
inhibits an efficient dispersion of the pollutants. Moreover, the EM is
a hotspot of high solar radiation driving the photochemistry of the
atmosphere. In addition, the prevailing summer westerlies at shallow
tropospheric layers favor the transport of pollutant-enriched air masses from
central and eastern Europe to the eastern Mediterranean (EM). Based on the
above key factors, this review focuses explicitly on summertime. Lelieveld
et al. (2002) studied air pollutant transport over the EM in summertime. They
report that the synoptic flow is controlled by the strong east–west pressure
difference between the Azores high and the Asian monsoon low, with additional
influence in the upper troposphere from the Tibetan anticyclone. This yields
a contrasted situation in the tropospheric column with European influence in
the lowermost troposphere, a much longer-range transport from Asia and North
America at mid-tropospheric levels, and a major impact from Asia in the upper
troposphere and lower stratosphere.
Desert dust is abundant over the EM, transported from two major source
regions: the north African Sahara and the Arabian deserts. However, in
general and predominantly, mineral dust affects the EM during all seasons
except summer (e.g., Dayan et al., 1991; Moulin et al., 1998; Sciare et al.,
2003), which is the reason why mineral dust is not in the scope of this study which is
focused on summer conditions.
In this review, we first describe the atmospheric dynamic conditions favoring
the build-up of tropospheric air pollutant concentrations. Secondly, we
propose a synthesis of the essential studies on air pollutant concentrations
including O3, sulfate (SO4) aerosols, total reactive
nitrogen (NOy), carbon monoxide (CO), and CH4. The
sources of the data reported include in situ observations, balloon-sounding,
aircraft, and space-borne observations as well as model data, the results of which,
in terms of dynamics, are mostly updated over 1948–2016 based on availability.
Summer atmospheric dynamic conditions favoring the build-up of tropospheric pollutants concentrations
Different spatial and temporal scales of motion affect pollutant transport
and dispersion: the microscale, mesoscale, synoptic scale, and macroscale, or
global scale. At the scale of a few months, the planetary boundary layer is
relatively well mixed. However, on shorter timescales and near the Earth's
surface (where pollutants are emitted), transport and dispersion are often
limited by atmospheric conditions. In this section, we will focus on the
global- and synoptic-scale processes that favor a potential accumulation of
pollutants in the EM troposphere.
Composite
long-term mean sea-level pressure (hPa) for July–August over 1948–2016.
“PT” indicates the Persian Trough
position. “H” indicates the anticyclone position. NCEP reanalysis
data are provided by the NOAA/OAR/ESRL PSD, Boulder,
Colorado, USA; http://www.esrl.noaa.gov/psd/.
Global and synoptic scales inducing subsiding conditions over the eastern Mediterranean
In general, the atmospheric conditions over the EM are persistent during the
summer and subject to two essential processes. The first is the cool
advection at shallow tropospheric layers caused by the strong, dry north
Etesian winds generated by the east–west pressure gradient manifested by
large-scale circulation features, low pressures over the EM as an extension
of the Persian Trough (PT), and the high pressure over central and
southeastern Europe (Tyrlis and Lelieveld, 2013). This surface low-pressure
trough extends from the Asian monsoon through the Persian Gulf and further
along southern Turkey to the Aegean Sea (Figs. 1 and 2). The second is the
dynamic subsidence generated by several global-scale processes: the African
monsoon as part of the subtropical descending branch of the Hadley cell
(Fig. 3a), the Asian monsoon as part of the Walker cell (Fig. 3b)
and subsidence caused by the negative relative vorticity characterizing this
region, during summer, as explained further on.
NCEP/NCAR reanalysis composite long-term mean temperature at
850 hPa (∼1500ma.s.l.) with wind vectors, averaged
over 1948–2016, for July–August. Note the southward penetration of the
European cold air over the Mediterranean Basin. This cold air mass is
transported at shallow tropospheric layers towards the eastern Mediterranean
by the Etesian northwesterlies characterizing the Persian Trough.
NCEP reanalysis data are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado,
USA; http://www.esrl.noaa.gov/psd/.
Rodwell and Hoskins (1996) used a hydrostatic primitive equation model
initialized by a 6-year June to August climatology derived from
European Center for Medium Range Weather Forecasts (ECMWF) analyses to
investigate the monsoon desert mechanism enhancing summertime descent in the
Mediterranean subtropics. They argued that the subsidence center in the EM is
governed by the Asian monsoon rather than by the Hadley circulation and
explained it by diabatic heating in the Asian monsoon region that induces
a Rossby wave to its west, which generates air masses' descent. This adiabatic
descent balances the horizontal advection on the southern flank of the
midlatitude westerlies. Among the summertime descent regions, the strongest
is located over the EM. Initiation of the descent over the EM coincides with
the northward movement of heating during the onset of the monsoon. The
anticyclonic center over northwest Africa and the monsoon result in an
adiabatic warming that reduces the specific humidity and consequently
enhances further the descent due to diabatic radiative cooling under
cloudless sky conditions. Moreover, trajectory calculations performed by
Rodwell and Hoskins (1996) revealed that the bulk of the sinking air masses
originate from midlatitude regions rather than over the intense monsoon
convection areas over northern India. This is consistent with Tyrlis
et al. (2013), who analyzed the thermodynamic state over the EM and calculated
the temperature changes caused by horizontal advection by using ECMWF
forecasts for diabatic heating over this region. They found that subsidence
at middle and lower levels is primarily driven by the midlatitude westerly flow.
Furthermore, Tyrlis et al. (2013) pointed at the steep slopes of the
isentropes in the free troposphere caused by the westward migration of the
mid- and upper-level warming of the atmosphere away from the diabatic heating
sources, which further enhance subsidence over the EM.
(a) Closed Hadley cell circulation of the African monsoon depicted
by the vertical cross section of wind vectors for July–August, averaged over
the 30–40∘ E longitudinal band. (b) Closed Walker cell
circulation of the Asian monsoon depicted by the vertical cross section of
wind vectors for July–August, averaged over the 20–35∘ N
latitudinal band. The two figures are based on the NCEP/NCAR long-term
averages (1948–2016) with the position of the eastern Mediterranean (EM) in
red. NCEP reanalysis data are provided by the NOAA/OAR/ESRL PSD, Boulder,
Colorado, USA; http://www.esrl.noaa.gov/psd/.
NCEP/NCAR reanalysis long-term averages (1948–2016) of the relative
vorticity at 200 hPa (∼12kma.s.l.) for July–August.
The relative vorticity vector is generally perpendicular to the ground,
positive when the vector points upward, and negative when it points downward.
Note the negative relative vorticity region located over the southeastern
Mediterranean as a result from both shear and curvature negative relative
vorticity. Relative vorticity units are 10-5s-1.
NCEP reanalysis data are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado,
USA; http://www.esrl.noaa.gov/psd/.
However, subsidence is neither restricted to mid-tropospheric levels nor
solely associated with the descending branch of both of these general circulation
cells. In summer, at higher atmospheric layers, air masses converge and
subside over the EM as contributed by an anticyclonic curvature caused by
anticyclonic centers formed over the Balkans. Such centers cannot be
considered as extensions of the Azores high since they exhibit typical
warm-core high structures from the surface up to mid-tropospheric levels
(Anagnostopoulou et al., 2014). Tyrlis and Lelieveld (2013) point at wave
disturbances originating over the North Atlantic that activate intense ridges
over the Balkans. These ridges are further amplified by anticyclonic
vorticity advection from northwestern Africa and, in tandem with diabatic
cooling under clear skies, form such centers over central and southeastern
Europe. The second dynamic factor inducing subsidence is an anticyclonic wind
shear as related to the position of the subtropical jet. Under these
circumstances, the southeastern part of the EM is exposed to the southern
flank of the jet and therefore prone to negative shear vorticity. Although
shear vorticity is an order of magnitude smaller than planetary vorticity,
the nearby jet streak makes this relative vorticity component significant due to
the strong change in wind speeds across the jet. Contribution of both
components enhances negative vorticity, resulting in a total long-term mean
negative vorticity of -1 to -3×10-5s-1 at
200 hPa (∼12kma.s.l.) featuring the summer season
over the EM (Fig. 4).
The contribution of the above-mentioned dynamic subsidence generated by all
processes results in positive omega values, defined as the Lagrangian rate of
change in pressure with time, indicating a downward air motion over the whole
EM with its highest core of maximum subsidence over Crete, as depicted over
mid-tropospheric levels (500 hPa geopotential height) (Fig. 5).
Following the subsidence caused by the large-scale downward motion, the
warming and drying up are manifested by the delimiting sharp decrease in
relative humidity over the EM basin (Fig. 6).
Based on National Centers for Environmental Prediction/National Center for
Atmospheric Research (NCEP/NCAR) reanalysis for 2000–2012, Lensky and Dayan
(2015) have recently shown that the coincidence of negative vorticity
advection aloft accompanied by cold horizontal advection, at lower
tropospheric levels, featuring the EM during PT synoptic conditions, drive the
wind flow out of the thermal wind balance inducing a vertical downward motion
(Figs. 2 and 7).
Ziv et al. (2004) found that the cool advection associated with the PT (Fig. 2)
and the subsidence related to both descending branches of the African and
Asian monsoons (Fig. 3) are interrelated and tend to balance each other. They
suggest that this compensation mechanism explains the reduced day-to-day
temperature variations over the EM in summer (Fig. 8).
NCEP/NCAR reanalysis long-term averages of omega (Pa s-1)
at 500 hPa (∼5.5kma.s.l.) designating vertical
motion for July–August 1948–2016. The maximum subsidence of
0.1 Pas-1 is equivalent to a downward air motion of ∼1.5cms-1. NCEP reanalysis data are provided by the
NOAA/OAR/ESRL PSD, Boulder, Colorado, USA;
http://www.esrl.noaa.gov/psd/.
Long-term mean vertical cross section of relative humidity, averaged
over the 31–36∘ N latitudinal band for July–August 1948–2016 with
the eastern Mediterranean position, in dashed black lines. NCEP
reanalysis data are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado,
USA; http://www.esrl.noaa.gov/psd/.
However, this monotonic regime is interrupted by the occurrence of hot-day
events resulting from an expansion of the subtropical high from north Africa
towards the EM, which are prone to elevated concentration of air pollutants.
Harpaz et al. (2014) found that such episodes are confined to the lower
4 km and controlled by the intensity of the negative temperature
advection rather than by the prevailing subsidence.
(a) Blue contours display positive omega values
(cm s-1) representing the vertical descending air motion at
a mid-tropospheric level (700 hPa) (∼3kma.s.l.)
pointing at a core of 1 cms-1 located over Crete. Red contours
are negative omega values. (b) Blue contours display cold advection
calculated as multiplication of the horizontal thermal gradient by the wind
vector. Red contours indicate warm advection, both at 995 hPa level,
equivalent to about 140 ma.s.l. at 12:00 UTC during Persian Trough
summer synoptic conditions. NCEP reanalysis data for 2000–2012 are
provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA;
http://www.esrl.noaa.gov/psd/.
Schematic of the proposed mechanism during intensification of the
Asian monsoon (reproduced from Ziv et al., 2004).
Atmospheric dispersion conditions over the eastern Mediterranean
The vertical velocity involved in the mixing process within the turbulent
layer near the surface and specifically its depth are important parameters in
determining air pollutant concentrations at shallow tropospheric levels
(Zhang and Rao, 1999). The changes in the mixing layer depth (MLD, i.e., the
height of the convective atmospheric boundary layer marked by the base of
a thermal inversion) is governed by several factors: surface heating
(Holtslag and Van Ulden, 1983), horizontal advection determined by the
intensity of the sea breeze in coastal areas (McElroy and Smith, 1991; Lensky
and Dayan, 2012), local terrain over the continent (Kalthoff et al., 1998),
and the strength of the subsiding atmospheric air mass capping the mixed
layer, defined by the temperature profile within this stable layer and
synoptic-scale vertical motion (Dayan et al., 1988). Beside these factors,
the MLD is controlled also by thermal advection associated with synoptic
weather systems and therefore develops under strong forcing by synoptic-scale
circulations (Businger and Charnock, 1983; Holt and Raman, 1990;
Sinclair et al., 2010). Consequently, both the surface synoptic systems and
their associated upper tropospheric conditions should be taken into
consideration for understanding the behavior of the MLD over the EM basin and
its adjacent coastal region.
Within the EM, numerous studies on the relationship between synoptic
circulation and the structure of the MLD over the continental EM were
conducted in Israel, the southeastern part of the basin. In particular,
several studies were undertaken to characterize the spatial and temporal
behavior of the MLD (Neumann, 1952; Halevy and Steinberger, 1974;
Rindsberger, 1974, 1976; Dayan et al., 1988, 1996, 2002; Glaser et al., 1993;
Lieman and Alpert, 1993; Dayan and Rodnizki, 1999; Ziv et al., 2004) using
sounding measurements at the Israel Meteorological Service permanent site in
Beit Dagan (31.99∘ N, 34.82∘ E; 39 ma.s.l.),
8 km southeast of Tel Aviv and at other sporadic sounding sites.
The atmospheric noon-time mixed layer during the summer over the EM region is
indicated by a persistent elevated inversion base formed by a clear boundary
line separating two differing air masses, a cool and humid mass above ground
capped by much warmer and subsiding dry air. The MLD is controlled by the
interlaced relationship between the downward motion of the subsiding air
aloft and the depth of the PT at the surface (Fig. 9).
Due to the existing correlation between the MLD featuring the PT and air
pollution episodes over the EM evidenced in previous studies (Dayan and
Graber, 1981; Dayan et al., 1988; Koch and Dayan, 1992), this barometric
system was classified into three essential types (Fig. 10) defined by the
surface-pressure difference between Nicosia (35.16∘ N,
33.36∘ E; 149 ma.s.l.) in Cyprus and Cairo
(30.1∘ N, 31.4∘ E; 75 ma.s.l.) in Egypt, and the
temperature at 850 hPa in Beit Dagan (Israel): moderate PT, shallow
PT, and deep PT (for details, see Dayan et al., 2002).
Analyses of upper-air measurements carried out regularly at Beit Dagan, in
the central coastal plain of Israel, point at significant differences in the
MLD for the several modes of the PT. The overall summer mean noon-time mixing
depth values for 1981–1984 is 764±320m (Dayan et al., 1988).
A classification with respect to the modes defined above resulted in mean and
SD values of 428±144m and 1010±214m for the
shallow and deep PT modes, respectively (Koch and Dayan, 1992). The spatial
distribution of the mixing depth is rather homogeneous under deep PT
conditions over the central coastal plain of Israel as compared to the
shallow mode where its value is kept almost uniform above sea level while
penetrating inland. Due to the important implication of this behavior on the
built-up concentration of air pollutants, the lateral variance of the
mixing depth was tested for part of the upper-air measurements performed at
four sites concurrently during the 1981–1984 campaign (Dayan et al., 1988). These
sites on a west–east transect were Nizanim (31.7∘ N,
34.63∘ E; 10 ma.s.l.) on the southern coastal shore of
Israel; Beit Dagan (31.99∘ N, 34.82∘ E;
39 ma.s.l.) on the coastal plain; Ruhama (31.5∘ N,
34.7∘ E; 210 ma.s.l.) ∼20km inland in the
northern Negev Desert; and Jerusalem (31.77∘ N, 35.21∘ E;
786 ma.s.l.). The average thickness of the mixed layer when moving
from the coast inland is reduced by 350 m while reaching Jerusalem
(Fig. 11). The longitudinal variance of the MLD north–south vertical cross
section on 21 summer noon-time upper-air measurements performed
simultaneously at three sites ∼60km apart along the Israeli coast
revealed that the MLD decreases gradually from north to south (Dayan et al.,
1988). This finding is explained by the greater distance of the southern
sites from the cyclonic core of the PT (which persists in summer to the
northeast of Israel) as well as the decreased distance from the anticyclonic
center of the north African subtropical high (which persists during all
seasons to the southwest of Israel). This lateral and longitudinal variance
indicates that the most reduced summer MLDs are expected over the
southeastern coast of the EM.
Most of the boundary-layer studies from other coastal regions in the EM were
conducted over the Greek peninsula and the Aegean Sea. Kassomenos
et al. (1995) analyzed the seasonal distribution of the MLD over the greater
Athens area as obtained from the upper-air station of the Greek
Meteorological Service at the Hellinikon airport for the period 1974–1990.
They point at a noticeable annual variability in the afternoon MLD with
maximum values (∼800–1100 ma.s.l.) being observed by the end
of July. They explain these high values observed during summer and the
elevated inversion formed by the higher incoming solar radiation
characterizing this season, which is efficiently converted into sensible heat
flux, favoring the development of a deep mixing layer and the horizontal
transport of warm air masses. Nevertheless, few summer days with stably
stratified atmosphere and very low MLDs (∼300ma.s.l.)
inducing high surface pollution levels in the Athens basin (Greece) were
identified as well. This is consistent with Svensson (1996) who analyzed such
a summer day over the Athens basin by applying a three-dimensional coupled
mesoscale meteorological and photochemical model. Tombrou et al. (2015)
mapped the MLD as part of the Aegean-GAME (Aegean Pollution: Gaseous and
Aerosol airborne Measurements) for two summer days under Etesian flow
conditions over the Aegean Sea. The thermal profiles they analyzed
demonstrate a well-inflated MLD of 700 to 1000 ma.s.l. during
noon time over Crete as compared to the shallow marine boundary layer (∼400–500 ma.s.l.) observed over both the east and west Aegean
marine regions.
Characterizing the structure of the MLD spatial variation offshore over the
EM basin is important for getting better insight into the processes
which control the dispersion of contaminants over the sea. A few investigators,
including Gamo et al. (1982) and Kuwagata et al. (1990) for Japan, Stunder and
Sethuraman (1985) for the United States, and Gryning (1985) for Denmark, have
analyzed the spatial variations of the atmospheric mixing layer in coastal
areas. Similar studies as related to the EM basin are quite limited and deal
also mainly with the conditions not directly located over the open sea but
rather at sites distant from the coastline.
In a 2006–2011 study based on a remote sensing tool, the ECMWF model, and
radiosonde observations launched at Thessaloniki airport (Greece,
40.6∘ N, 22.9∘ E; 10 ma.s.l.) ∼1km
from the coastline, Leventidu et al. (2013) found the MLD seasonal cycles
peak with a summer maximum of 1400, 1800, and 2100 ma.s.l. in June,
July, and August, respectively.
Seasonal map of the mixing layer depth (m) for the summer (June, July,
August) of 1987 over the Mediterranean region at 12:00 UTC
(from Dayan et al., 1996; used with permission from Kluwer Academic Publishers).
Much earlier in the unique study of this type that we are aware of, Dayan
et al. (1996) evaluated the spatial and seasonal distribution of the MLD
over the whole Mediterranean Basin. Based on ∼65 000 air measurements
from 45 radiosonde stations within and surrounding the basin from spring 1986
through winter 1988, the MLD was derived from the potential temperature
gradient measured within the boundary layer and the capping stable layer
above it. As expected, the summer values proved to be generally higher over
land and minimum over the most eastern and western limits of the
Mediterranean Basin (Fig. 12). They concluded that the distance from the
coastline and topography are the main factors influencing the spatial
distribution of the MLD. The steep gradient in MLD values observed moving
onshore is consistent with the elevated summer values in Thessaloniki
(Greece) reported by Leventidu et al. (2013).
Dayan et al. (1996) found that the most striking temporal effect on MLD
distribution over the basin is caused by synoptic weather systems and the
intensity of the sea breeze along the coast. The diminishing of the MLD over
the Mediterranean Basin moving from its center eastwards toward the EM coast
that they have observed is consistent with the unique series of measurements
of the temperature profiles performed during the summer of 1987 near the Port
of Ashdod (31.82∘ N, 34.65∘ E), some 40 km south of
Tel Aviv (Israel) at 2 to 22 km from shore using a tethered balloon
where prominent inversion bases of 350 to 600 ma.s.l. were observed
(Barkan and Feliks, 1993). Moreover, such limited MLD values over the sea
were obtained in the airborne Gradient in Longitude of Atmospheric
constituents above the Mediterranean Basin (GLAM) campaign in August 2014
(Zbinden et al., 2016): the MLD over the sea measured in the period 6–10
August 2014 was approximately 800 ma.s.l. over Crete, diminishing
to about 400–500 ma.s.l. over Cyprus.
The diurnal behavior of the MLD is assessed in the Israeli coastal plain
based on routine radiosonde ascents that are, unfortunately, of coarse
temporal resolution. The hourly maximum MLD is between 23:00 and 05:00 UTC
for all seasons and decreases gradually toward its minimal value at
18:00 UTC (Dayan and Rodnizki, 1999).
However, since this cycle is governed mainly by synoptic weather systems, and
the strength of the sea breeze, this behavior would be more significant for
the summer. During this season, the variation of the mixed-layer height due
to diurnal variations of solar radiation and local terrain effects is not
obstructed by large-scale variations caused by frequent transitions between
different synoptic configurations, as indicated by other seasons.
Consequently, MLD variation is most evident during the summer, mainly
controlled by the daily sea breeze cycle and heat fluxes that are most
intensive at this time. The layer minimal depth of
760 ma.s.l. along the coast is usually observed during late afternoon hours when
heat fluxes dissipate rapidly and the wind speed of the cool sea breeze
reaches its minimal rate. This process results in a decrease of the marine
turbulent boundary-layer depth (Dayan and Rodnizki, 1999). These MLDs are
less developed as compared to the mean MLDs of 850 ma.s.l. observed
over the Athens basin by Kassomenos et al. (1995).
Assessing the atmospheric dispersion conditions is commonly derived from the
ventilation rates' calculation. This term is the MLD multiplied by the mean
wind speed in the mixed layer, representing the potential of the atmosphere
to dilute and transport contaminants away from a source region. Matvev
et al. (2002) have calculated over 1948–1999 the mean and SD of the mixing
depth, wind speed, and long-term range of ventilation rates at the Israel
Meteorological Service sounding site in Beit Dagan (Israel) for the summer.
A criterion usually adopted is that if the ventilation coefficient is less
than 6000 m2s-1, the site has limited ventilation (Dobbins,
1979; Pielke and Stocker, 1991).
Monthly long-term means (LTMs) and Sds of the mixing layer depth
(MLD), wind speed, and range of ventilation rates over
Beit Dagan in the central coast of the EM. LTM and SD values for MLD include the years 1955–1968 (Rindsberger, 1974), 1981–1984
(Dayan et al., 1988), and 1987–1989 (Dayan and Rodniski, 1999). LTM and SD values for wind speeds are from the NCEP/NCAR reanalysis
project (NOAA-CIRES Climate Diagnostics Center) for a 51-year data record over 1948–1999 from http://www.esrl.noaa.gov/psd/
(adapted from Matvev et al., 2002).
MonthMLD Wind speed Ventilation(m) (ms-1) rates (m2s-1)LTMSDLTMSDRange of LTMJune8104705.52.251105–9920July8704505.01.651365–8780August8203954.51.501275–7290
Their results (Table 1) clearly show that the monthly long-term mean
ventilation rates of ∼4500m2s-1 characterizing the EM
coastal zone during summer are reduced and therefore inhibit an efficient
dispersion of pollutants as compared to the summer mean values of ∼7000m2s-1 obtained by Kassomenos et al. (1995) over the
greater Athens area.
Air mass origins over the eastern Mediterranean
The chemical composition of an air mass is inevitably related to its origin
and pathway. Consequently, both of these terms are indispensable in explaining
its composition (Fleming et al., 2012). Studies of the long-range transport
(LRT) of pollution by trajectory models help us to interpret and better
define the movement and removal processes affecting atmospheric
concentrations. Although changes in wind direction are observed on a diurnal
and seasonal basis depending on the synoptic conditions affecting the region,
the prevailing wind flows over the EM are from the west towards the east.
Therefore, air pollutants emitted from upwind sources to the west of the EM
will reach the EM and will be added to those emitted locally. Indeed,
numerous observational and modeling studies have confirmed that the EM is
affected by the long-range transport of air pollutants originating from
Europe (e.g., Dayan, 1986; Luria et al., 1996; Wanger et al., 2000; Erel
et al., 2002, 2007, 2013; Matvev et al., 2002; Rudich et al., 2008; Drori
et al., 2012).
To get insight into the LRT over the EM, the Air Resources Laboratory's
trajectory model (GAMBIT – Gridded Atmospheric Multi-Level Backward Isobaric
Trajectories; Harris, 1982) was applied over 1978–1982 (Dayan, 1986). The
duration of each trajectory was chosen as 5 days backward in time, enabling
the tracing of air masses originating from Europe, the Mediterranean Basin,
northern Africa, and the Near East close to the EM central coast of Israel
(Fig. 13).
The 850 hPa level (∼1500ma.s.l.) was chosen as the most representative of the transport layer. This level is
selected as the intermediary level between the surface wind regime and the regime of upper winds relatively free from local surface
effects. Trajectory direction was divided into five distinctive geographical classes as shown in Fig. 13. Respective occurrences and
seasonal distributions can be summarized as follows:
The long fetch of maritime air masses from northwest Europe crossing the Mediterranean Sea, accounting for 36 %, was the
most frequent on average and fell evenly throughout the whole year.
Northeast continental flow that originated in eastern Europe, accounting for 30 %, was the most frequent during the
summer season.
Southeast flow from the Arabian Peninsula, accounting for 5 %, was infrequent, occurring mainly during the
autumn.
Southwest flow along the north African coast, accounting for 11 %, was the most frequent during late winter and spring.
South–southwest flow from inland north Africa accounted for 7 %, with a late winter and spring maximum.
Therefore, (1) and (2) trajectory types are indeed predominant with a summer
maximum occurrence (>66 %) over the EM coastal zone.
The 5-year (1983–1987) flow climatology study of back trajectories at
Aliartos, Greece (38.22∘ N, 23.00∘ E), revealed that about
40 % of the 850 hPa back trajectories arriving at this site
during summer originate from northwest and north sectors (Katsoulis, 1999),
which is consistent with the flow patterns reported by Kubilay (1996) for
Mersin, Turkey. Katsoulis (1999) suggested that these predominant flow
directions point at northeastern Europe and northwestern Asia as potential
source regions.
These studies show that the main flow direction to the EM observed during
summer lies between west and north wind sectors. This implies that the most
probable source areas reaching and affecting the northern and eastern parts
of this basin are the industrialized countries of eastern and central Europe
located upwind of this part of the basin.
Summer atmospheric air pollutant concentrations
The EM is one of the regions in the world where elevated concentrations of
primary and secondary gaseous air pollutants have been reported frequently.
This region is influenced not only by local atmospheric dispersion conditions
but also by the ability of the atmosphere to inherit a significant proportion
of pollutants from European sources.
After reviewing the atmospheric dispersion and transport conditions
characterizing the EM during the summer, a summary of the essential results
published over the last decade dealing with trace gases and anthropogenic
sulfate aerosol concentrations over this region is presented. These studies
demonstrate how the above-described global- and synoptic-scale processes
control the extent of transboundary transport of air pollutants and chemical
composition and concentrations over the EM.
Processes controlling O3 formation
Most tropospheric O3 formation occurs when nitrogen oxides
(NOx), CO, and volatile organic compounds (VOCs) react in
the atmosphere in the presence of sunlight. Due to cloud-free conditions,
high incoming solar radiation characterizes the EM during summer (Lelieveld
et al., 2002), which enhances the build-up of O3 concentrations.
Numerous researchers have identified the EM as a “hot spot” of summertime
tropospheric ozone (e.g., Stohl et al., 2001; Roelofs et al., 2003; Zbinden
et al., 2013; Zanis et al., 2014; Doche et al., 2014; Safieddine et al.,
2014).
Zbinden et al. (2013) derived the climatological profiles and column contents
of tropospheric O3 from the Measurement of Ozone by Airbus in-service
Aircraft program (MOZAIC) over the mid-northern latitudes (24 to
50∘ N) over the 1994–2009 period. Among the 11 most visited sites
by the MOZAIC aircrafts is the EM cluster, which comprises 702 profile data
from the two airports in Cairo (31.39∘ E, 30.10∘ N, in
Egypt) and Tel Aviv (34.89∘ E, 32.00∘ N, in Israel), from
which monthly means were derived. The O3 volume mixing ratio obtained
were converted to Dobson units (DUs) and validated against coincident
ozonesonde profiles. Considering all sites, the EM reaches the largest
tropospheric O3 column concentration of 43.2 DU in July, which is
related to an extreme summer maximum within 1–5 km, in agreement
with the results derived from the space-borne Ozone Monitoring Instrument
(OMI) and Microwave Limb Sounder (MLS) by Ziemke et al. (2011), pointing at
the favorable photochemical conditions characterizing this region.
Zanis et al. (2014) identified a summertime pool with high O3
concentrations in the mid-troposphere over the EM over the 1998–2009 period
as derived from the ERA-Interim reanalysis O3 data, the Tropospheric
Emission Spectrometer (TES) satellite O3 data, and simulations with
the EMAC (ECHAM5–MESSy) atmospheric chemistry–climate model. They indicated
that the high O3 pool over the mid-troposphere is controlled by the
downward transport from the upper troposphere and lower stratosphere over
this part of the Mediterranean Basin (MB), which is characterized by large-scale subsidence. This
subsidence is regulated by the Asian monsoon as described in Sect. 2.1.
Furthermore, Zanis et al. (2014), based on previous case studies (e.g., Galani
et al., 2003; Akritidis et al., 2010), climatological studies (e.g.,
Sprenger and Wernli, 2003; James et al., 2003), and their own results, deduced
that the mechanism leading to high tropospheric O3 over the EM
consists of two essential consecutive phases. In the first stage, an enrichment
in stratospheric O3 occurs into the upper troposphere via
a stratosphere-to-troposphere transport process. In the second stage, these
O3-rich air masses are transported downward by the strong summertime
subsidence characterizing this region.
June, July, and August monthly means of O3 concentrations
(ppbv) at 3 km partial column measured by IASI in
summer (June, July, August) within the 2007–2012 period over the Mediterranean (IASI morning overpasses). Only the observations over
the sea are considered in the averages. The monthly means referred to as “CLIM” represent the averages over the whole period
(adapted from Doche et al., 2014).
Doche et al. (2014) analyzed tropospheric O3 concentrations for the
2007–2012 period as observed over the MB by the space-borne Infrared
Atmospheric Sounding Interferometer (IASI). They identified an abrupt
west–east O3 gradient in the lower troposphere over the
Mediterranean Basin with the highest concentrations observed over its eastern
part. These concentrations were observed at mid-tropospheric layers
(3 km) caused by subsiding O3-rich air masses from the
upper troposphere typifying summer. A clear and consistent seasonal
variability emerges from their measurements, showing a maximum of the
3 km partial column O3 concentration in July (Fig. 14). This
is consistent with the study of Tyrlis and Lelieveld (2013) who found that
the key dynamic driving factors yielding to high O3 concentrations in
late July and early August, in the middle and lower free troposphere, are maximum
tropopause folding activities, i.e., stratospheric air intruding into the
troposphere and the subsidence over the EM, featuring Etesian outbreaks,
which are temporally well correlated with the Indian monsoon. This tropopause
folding is manifested by a slightly lower tropopause in the middle and lower free
troposphere observed during such outbreaks over the latitude of the Aegean,
forming a narrow “transport corridor” of positive potential vorticity
anomalies. Tyrlis and Lelieveld (2013) argue that such frequent subsidence of
high potential vorticity illustrates the important role of stratospheric
intrusions in the summer dynamic conditions over the EM. Furthermore,
a climatology of tropopause folds over this region based on the ERA-Interim
data spanning the period 1979–2012, identified the Anatolian plateau as a hot
spot of fold development that occurs ∼25 % of the time during July
and August, and a seasonal evolution linked with the south Asian monsoon
(Tyrlis et al., 2014). The contribution of tropopause folds in the summertime
pool of tropospheric O3 over the EM was confirmed by Akritidis
et al. (2016) as simulated with the EMAC atmospheric chemistry model.
Based on IASI measurements and the Weather Research and Forecasting Model
with chemistry (WRF-Chem), Safieddine et al. (2014) have shown that the air
column of the first 2 km above ground is enriched by anthropogenic
O3. Above 4 km, O3 is mostly originating from outside
the Mediterranean Basin by LRT process or generated through
stratosphere-to-troposphere exchange characterizing the EM during the summer.
Air masses from surrounding regions in the EM atmosphere have a great impact
on surface O3 concentrations. In a recent study, Myriokefalitakis
et al. (2016) have investigated the contribution of LRT to the O3 and CO
budget in the EM basin using a global chemistry-transport model (CTM), the
TM4-ECPL, driven by ECMWF interim reanalysis project (ERA-Interim)
meteorology. They found that about 8 % of surface O3
concentrations are affected by local anthropogenic emissions, whereas
subsiding air masses from the free troposphere and horizontal transport from
surrounding regions provide about 38 and 51 % of O3 sources,
respectively, into the EM mixed-layer depth. Although elevated O3
concentrations over the EM during the summer are mainly attributed to LRT of
polluted air masses originating from Europe and lingering over the
Mediterranean Basin, their enhancement as a secondary pollutant is also caused
by their precursors emitted along the coasts of the EM. Consequently, several
studies dealing with O3 concentrations measured over coastal sites
surrounding the EM and its inland penetration are presented.
Measurements of O3 were performed at several sites in Crete and
Greece and for rather long periods: over the northern coast of Crete in
Finokalia (35.50∘ N, 26.10∘ E), 70 km northeast of
Heraklion, from September 1997 to September 1999 (Kouvarakis et al., 2000);
from a rural area (40.53∘ N, 23.83∘ E) close to
Thessaloniki in the north of Greece from March 2000 to January 2001; and from
an O3 analyzer installed in a vessel traveling routinely from
Heraklion, Crete, to Thessaloniki, Greece, from August to November 2000. Based
on these measurements, Kouvarakis et al. (2002) pointed out the existence of
a well-defined seasonal cycle in boundary-layer O3, with a summer
maximum both above the Aegean Sea and at Finokalia. They indicated that LRT
is the main factor accounting for the elevated O3 levels above the
EM. This finding is consistent with the 1997–2004 surface O3 time
series at Finokalia (Crete) of Gerasopoulos et al. (2005), who investigated
the mechanisms that control O3 levels and its variability. They
identified transport from the European continent as the main mechanism
controlling the O3 levels in the EM, especially during summer when
O3 reaches a July maximum of 58±10ppbv. Moreover, on
a larger regional scale, Kourtidis et al. (2002) used ozonesonde ascents,
lidar observations, ship cruises, and aircraft flights to show that south and
southwestern synoptic flows associated with Saharan dust events result in
lower O3 above the planetary boundary layer by 20–35 ppbv,
as compared to northerly flows, which transport air from continental Europe.
Based on 16 years of O3 concentrations measured at the EMEP Agia
Marina Xyliatou rural background station in Cyprus and three other remote marine
sites over the western, central, and eastern parts of the island, Kleanthous
et al. (2014) have shown that local precursors contribute to only about
6 % (∼3ppbv) of the observed O3. However, elevated
concentrations of this secondary pollutant occurring in summer are attributed
to LRT of air masses mainly originating from northerly and westerly
directions. The summer average annual maximum of 54.3±4.7ppbv
was observed to be related to the transport of polluted air masses from the
Middle East, and eastern and central Europe toward Cyprus.
Despite the prevailing synoptic meteorological conditions featuring the EM in
summer, the differing pathways of the LRT of polluted air masses can affect
differently the build-up of pollutants concentrations. To investigate such
changes, Wanger et al. (2000) performed a comprehensive study that included
150 h of instrumented aircraft monitoring flights comparing two events of
air mass transport (September 1993 and June 1994) representing two distinct
types of LRT. This airborne study comprised flight paths performed
approximately 70 km offshore parallel to the Israeli coastline and
180 km in length with Tel Aviv in the center.
These flights were performed during midday under westerly wind flow
conditions at an altitude of 300 ma.s.l. (well within the
atmospheric mixed layer). While both wind flow conditions were nearly similar
through the measurement periods and along the 180 km flight path, the
air mass sampled in September 1993 was much “cleaner” than the one sampled
in June 1994. The averaged O3 concentration of the first campaign was
39 ± 7 ppbv compared to 48 ± 9 ppbv in the second
period. Wanger et al. (2000) model simulations showed that the pollution
sources in southern Europe and the Balkans did not affect the EM coasts in
September 1993, contrarily to the synoptic conditions and simulation results
for the June 1994 period where the winds over the EM tended to be
northwesterly and thus forced the polluted air masses toward the coasts of
the EM.
The summer synoptic and dynamic conditions prevailing over the EM supply the
essential ingredients for the build-up of O3 concentrations. Based
on the similar climatic conditions between the Los Angeles Basin (USA) and
the EM, Dayan and Koch (1996) proposed a theoretical description of the
cyclic mechanism in summer, leading to fumigation (i.e., a downward dispersion
of an enriched O3 cloud toward the ground) further inland from the EM
coast. Under the deep mode of the PT, stronger westerly winds, acting as
a weak cold front (Fig. 15, panel A1), penetrate far inland, undercutting the
mixed layer polluted by O3 from the previous day (Fig. 15, panel A2).
In this way, part of the mixed layer containing O3 is pushed upward
and isolated from the ground. If the pressure gradient weakens on the
following day, the western flow weakens (Fig. 15, panel B1). The cooling
effect of the cool and moist marine air is consequently reduced and the
convective boundary layer inflates rapidly. When the top of the mixed layer
reaches the elevated O3 cloud, the latter is penetrated by convective
currents (Fig. 15, panel B2) and parts of the cloud are entrained toward the
ground in this fumigating process.
Elevated O3 concentrations (>117ppbv) were measured at
inland rural sites of central Israel during the 1988–1991 early summer
months (Peleg et al., 1994). Based on air mass back-trajectory analyses,
these elevated O3 mixing ratios were found only in the event of air masses
passing over the Tel Aviv metropolitan area. Furthermore, the very low ratio of
SO2 to NOx (sulfur dioxide, SO2)
clearly indicates that O3 precursors such as NOx,
CO, and VOCs originate mainly from fossil-fuel combustion from mobile sources
(Nirel and Dayan, 2001). These pollutants are subjected to chemical and
photochemical transformations in the presence of solar radiation and
atmospheric free radicals to form O3.
Over central Israel, the main source for these precursors emitted along the
Israeli coastline is transportation (Peleg et al., 1994). Since O3
and other secondary pollutant formation takes several hours, significant
transport and mixing occur simultaneously with the chemical reactions
(Seinfeld, 1989; Kley, 1997). Thus, increasing urban and commercial activity
along the highly populated Israeli coastal region, together with expanding
transportation activity in the Gaza region, was found to strongly deteriorate
inland air quality and, specifically, cause increasingly elevated inland
O3 levels. Model results showed that traffic emissions during the
morning rush hour from the Tel Aviv metropolitan area contribute about
60 % to the observed O3 concentrations (Ranmar et al., 2002).
Moreover, their study showed the summer season features a shallow mixed layer
and weak zonal flow, leading to poor ventilation rates, which restrict
O3 dispersion efficiency. These poor ventilation rates result in the
slow transport of O3 precursors, enabling their photochemical
transformation under intense solar radiation during their travel inland from
the EM coast.
However, elevated O3 concentrations are not limited to the summer
over the EM. Dayan and Levy (2002) found 103 “high-ozone days” where
O3 is >80ppbv for at least 2 h based on 24 Israeli sites
over 1997–1999. From their O3 temporal analyses, they concluded that
the highest values are more frequent during the transitional (spring and
autumn) seasons (65 % of 103 days) than during the summer season
(35 %).
Based on the recent remote sensing tools in conjunction with meteorological
observations and models, we conclude on the three essential processes that
control the O3 concentration during summer at various tropospheric
levels over the EM: (1) in the shallow troposphere, the horizontal transport
of O3-enriched air masses from eastern continental Europe to the
region controlled by the anticyclonic center over central and southeastern
Europe and the PT causing the Etesians; (2) the dynamic subsidence at
mid-tropospheric levels; and (3) the stratosphere-to-troposphere exchange in
the upper troposphere. At the surface of the EM coast, during transitional
seasons, high O3 episodes are associated with hot and dry air masses
originating east of Israel, where O3 precursor emissions are
negligible, demonstrating that high O3 levels are more dependent on
air mass characteristics than on upwind precursor emissions.
Particulate sulfate (SO4) abundance
Globally, the two main particulate SO4 precursors are SO2
from anthropogenic sources and volcanoes, and dimethyl sulfide (DMS) from
biogenic sources, especially marine plankton. In the EM atmosphere,
particulate SO4 contributes more than 50 % to the submicron
aerosol mass (Bardouki et al., 2003a, b; Sciare et al., 2005). A first
attempt to quantify the biogenic contribution caused by the oxidation of
marine DMS as a possible source to particulate SO4 observed over
the EM coastal region was carried out by Ganor et al. (2000). They used an
instrumented aircraft during August 1995 to sample DMS and methane sulfonic
acid (MSA) offshore and over land in Israel. Being exclusively produced by
oxidation of DMS, MSA was used as tracer. Ganor et al. (2000) found this
source as a rather limited contributor: between 6 and 22 % of the
non-sea-salt SO4 (nss-SO4) measured during summer was
attributed to marine biogenic production. Evidently, several other factors
favor particulate SO4 abundance over the EM. The homogeneous
conversion of gaseous SO2 to particulate SO4 is rather
slow, i.e., about 1–3 % h-1 (Meagher et al., 1981). Wet deposition
chiefly governs the atmospheric lifetime of SO4, estimated to be up
to 6 days on a global average (Chin et al., 2000). Due to rainless conditions
and associated wet deposition in summer, and the slow dry deposition velocity
of SO4 aerosol (∼0.01–0.4 cms-1), SO4
aerosols account for 50–90 % of the total sulfur (S) in transported air
masses toward the EM (Matvev et al., 2002). Two additional factors favor late
spring and summer particulate SO4 regional abundance. First is the
intense radiant energy emitted by the Sun under clear-sky conditions that
leads to an efficient oxidation of SO2 to SO4 via hydroxyl
radical (OH) as the predominant oxidant during daytime (Mihalopoulos et al.,
2007). Second is the prevailing summertime westerly winds that transport
SO4-rich air masses from sources over central Europe before
significant removal occurs. A pioneering study to measure particulate
SO4 in the background atmosphere of the EM was carried out by
Mihalopoulos et al. (1997) in Finokalia, Greece. They reported a mean
SO4 aerosol concentration of 188 neqm-3 (∼9µgm-3) with a minor marine contribution of about 5 %,
resulting in a concentration of 178 neqm-3 (∼8.5µgm-3) for nss-SO4. These summer
concentrations, about 10 % higher than those observed in Thessaloniki
(Tsidouridou and Samara, 1993), were associated with transport from eastern
and central Europe. This is consistent with Sciare et al. (2003), who
measured particulate nss-SO4 during a 1-month experiment in summer
2000 at a background site on Crete. They found a high average concentration
of 6 µgm-3 (∼62nmolem-3) for air masses
originating from Turkey and central Europe. Identical results were obtained
by Koulouri (2008), who measured similar nss-SO4 concentrations
during the period July 2004–July 2006.
Another source of SO4 aerosols is ship emissions, which contribute
substantially to atmospheric pollution over the summertime Mediterranean
region. Based on a regional atmospheric-chemistry model and a radiation
model, Marmer and Langmann (2005) found that the summer mean SO4
aerosol column burden over the Mediterranean is 7.8 mgm-2, with
54 % originating from ship emissions.
Concentrations of SO4-rich air masses have been measured
intermittently at various downwind ground sites in Israel, the easternmost
Mediterranean region, from an instrumented aircraft for a 10-year period
between 1984 and 1993 by Luria et al. (1996). They found that the
concentration of particulate SO4 observed during the summer was
relatively high compared to other world locations, exceeding occasionally
500 nmolem-3 as compared to wintertime levels that were in the
range of 50–100 nmolem-3. From airborne observations, Wanger
et al. (2000) measured an averaged SO4 concentration of
38±7nmolem-3 in their first series of measurement between 5
and 9 September 1993, and up to 108±63nmolem-3 between 15
and 21 June 1994. The annual average, calculated in Luria et al. (1996), is
100±15nmolem-3, which is twice as high as predicted for the
region by a global model and as high as reported for some of the most
polluted regions in the USA. They pointed to several indicators suggesting
that the origin of the particulate SO4 over the EM region is not
from local sources but the result of LRT. The indicators include the lack of
correlation between SO4 and primary pollutants, the high
SO4 to total S values, the origin of the air mass back
trajectories, and the fact that similar levels were observed during concurrent periods at
different sites. Throughout their study, a higher concentration of
SO4 was found during the afternoon hours, especially during the
summer and at the inland locations. However, aerosol chemical analyses from
a two-stage aerosol sampler from a receptor site in Sde Boker
(31.13∘ N, 34.88∘ E; 400 ma.s.l.) in southern
Israel, point at a significant decline of 24 % of these elevated
nss-SO4 mean concentrations for the summer months (July and August)
from ∼3µgm-3 in 1994 to ∼2.3µgm-3 for 2004. This decline is attributed to the
decrease of S emissions in central and eastern Europe over the past 3
decades. Indeed, the majority (60 %) of the calculated air mass back
trajectories related to extreme events (during which the fine fraction S
concentration at Sde Boker exceeded a threshold of 3 µgm-3)
originated from Russia, Ukraine, and the northern Black Sea region (Karnieli
et al., 2009).
The effect of land and sea breeze on coastal meteorology in general and the
interaction between land and sea breeze and air pollutants in particular
plays an important role in determining many aspects of coastal environments
around the world. A meteorological phenomenon that is often associated with
the land and sea breeze is air mass recirculation in coastal regions (Miller
et al., 2003; Levy et al., 2008). Sulfate particles measured along the
central coast of Israel in mid-August 1987 and mid-August 1995, and identified
by lesser microprobe analysis, have shown that the concentration during land
breeze was 6–10 times higher (34.6–64.1 µgm-3) as
compared to sea breeze conditions (4.3–7.1 µgm-3) (Ganor
et al., 1998).
In another attempt to quantify the S flux arriving at Israel's western coast
from Europe and the Israeli pollution contribution to the air masses leaving
its eastern borders towards Jordan, Matvev et al. (2002) conducted 14
research flights at an altitude of approximately 300 ma.g.l.,
measuring SO2 and particulate SO4 during the summer and
autumn seasons. Two different legs were performed for each research flight:
the first over the Mediterranean Sea, west of the Israeli coast, and the
second along the Jordan Valley. Their results have shown that the influx of S
reaching the Israeli coast from Europe varied in the range of
1–30 mgSh-1, depending on the measuring season. The
SO4 level in the incoming LRT air masses was at least 50 % of
the total S content. The contribution of the local pollutant sources to the
outgoing easterly fluxes also strongly varied with the season. The Israeli
sources contributed an average of 25 mgSh-1 to the total
pollution flux during the early and late summer as compared to only
approximately 9 mgSh-1 during the autumn period. The synoptic
analysis indicates that conditions during the summer in Israel favor the
accumulation of pollution species above the Mediterranean Basin from upwind
European sources. This season is characterized by weak zonal flow within
a shallow mixed layer that led to poor ventilation rates, limiting an
efficient dispersion of these pollutants during their transport eastward.
Under these summer conditions, influx local contribution and the total
outflux of these pollutants are elevated as opposed to other seasons. To
illustrate, during autumn, the EM is usually subjected to weak easterly
winds, interrupted at times by strong westerly wind flows inducing higher
ventilation rates. Such autumnal meteorological conditions and the lack of
major emitting sources eastwards of Israel result in lower S budgets to and
from Israel.
Compilation by Rudich et al. (2008) of sulfate particulate
concentrations and yearly fluxes from (a) Luria et al. (1996), (b) Wanger
et al. (2000), and (c) Matvev et al. (2002).
1 Following Matvev et al. (2002), conversion from nmolem-3 to
yearly fluxes takes into account the vector component of onshore wind speed,
length of flight leg, and the MLD. 2 The June 1994 flight has been
performed during a highly polluted month over Israel.
An estimate of the yearly flux showed that approximately 0.06 TgS
arrived at the Israeli coast from the west (Matvev et al., 2002). This is
approximately 15 % of the pollution leaving Europe towards the EM. The
outgoing flux towards Jordan contributed by local sources was calculated to
be 0.13 TgS yr-1, i.e., almost all the S air pollution
emitted in Israel. The results of the flux rates for the S compounds over
Israel are summarized in Table 2 for the different research flights and field
campaigns. These latter results show for the early summertime that the
uppermost fluxes from the west were averaging 0.19 Tgyr-1.
During this season, the levels doubled the averages for late summer
(0.085 Tgyr-1) and were over 5 times the average levels measured
for the autumn (0.035 Tgyr-1). The wide range in fluxes derived
is explained by the varying distance from the polluted coastline.
The aerosol optical depth (AOD), the vertical integral over an atmospheric
column of the incident light scattered and absorbed by aerosols, is often
used to estimate the aerosol loading in the atmosphere. Particulate
SO4 is among the numerous aerosol types. Nabat et al. (2013)
compared AOD from several model data to satellite-derived data for the period
2003–2010 over the Mediterranean region. They found that the AOD seasonal
cycle obtained from the Monitoring Atmospheric Composition and Climate (MACC)
reanalysis model, which includes Moderate-Resolution Imaging
Spectroradiometer (MODIS) AOD assimilation at 550 nm, much resembles
the satellite-derived AOD variability and has the best spatiotemporal
correlation compared to AErosol RObotic NETwork (AERONET) stations. Based on
these models and satellite-derived data, Nabat et al. (2013) have clearly
shown that particulate SO4 has a maximum during spring and summer
over the EM (Fig. 16). Matvev et al. (2002) performed airborne measurements
along a 150 km line west of the Israeli coast. They derived an annual
flux of the order of 0.06 Tgyr-1 of (dry) S across the
corresponding surface. Given the observed ratio of SO4 to total S
of 40–90 % in the region (Matvev et al., 2002; Sciare et al., 2003), the
annual flux of SO4 based on field measurements is
0.024–0.054 Tgyr-1. Rudich et al. (2008) used satellite data to
estimate the pollution transport toward the EM. MODIS Terra- and Aqua-derived
estimates of the annual SO4 flux along the same transect are 0.038
and 0.040 Tgyr-1, respectively, in the middle of the range
obtained from field observations.
Average aerosol optical depth (AOD) contributed by particulate
sulfate validated against AERONET AOD observations over the
period 2003–2009. As mentioned at http://www.esrl.noaa.gov/gmd/grad/surfrad/aod/, a value of 0.01 corresponds to an extremely
clean atmosphere, and a value of 0.4 to a very hazy condition (the 2003–2010 average AOD over the Mediterranean Basin is ∼0.20) (adapted from Nabat et al., 2013).
Rudich et al. (2008) also found that MODIS-based estimates (from Terra and
Aqua satellites) of the SO4 flux agree reasonably well with the
Goddard Chemistry Aerosol Radiation and Transport (GOCART) model simulations
of anthropogenic SO4, as shown in Fig. 17 for seasonal averages.
The annual SO4 flux from the GOCART model is
0.181 Tgyr-1, about 18 % higher than the MODIS/Terra
estimate of 0.153 Tgyr-1. Similar comparison on a seasonal basis
demonstrates that the GOCART model overestimates the winter (by ∼85 %) and
spring (by ∼30 %) fluxes while it underestimates the summer and
autumn fluxes by 10–25 %. If we consider the comparison between the
GOCART model and MODIS/Aqua, the model annual flux is
0.201 Tgyr-1, about 25 % higher than the MODIS/Aqua estimate
of 0.159 Tgyr-1. On a seasonal basis, their estimates are in
excellent agreement in summer and autumn but about 50 % higher in the
MODIS/Aqua winter and spring estimates. Based on the comparison of the two
instruments, the model results, and the consistency with the aircraft
measurements, they concluded that both MODIS instruments can be used for
estimating the flux of pollution based on their daily AOD retrievals.
Seasonal flux (Tg season-1) of dry sulfate as derived from MODIS/Terra and MODIS/Aqua space-borne observations compared to
GOCART model-derived results, along the 150 km Israeli coastline of the eastern Mediterranean Sea. The seasons on the x axis are
winter (DJF), spring (MAM), summer (JJA), and autumn (SON) (adapted from Rudich et al., 2008).
Local formation and long-range transport of total reactive nitrogen (NOy)
Total reactive nitrogen (NOy) is a collective term for
oxidized forms of nitrogen in the atmosphere such as nitric oxide (NO),
nitrogen dioxide (NO2), nitric acid (HNO3), nitrous acid
(HNO2), nitrate (NO3), nitrogen pentoxide
(2N2O5), peroxynitric acid (HNO4), peroxyacetyl
nitrate (PAN), and other organic nitrates (Emmons et al., 1997). Research
studies measuring inorganic reactive nitrogen compounds over marine areas, in
general, and more specifically over the EM basin, are scarce (Lawrence and
Crutzen, 1999; Corbett et al., 1999; Veceras et al., 2008). Measurements of
NO2, HNO3, and HNO2 undertaken with instrumentation
aboard a research vessel in the Aegean Sea between 25 and 29 July 2000 revealed
typical NO2 concentrations of 4–6 ppbv with a broad maximum
of 20–30 ppbv. The level of NO2 was relatively high during
the night and low during the day due to enhanced photochemical activity,
vertical mixing, and the daily wind characteristics. Extreme NO2
concentrations were caused by upslope wind bringing air from marine traffic
emissions trapped within the marine atmospheric boundary layer. The
concentration of both nitric and nitrous acids in ambient air of the Aegean
Sea was low (below 50 pptv). Večeřa et al. (2008) explained
these results by the lack of precursors for these acids (Cohen et al., 2000),
the high solar irradiation leading to HNO3 dissociation, and the
reaction of HNO3 with sodium chloride aerosol.
NOy, identified as a precursor in the O3 formation,
was measured by Wanger et al. (2000) for two summer airborne campaigns over
the EM at an altitude of about 300 m (well within the MLD) using
a high-sensitivity NO-NOy analyzer (TEII 42 S,
chemiluminescence method, ±0.1ppbv sensitivity). In the first
campaign of September 1993, characterized by cleaner air mass conditions, an
average NOy concentration of 1.0±0.6ppbv
was measured as compared to 3.9±1.8ppbv sampled during the
June 1994 campaign.
The Mediterranean Intensive Oxidant Study (MINOS) campaign, performed in the
summer of 2001, allowed Lelieveld et al. (2002) to examine the air pollution
conditions at shallow and mid-tropospheric levels over the EM basin. During
this experiment, elevated concentrations, typically 0.1 to 0.2 ppbv,
of NO in the upper troposphere and only about 20 pptv within the MLD
were observed at the Finokalia station. However, the value measured within
the MLD at Finokalia was rather low and not typical for this site. From
autumn 1998 to summer 2000, a Thermo Environmental Model 42C high-sensitivity
chemiluminescence NOx analyzer with a detection limit of
50 pptv was operated at Finokalia in parallel with the O3
analyzer to monitor NO and NOx (Kouvarakis et al., 2002).
During the whole examined period, NO concentrations ranged between
50 pptv (most of the time) and 100 pptv, and
NOx′ (NOx′=NO+NO2+ PAN) between 0.1 and
4 ppbv. Kouvarakis et al. (2002) interpreted the very low
NO/NOx′ ratio obtained, which might indicate that the Finokalia station is affected by aged air masses. Furthermore,
they argued that the similar diurnal amplitude of O3 above the Aegean
Sea and at Finokalia during summer indicates that the regime of
NOx above the Aegean is similar to that observed at
Finokalia.
The observed diurnal evolution at Finokalia of NO and
NOz′ – the latter expressing mainly the sum of
NO2, NO, PAN-like compounds, organic nitrates, and HNO3 –
was used as a tracer of pollution by Gerasopoulos et al. (2006) to analyze
the diurnal variability of O3 over the EM. The diurnal cycles of
these two tracers based on 3.5 years of measurements point at a maximum
value of ∼70pptv for NO and up to ∼1.55ppbv for
NOz′. These maxima were observed 1–2 h
after the minimal O3 concentration was measured at about 06:30 UTC.
Ambient concentrations of NO, NO2, and NOx have
been also reported over the northwestern parts of Turkey. An
NO2 concentration of 8.5±4.8ppbv was obtained for the
summer of 2005 by collecting weekly average data in a sampling site in the
city Eskişehir, located 230 km to the west to the capital of
Turkey, by use of passive samplers (Özden et al., 2008). Im et al. (2008)
studied O3 pollution and its relationship with NOx
species based on hourly concentration levels of O3, NO, and
hydrocarbon measured between 2001 and 2005 in Kadıköy, an urban
district in the Anatolian side of Istanbul. The mean and SD for the
summer (June–August) NO, NO2, and NOx
concentrations reported for this 5-year period were 14.4±6.2,
22.7±2.7, and 37.7±14.3ppbv, respectively. Moreover, they
suggested that the very strong correlation they found between NO and
NOx implies that the NOx species are
mainly from local sources.
Traub et al. (2003) analyzed several trace gas concentrations measured along
flight tracks of the Deutsches Zentrum für Luft- und Raumfahrt (DLR)
Falcon aircraft over the eastern and central Mediterranean Sea during MINOS
in August 2001. In order to inquire into the role of LRT of pollutants in the
air masses above the Mediterranean area and to determine their source
regions, 5-day backward trajectories were computed and initialized along the
Falcon flight tracks. They found that all trajectories with source regions in
eastern Europe were associated with higher mean concentrations than those
from westerly directions. Traub et al. (2003) measured mean NO and
NOy concentrations of 0.05±0.02 and
1.4±0.4ppbv, respectively, for the computed trajectories within
the MLD originating from eastern Europe as compared to 0.04±0.01 and
1.1±0.5ppbv, respectively, for trajectories originating from
western Europe.
Increasing urban and commercial activity along the highly populated Israeli
coastal region, together with expanding transportation activity, has yielded
few ground-based measurement studies in order to quantify the impact of
local urban versus regional and foreign sources on the concentrations of the
NOx species, which vary in their atmospheric fate.
Results of half-hourly NOx concentrations recorded from
nine monitoring stations from 2002 to 2005 in the Haifa Bay, Israel, resulted
in a typical mean mixing ratio of 25 ppbv (Yuval et al., 2007) and
a typical background value below 0.5 ppbv for the summer over the EM
(Alper-Siman Tov et al., 1997). This background value was further evidenced
by Dayan et al. (2011) who analyzed NOx concentrations
during the Day of Atonement. On this day, all traffic and most of the
industrial activities cease in the Jewish populated parts of the country,
which provides a unique opportunity to test the relative contribution of
pollution sources within urban centers versus regional and foreign sources.
In a study aimed at analyzing the sources and sinks of HONO in urban areas,
and their seasonal dependency, Amaroso et al. (2008) carried out measurements
of HONO, NOx, O3, and SO2 during autumn
and summer in Ashdod (31∘49′ N,
34∘40′ E, 10 ma.s.l.) (south of Tel Aviv,
Israel), a typical coastal Mediterranean urban area. The 15-day July campaign
consisted of 4320 5 min averaged measurements, of HONO, NO, and NO2.
HONO analyses were performed with a liquid coil scrubbing/UV-vis instrument
(see Amaroso et al., 2008). NO and NO2 measurements were performed by
a Thermo Model 42C NO-NOx analyzer. The mean
concentrations obtained for this campaign were 1.4±2.0, 6.0±8.8, and
14.8±7.3ppbv for HONO, NO, and NO2, respectively. The
HONO mixing ratios obtained clearly point at the typical diurnal cycle with
nighttime maxima and daytime minima (Lammel and Cape, 1996).
Ranmar et al. (2002) addressed the dynamics of transboundary air pollution,
where transportation emissions (such as NOx and VOC)
originating from Israeli major coastal sources impact the onshore mixing
layer. Analysis of NOy data (here, the sum of all nitrogen
oxide species, excluding N2O), collected from 1 June to 30 September
for the years 1999 and 2000 at a monitoring station located in metropolitan
Tel Aviv, yielded an average of 24.5±15.1ppbv. They noted the
higher initial NOy levels during the morning rush hour
emissions that were subjected to a noticeable bleaching by the late morning
sea breeze in comparison to inland locations, which leveled off at relatively
higher midday concentrations. Ranmar et al. (2002) argued that this may
indicate, in the absence of any alternative NOy source,
that the early morning NOx produced by transportation
sources in Tel Aviv is transported inland, providing additional
NOy to the regions along its path.
Seasonal average over June–August 2006 of OMI NO2 columns
over the Mediterranean Sea
(1015moleculescm-2), retrieved from the OMI satellite and considering only maritime pixels (reproduced from
Marmer et al., 2009).
Besides cruises of research vessels, airborne campaigns, and ground truth
measurements, satellite-borne initiatives have been undertaken to get better
insight into the reactive nitrogen concentrations over the EM. Marmer
et al. (2009) used OMI (Boersma et al., 2004) as an observation tool to
measure atmospheric NO2 column concentrations in order to validate
ship emission inventories over the Mediterranean Basin. Figure 18 shows the
average OMI NO2 tropospheric columns (gridded to 0.125∘×0.125∘) over the Mediterranean Sea for June–August 2006. The
most prominent feature here is the elevated NO2 monthly mean. Under
cloud-free conditions, typical values ranged from 1.2 to
2.0×1015moleculescm-2 over the northeastern African
coast, the EM coast, the southern coast of Turkey, and the whole Aegean Sea,
as compared to over 6×1015moleculescm-2 for European
inland congested regions. Based on OMI NO2 tropospheric columns and
the Goddard Earth Observing System chemistry-transport (GEOS-Chem) model,
Vinken et al. (2014) attributed the elevated NO2 column regions over
the Mediterranean to NO2 emissions along ship tracks.
Carbon monoxide sources and pathways
CO has a global-average lifetime of about 2 months in the troposphere and
its molecular weight is close to that of air. This molecule is considered as
an excellent tracer for pollution sources and pollution pathways through the
troposphere. In addition to production by chemical oxidation in the
atmosphere, CO is emitted by biomass burning, man-made sources, vegetation,
and ocean. The CO seasonal cycle is mainly governed by the concentration of
OH in the troposphere (Novelli et al., 1992) and is expected to be the lowest
in the summer when photochemistry is active and the highest during late
winter or spring.
An assessment of CO baseline concentration levels at the surface over the EM
is presented based on few observational studies that have been conducted for
this pollutant. As part of a comparative air quality study, CO was analyzed
at Patras (38.25∘ N, 21.74∘ E) and Volos
(39.36∘ N, 22.94∘ E), two Mediterranean Greek coastal urban
sites (Riga-Karandinos and Saitanis, 2005). They observed an annual average
hourly mean concentration of 1.14 ppm over 1995–2003 at Volos as
compared to 0.95 ppm at Patras over 2001–2003. The diurnal pattern
at both sites during summer showed that vehicle-induced emissions contribute
significantly to CO levels, with peak concentrations of 1.14 and
0.96 ppm measured at 09:00 UTC at Volos and Patras, respectively.
Over the EM coast, hourly average CO measurements conducted by Saliba
et al. (2006) in the city of Beirut (33.89∘ N, 35.50∘ E),
Lebanon, point at an average monthly CO concentration during summer of
1.05 ppm, similar to the concentrations observed at Volos and Patras,
Greece (Riga-Karandinos and Saitanis, 2005).
CO concentrations were measured by Elbayoumi et al. (2014) from the autumn of
2011 through mid-2012 in the Gaza strip, in the southeastern coast of the EM
as part of an exposure study to assess the effect of seasonal variation on
the mean daily indoor–outdoor ratio at 12 schools located over the northern,
central, and southern strips of Gaza. They observed a 6 h average daily
outdoor CO concentrations of 0.96±0.91ppm for all the schools.
They further reported that the outdoor CO concentration spanned from 0.10 to
2.46 ppm with a mean of 0.88 ppm for urban sites and from
0.10 to 2.71 ppm with a mean of 1.02 ppm for overpopulated
sites along the Gaza strip.
Due to the key role CO plays in atmospheric chemistry, several
chemistry-transport modeling studies were devoted to this subject. CO was
measured and used as a tracer in such a model (Lelieveld and Dentener, 2000)
during the summer 2001 MINOS campaign (Lelieveld et al., 2002). The model
diagnosed CO from anthropogenic sources in different parts of Europe, North
America, and Asia. Trajectory calculations in the lower troposphere
identified western and eastern Europe as the main source emissions.
Consequently, model simulations were performed for August 2001 over Sardinia
(40∘ N, 8∘ E) in the western Mediterranean and over Crete
(35∘ N, 25∘ E). Considering the negligible impact of local
pollution sources, the high CO levels observed over Crete, in excess of
150 ppbv, were surprising. The model results indicated that regions
surrounding the Mediterranean such as southern Italy, Greece, Serbia,
Macedonia, the Middle East, and north Africa contribute relatively little to
the CO pollution, typically about 20 %. Furthermore, Lelieveld
et al. (2002) found that the EM is affected by CO-polluted air emitted from
eastern Europe, Poland, Ukraine, and Russia. This pollution flow, east of
the Carpathian Mountains, is channeled over the Black Sea and the Aegean Sea,
and contributes 60 to 80 % of the boundary-layer CO over the EM. Their
model results are consistent with aircraft measurements, showing that the
entire Mediterranean lower troposphere is polluted.
In the free EM troposphere, where westerly winds predominate, they revealed
quite a different situation as compared to concentrations measured within the
MLD. The mid-tropospheric CO measurements were ∼75–80 ppbv.
From their model tracer analysis, the largest contribution over the
Mediterranean is found originating from Asia (40 to 50 %). The CO typical
lifetime (∼2 months) enables air mass to circumnavigate the globe,
which results in a low variability of its concentrations. Lelieveld
et al. (2002) found that contributions by pollution from western and eastern
Europe to mid-tropospheric CO were only about 10 %.
Locations and elevations of NOAA Earth System Research Laboratory
Global Monitoring Division (ESRL/GMD) background sites for CO measurements
plotted in Fig. 19.
CodeNameLat. (∘ N)Long. (∘ E)Elev. (m)CountryWISWIS station Negev Desert31.1334.88400.0IsraelHUNHegyhátsál46.9516.65248.0HungaryLMPLampedusa35.5212.6245.0ItalyBSCBlack Sea Constanta44.1728.683.0RomaniaOXKOchsenkopf50.0311.801022.0GermanyBALBaltic Sea55.3517.223.0PolandMHDMace Head County Galway53.33-9.905.0Ireland
Monthly mean CO concentrations over 1996–2009 at Sde Boker (red)
and at seven European ESRL/GMD background stations (listed
in Table 3, multiple colors), compared to the 5-year averaged CO surface concentrations at Sde Boker (black) over 2003–2007 from
the MOZART-4 chemistry-transport model (adapted from Drori et al., 2012).
Drori et al. (2012) conducted a study to locate the various CO sources
converging from Europe, north Africa, and the Middle East and quantify their
respective contributions to the EM. Background CO concentrations are
monitored regularly over the southern part of Israel in Sde Boker (Weizmann
Institute of Science – WIS station Negev Desert: 31.13∘ N,
34.88∘ E; 400 m a.s.l.) as part of the National Oceanic and
Atmospheric Administration (NOAA) Earth System Research Laboratory Global
Monitoring Division (ESRL/GMD), which aims at representing the EM. While
comparing the seasonal cycle of Sde Boker to other European ESRL/GMD
background sites (see Table 3), one essential feature is eminent from their
results (represented in Fig. 19): CO concentrations are high over winter
months, decreasing abruptly during April, and increasing again from November.
A second maximum is observed during August compared to July and September
(Drori et al., 2012).
To get insight into the spatial distribution of CO concentrations over the
EM, the version 4 Measurement of Pollution in the Troposphere (MOPITT)
level-2 CO retrievals (Deeter et al., 2010) were employed by Drori
et al. (2012) using a priori information based on
the Model for OZone and Related chemical Tracers (MOZART-4)
chemistry-transport model simulation climatology (Emmons et al., 2010). The
averaging kernel profile obtained for a retrieval near the Sde Boker ESRL/GMD
station shows that, during the day, the 900 hPa retrieval sharply
peaks at the same level, indicating that there is a good sensitivity to lower
tropospheric concentration. The anomalous high concentration observed at the
WIS ESRL/GMD Sde Boker station, and calculated by the MOZART-4 model during
August (Fig. 19), might be limited to lower levels, and therefore averaging
over several layers might hide this signal. Furthermore, Drori et al. (2012)
compared the in situ measurements at Sde Boker and CO retrieved from MOPITT
to MOZART-4 model results. CO sources included direct emissions and secondary
production from hydrocarbons' oxidation, while CO sinks included a reaction
with OH and dry deposition. The seasonal cycle of surface CO at Sde Boker
simulated by MOZART and averaged for 5 consecutive years shows a similar
pattern exhibiting CO concentration reaching a maximum in February and
a second peak in mid-summer months (i.e., July and August) that surpasses
those of the early summer (i.e., May–June) (Fig. 19).
Monthly time series of total surface CO (black) at Sde Boker, Israel,
and contributions from specific sources (anthropogenic
in purple, chemical production in orange, biogenic in green, and fires in red; ocean is negligible and not shown) as simulated by
MOZART for 2006–2007 (adapted from Drori et al., 2012).
To attribute the CO sources affecting the EM, Drori et al. (2012) partitioned
these sources using a tagging method into five types: anthropogenic,
biogenic, fire, chemical production, and ocean. The total CO concentration
and specific contributions 2006–2007 time series of MOZART at the surface
at 30∘ N and 33.75∘ E are shown in Fig. 20 where ocean
sources' contributions are not shown (negligible). Both biogenic (green line)
and biomass burning sources (red line) have a minor contribution. Biogenic
sources are characterized by a distinct seasonal cycle with high contribution
over winter and low daily variability. Biomass burning has no defined
seasonal signature and contributes on an episodic event basis. CO from
chemical production (orange) contributes substantially (50–80 ppbv)
with a defined seasonal cycle: low during winter and autumn and high during
summer indicated by a low daily variability. Anthropogenic sources were found
to be the main contributor to the total CO (purple, 50–180 ppbv). As
expected, their seasonal cycle is indicated by elevated winter concentrations
decreasing during spring, slightly increasing during summer, and decreasing
again during autumn. The daily variability is high and similar to the total
CO daily variability. Comparing the daily variability of the various sources,
Drori et al. (2012) concluded that anthropogenic sources mainly govern total
CO daily variability over the EM.
To further attribute the CO surface daily variation, Drori et al. (2012)
tagged the anthropogenic sources for the three northern continents, i.e.,
North America, Europe, and Asia. Figure 21 shows the results of these
anthropogenic sources' attribution to the CO surface. European anthropogenic
sources contribute substantially (10–80 ppbv) to local CO
concentrations with the greatest daily variability all year round. Asian and
North American sources are in the same order of magnitude
(10–25 ppbv) with low daily variability during most of the year and
very small variability during summer. Obviously, daily summer CO variations
in the EM are mainly caused by European anthropogenic sources. The seasonal
cycle of the European contribution is very similar to the seasonal cycle of
total CO, indicated by a high concentration in winter, spring, and autumn and
a lower summer concentration. The contribution of European emissions to CO
surface concentrations is comparable to that from EM local emissions.
Drori et al. (2012) found, however, that local and European emission
contributions to local CO concentrations are generally negatively correlated,
meaning that either local or European sources are dominant, except during
summer, when both sources simultaneously affect the local CO concentration.
A possible reason for the positive summer correlation might be explained
by the short range of air mass transport caused by the dominant summer
synoptic system, i.e., the PT in its weak mode recirculating local and
European emissions, and by the fact that summer chemical production is
a major CO source over the EM.
Monthly time series of the European (red), Asian (blue), and North
American (green) anthropogenic contribution to the total
surface CO (black) at Sde Boker as simulated by MOZART over 2006–2007. Distinct continents are scaled on the left vertical axis and
total CO on the right vertical axis (adapted from Drori et al., 2012).
Another recent modeling study focused on CO concentrations was conducted by
Myriokefalitakis et al. (2016). They compared and validated model results
against in situ observations at the surface, in the mixed layer, and in the
free troposphere (between 850 hPa and the tropopause) in the
countryside and remote atmosphere over Europe for 2008. This study analyzes
the total CO budget and the partial contribution of regional anthropogenic,
biogenic, and biomass burning CO emissions in the EM. The budget calculated
for 2008 in the EM mixed layer, using a basic simulation relying on
anthropogenic emissions and meteorology, points at a load of 0.6 Tg
of CO, a chemical production of 10 Tgyr-1, primary emissions in
the region of 8 Tgyr-1, and a dry deposition flux of
3 Tgyr-1. Moreover, Myriokefalitakis et al. (2016) found that
subsidence from higher atmospheric layers typifying the EM summer is an
important CO source (12 Tgyr-1) in the EM free troposphere. At
the surface, anthropogenic local emissions in the EM were found to contribute
18 % to surface CO levels on an annual average. Over Cairo, out of the
total surface CO concentration, roughly 32 % are contributed by
anthropogenic sources. These EM CO concentration results are consistent with
previous modeling studies (e.g., Kanakidou et al., 2011; Drori et al., 2012;
Im and Kanakidou, 2012).
Methane concentrations
CH4 is the most abundant hydrocarbon in the atmosphere with
concentration originating from natural and anthropogenic sources. It is also
the biggest contributor to GHG after water vapor and CO2 due to its high
global warming potential relying on its infrared absorption and long
atmospheric lifetime of ∼8 years (Lelieveld et al., 1998), which
allows its mixing throughout the atmosphere. CH4 emissions are
primarily caused by microbiological decay of organic matter under depletion
of dissolved oxygen in wetlands, followed by decomposition of solid waste and
enteric fermentation from domestic livestock. As for the geologic sources,
a total geological CH4 flux of 53±11Tgyr-1 was
suggested, which accounts for 7–10 % of the total global CH4
budget (Etiope et al., 2008). The geological formations contributing to
CH4 over the greater area of the EM (25–50∘ N,
5–55∘ E) are mud volcanoes with essential hot spots located over
eastern Romania, the Black Sea, central and eastern Azerbaijan, and the
Caspian Sea.
In contrast to trace gases of short lifetimes such as NOx
and NOy, the long lifetime of CH4 over the EM may
lead to interannual fluctuations of concentrations caused by circumglobal
phenomena such as low-frequency global circulation patterns, i.e., the El
Niño–Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO),
or changes in global temperature. Langenfelds et al. (2002) point at major
biomass burning events linked to ENSO dry periods, which increased the growth
rate of CH4 over other parts of the world. Artuso et al. (2007)
compared the global average temperature anomaly to the growth rate of
CH4 in Lampedusa (35.5∘ N, 12.6∘ E), Italy, for the
period 1995–2005. The 0.71 positive correlation they found reflects the
strong relationship between these two factors. Over the EM, the NAO may
possibly affect the concentration evolution through changes in the
circulation (e.g., weakening of the northwesterly flow). However, so far, no
association was found between the NAO index trend and the CH4
concentration growth over this part of the basin. The only study analyzing
directly a possible association between the NAO index and CH4
concentration growth carried out by Chamard et al. (2003) in Lampedusa has
not found any relationship between these two factors.
Satellite ability to monitor the concentration of trace gases in the
atmosphere is important for completing the picture in regard to their
budget. Among the space-borne measurements of trace gases, the Scanning
Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY)
instrument was proven as a feasible tool to detect CH4 concentrations
(Bovensmann et al., 1999). Measurements of column-average volume mixing
ratios of CH4 were retrieved on a global basis (Frankerberg et al.,
2005).
Maps by 1∘×1∘ resolution of dry air
column-averaged mole fractions, denoted as SCIAMACHY WFM-DOAS
XCH4 levels, in 2003, including a yearly average (a),
a summer average (b), and an August average (c) in ppbv
(from Georgoulias et al., 2011; used with permission from Taylor and Francis).
Georgoulias et al. (2011) used data from the SCIAMACHY instrument aboard
the European environmental satellite (ENVISAT). SCIAMACHY's spectral
near-infrared nadir measurements are sensitive to CH4 and CO2
concentration changes at all atmospheric altitudes, including the one in the
mixed layer where the signal emitted from the surface source is the largest.
Annual, seasonal, and monthly spatial distribution of CH4 were
displayed for 2003 and 2004 based on the analysis of Weighting Function
Modified Differential Optical Absorption Spectroscopy (WFM-DOAS) version 1.0
(Schneising et al., 2009) dry air column-averaged mole fractions, denoted as
XCH4 (ppbv). The reflectivity of water surfaces is very low;
therefore, Georgoulias et al. (2011) mapped the concentration of CH4
over the EM basin discarding the Mediterranean Sea. To reduce the noise
inserted by the single pixel retrieval error and the temporal and spatial
sparsity of the data, the data were averaged on 1∘×1∘ monthly mean grids. Annual, summer, and August spatial
distributions for 2003 are displayed in Fig. 22a, b, and c,
respectively. Those maps illustrate an eminent seasonal variation with
a summer maximum in XCH4 levels observed in both consecutive years
(2004 not shown). The northeastern African coast exhibits the highest
XCH4 values, with a hot spot over the Nile Delta in Egypt in summer
and August. The lowest XCH4 levels along the Arabian Peninsula, the
Zagros Mountains, and eastern Anatolia mountain barrier coincide spatially with
high altitude areas. To examine to what extent the warm period affects the
annual, seasonal, and latitudinal patterns, Georgoulias et al. (2011) further
proceeded to a monthly analysis. They observed an increase in XCH4
levels during the summer season, with August being the month with the highest
levels (1775–1780±24ppbv) for both 2003 and 2004. The
highest values are concentrated in the northeastern part of the area
primarily in July–August. From July to September, there is a shift of high
XCH4 levels from higher to lower latitudes. Despite the abundance of
mud volcanoes over the greater area of the EM region, Georgoulias
et al. (2011) ruled out the possibility that the CH4 total columns
from SCIAMACHY (2003–2004) measured over these EM regions were attributed to
volcano eruptions.
Summer averaged vertical profiles of CH4 as measured by AIRS
(blue lines) and GOSAT (green lines), and as calculated
by MOCAGE (thin red lines) over the eastern (dashed lines) and western (solid lines) Mediterranean basins in summer 2010. Also shown
are the seasonally averaged MOCAGE profiles convolved with the AIRS averaging kernels (thick red lines) for the summer over the
eastern (dashed lines) and western (solid lines) Mediterranean basins (adapted from Ricaud et al., 2014).
Fields of CH4 as calculated by MOCAGE (c, d) and as
measured by IASI (a) in total column and AIRS (b)
at 260 hPa averaged for summer (July, July, August) 2009. Horizontal winds are from ARPEGE averaged over the same period. The
two blue squares represent the western and eastern Mediterranean basins (adapted from Ricaud et al., 2014).
Seasonal evolution of the difference in CH4 fields between
the eastern and western Mediterranean basins: (b) around
300 hPa as measured by AIRS (blue) and GOSAT (green) and as calculated
by LMDz-OR-INCA (yellow) and CNRM-AOCCM (brown), and (a) in the total
column as measured by IASI and calculated by MOCAGE (adapted from Ricaud et al., 2014).
Ricaud et al. (2014) presented a thorough analysis of atmospheric CH4
distributions over the Mediterranean Basin in the troposphere, as part of the
ChArMEx program, using both
satellite measurements and model simulations. For this reason, they analyzed
space-borne measurements from (i) the Thermal And Near infrared Sensor for
carbon Observations – Fourier Transform Spectrometer (TANSO-FTS) instrument on
the Greenhouse gases Observing SATellite (GOSAT), (ii) the
Atmospheric InfraRed Spectrometer (AIRS) on the AURA platform, and (iii) the
Infrared Atmospheric Sounder Interferometer (IASI) instrument aboard the
MetOp-A platform. These space-borne tools were used in conjunction with the
results obtained from three global models: the chemical transport model (CTM)
MOCAGE (Teyssèdre et al., 2007) and the two chemical climate models
(CCMs) CNRM-AOCCM (Michou et al., 2011) and LMDz-OR-INCA (Hourdin et al.,
2006). The sensitivity of those space-borne sensors is mainly located in the
upper tropospheric layers, peaking around 300 hPa with an envelope as
defined by the half width at half maximum of the averaging kernels (see
Fig. 23) from 400 to 200 hPa. Consequently, the comparisons between
measurements and model outputs of CH4 are mainly concentrated on the
layer around 300 hPa for AIRS and GOSAT, or considering the total
column for IASI.
The 6-day back-trajectory climatology from the point at
33∘ N and 35∘ E located off Israel in the eastern
Mediterranean Basin (red filled circle) derived for July–August over 2001–2010 every 12 h. The position of the gravity center of
each distribution (i.e., the maximum in the probability density function) at each level is represented every 24 h by a star
(adapted from Ricaud et al., 2014).
Schematic representation of the processes impacting the mid-to-upper
tropospheric pollutants, including CH4 above the
eastern Mediterranean in summer (July–August) (adapted from Ricaud et al., 2014).
In summer, the horizontal distribution of CH4 in the upper
troposphere shows a clear longitudinal gradient between the east and the west
of the Mediterranean Basin, both in the space-borne measurements and in the
model calculations (Fig. 24). There is a maximum of CH4 in the
eastern MB compared to the western MB, both considering the upper
tropospheric layer and the total column information. The difference between
the east and the west of the MB has been calculated within all the datasets,
and the seasonal variations have been investigated (Fig. 25). This clearly
shows that the east–west difference peaks in summer, mainly in August.
The LRT conditions in the upper troposphere differ over both parts of the
Mediterranean Basin. In the western part, whatever the season considered, air
masses are basically coming from the west. However, in the EM, apart from the
westerlies' influence, air masses are also originating from northern Africa
and the Arabian Peninsula (Ziv et al., 2004; Liu et al., 2009), and even
farther away, from Asia.
To further examine the origin of air masses reaching the eastern MB, a 6-day
back trajectory from the point at 33∘ N, 35∘ E, located in
the EM (red filled circle in Fig. 26) was calculated, considering vertical
movement, using the British Atmospheric Data Centre (BADC) trajectory service
(http://badc.nerc.ac.uk/community/trajectory/) every 12 h in
July–August over 2001–2010. The position of the gravity center of all
trajectories (i.e., the maximum in the probability density function) is
displayed every 24 h in Fig. 26 at 850 (red stars), 700 (orange), 500
(green), 300 (blue), and 200 hPa (yellow). For this purpose, data
from the ECMWF archive (spatial resolution of 2.5∘ at the five standard pressure levels) were used in the
calculation.
Based on these studies focused on the EM, Ricaud et al. (2014) proposed
a scheme displaying the transport mechanism (Fig. 27) representing the
several-stage process: (1) capturing of lower tropospheric pollutants,
including CH4, in the Asian monsoon, (2) pollutants' ascent to the
upper troposphere by the Asian monsoon, (3) accumulation of pollutants
within the Asian monsoon in the upper troposphere, (4) long-range transport
and large-scale repartition of pollutants in the upper troposphere from the
Asian monsoon anticyclone to the Middle East and north Africa, (5) subsiding
air masses yielding to the build-up of pollutants at mid-tropospheric layers
above the EM.
Conclusions and perspectives
This review demonstrates the significant progress made in understanding the
atmospheric pollution over the MB. Measurements from space-borne and aircraft
instruments and outputs from chemistry–climate models and chemistry-transport
models clearly revealed that the general atmospheric dynamic summer
conditions characterizing the EM basin differ much from the western ones. The
impact of the different meteorological regimes together with the seasonal
variabilities of the emissions of various atmospheric pollutants results in
a longitudinal concentration gradient between the eastern and western
Mediterranean basins.
Several new campaigns have been recently organized to give more insight for
the understanding of the processes occurring in the western and eastern parts
of this basin in the framework of the ChArMEx program. The TRAnsport and Air
Quality (TRAQA) campaign (Attié et al., 2014; Di Biagio et al., 2015;
Sič et al., 2016) held in summer 2012 was dedicated to the export/import
of pollutants from the European continent to the Mediterranean Sea
by means of balloon and airborne measurements. The Aerosol Direct Radiative
Impact in the Mediterranean (ADRIMED) campaign investigated aerosols of
various origins and their optical properties over the western basin in summer
2013 (Mallet et al., 2016). The Secondary Aerosol Formation in the
Mediterranean (SAFMED) campaigns focused on the organic reactive gases and
aerosol over the northwestern basin and southeastern France in summer 2013
and 2014 (Di Biagio et al., 2015). Finally, the GLAM campaign (Ricaud et al.,
2017) held in August 2014 was dedicated to the study of the gradient of
chemical constituents (pollutants and GHGs) from Toulouse (France) to Larnaca
(Cyprus) and the impact of the Asian monsoon anticyclone on the EM pollutant
levels.
Surface background stations in the EM (e.g., Crete, Greece, and Larnaca,
Cyprus) and in the western Mediterranean Basin (e.g., Menorca, Spain, and
Lampedusa, Italy) deployed even more instruments to obtain a wide variety of
atmospheric parameters (meteorology, chemistry, dynamics, radiation, etc.).
These campaigns were organized in a close relationship with modeling studies
(forecasts and reanalyses) and space-borne observations. New airborne
campaigns are under analysis, e.g., Oxydation Mechanism Observation (OMO) in
summer 2015, or in progress (Radiative Impact of the Arabian Sea pollutants,
greenhouse gases and aerosols on the eastern MEditerranean climate in Summer
(RIMES) in summer 2019) in order to quantify the export of the Asian
pollutants to the EM basin and its impact on the chemical constituent
loading.
Concurrently to these intensive experiments, new sites have been
instrumented. In early 2015, the Agia Marina Xyliatou EMEP rural background
air quality station located at 532 m altitude in the center of
Cyprus (35.03∘ N, 33.05∘ E), and operated since
October 1996 (Kleanthous et al., 2014), has been augmented with a package of
atmospheric chemistry and physics monitoring instruments, thanks to the Cyprus
Institute and French laboratories, in order to initiate an enhanced
atmospheric chemistry observation period of several years in the easternmost
Mediterranean Basin. Unmanned aircraft vehicles are also deployed on
a regular basis to document the lower troposphere above the station, and the
German Leibniz Institute for Tropospheric Research (TROPOS) institute has
deployed a full set of aerosol–cloud–water vapor remote sensing instruments
for almost a year in October 2016. This unprecedented experimental effort is
expected to bring information on the variability of new compounds and
processes with a focus on VOCs and secondary and carbonaceous aerosols and
their origins, and on interactions between aerosols and the water vapor cycle
in this region.
NCEP reanalysis data provided by the NOAA/OAR/ESRL PSD,
Boulder, Colorado, USA, for the period 1948–2016 were used to calculate
the composite long-term mean of sea-level pressure, temperature at 850 hPa, wind
vectors, relative humidity, relative vorticity at 200 hPa, and omega at the 500 hPa
and 700 hPa levels; these data are available at
http://www.esrl.noaa.gov/psd/.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “CHemistry and
AeRosols Mediterranean EXperiments (ChArMEx) (ACP/AMT inter-journal SI)”. It
is not associated with a conference.
Acknowledgements
We would like to thank two anonymous reviewers for their valuable and
insightful comments, and Eric Hamonou for his efficient assistance in taking
care of the copyright permission process. The first author is grateful for
the partial funding from CNRS, France, for carrying out this study in the
framework of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx) program
and for the hosting granted by Météo-France in the initiation of the
program.
Edited by: Nikolaos Mihalopoulos
Reviewed by: three anonymous referees
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