Introduction
During summer and early autumn (warm period), the circulation over the
eastern Mediterranean (EM) is dominated by a persistent northerly flow known
as the Etesians (Tyrlis and Lelieveld, 2013). Under the prevalence of the
Etesians, the advection of the air masses is pronounced over the EM, rendering
the atmospheric conditions as the most important factor for high
concentrations of gases and aerosol particles even in remote areas. The
scientific interest in the Aegean Sea (AS), which is part of the EM, has
led to a number of experimental campaigns during the warm period
(Mihalopoulos et al., 1997; Paronis et al., 1998; Formenti et al., 2002a, b;
Kouvarakis et al., 2002; Lelieveld et al., 2002; Zerefos et al., 2002),
focusing initially on the interpretation of ozone (O3) enhancement under
the Etesian regime. Apart from the simultaneous contribution of local and
distant sources in the area, in the presence of enhanced photochemistry,
strong subsidence was also identified in most of these events (Kallos et al.,
1998, 2007; Lelieveld et al., 2002; Salisbury et al., 2003; Kalabokas et al.,
2007, 2008, 2013; Kanakidou et al., 2011; Bossioli et al., 2016). Airborne
measurements performed during an Etesian outbreak (Aegean-GAME campaign;
Tombrou et al., 2015) clearly showed that neutral to stable atmospheric
conditions prevailed over the northern and central AS, with reduced friction
velocities and absolute turbulent fluxes (momentum or heat) cumulating the
concentrations below the planetary boundary layer (PBL) and mainly inside the
shallow marine atmospheric boundary layer (MABL). Unstable conditions were
found only over the southeastern AS, in the vicinity of Crete, resulting in enhanced
friction velocities and large positive values of sensible heat flux.
Long-term aerosol observational studies in the EM have relied on ground measurements collected at Finokalia, a remote coastal site in the northeast of Crete (Bardouki et al., 2003;
Eleftheriadis et al., 2006; Lazaridis et al., 2006; Gerasopoulos et al.,
2007; Kalivitis et al., 2008, 2014, 2015; Koulouri et al., 2008; Querol et
al., 2009; Pikridas et al., 2010, 2012), and a few more at the Akrotiri station
on western Crete (Lazaridis et al., 2008; Kopanakis et al., 2012, 2013).
Most of these ground-based observations indicate that the mass of fine
aerosols presents a summer maximum; however, their appearance is season independent. These fine aerosols have been related to regional
sources of pollution enhanced by long-range transport during the Etesian
flow. In particular, a mixture of anthropogenic (Koçak et al., 2011),
biogenic (Im and Kanakidou, 2012), and biomass burning emissions (Sciare et
al., 2008; Bougiatioti et al., 2014), originating mainly from the Balkans and
central and eastern Europe, result in enhanced aerosol concentrations in
the southern AS (Kalivitis et al., 2014).
A few short-lived particle formation events (18–25 nm) were first recorded
at Finokalia by Kalivitis et al. (2008), arriving with low speed from the
west during autumn. Thereafter, new particle formation (NPF) events have
been frequently observed at Finokalia (Manninen et al., 2010;
Ždímal et al., 2011; Pikridas et al., 2012; Kalivitis et al., 2014,
2015) and Akrotiri (Kopanakis et al., 2013) throughout different
periods of the year, but more frequently during winter. NPF events are favored when air masses are
enriched by a reactant (e.g., NH3) prior to reaching the site at
Finokalia (Pikridas et al., 2012; Kalivitis et al., 2014). During Etesian
flow conditions, the particle size distributions were centered at the lower
end of the accumulation-mode size range (Kalivitis et al., 2014). This was
partly attributed to the production of sufficient sulfuric acid to increase
the condensation sink and suppress NPF events during the summer (Pikridas et
al., 2012). It has been only recently shown that NPF events could occur at
Finokalia during Etesians (Kalivitis et al., 2015). A large number of
PM1 particles (of the order of 104 cm-3) were also observed
in the northeastern AS during an Etesian outbreak (Tombrou et al., 2015),
whereas high number concentrations of nucleation-mode particles observed in
the northern AS by Triantafyllou et al. (2016) have been associated with
polluted air masses transported from Istanbul.
The above drives the need to understand the history of the air masses
as they transect the Aegean before arriving at Finokalia. In particular, we
need to elucidate the atmospheric and chemical processes that affect aging
of the air masses passing over the AS maritime area between the Cyclades and
Crete. Furthermore, we need to examine whether NPF events observed at Finokalia
would be stronger over the central Aegean during the northern Etesian flow.
Bougiatioti et al. (2009, 2011) observed high cloud condensation nuclei (CCN) concentrations at
Finokalia from air masses coming from the Balkans during a period
representative of an Etesian regime, while Kalivitis et al. (2015)
recently demonstrated that the NPF events are associated with an increase in
the concentration of CCN production in the EM
atmosphere. However, few studies to date have focused on understanding the
increase in cloud droplet number that results from NPF, which is the true
microphysical link between NPF and the aerosol indirect effect.
Driven by the arguments above, we chose to perform measurements at a remote
site on Santorini, which is located within the same path of air masses that
reach the station of Finokalia during the Etesians. Our aim is to
elucidate both atmospheric and chemical processes that affect aging of the
air masses passing over the AS before reaching its southern edge, the island
of Crete. Continuous ground measurements of particle properties,
concentration of gaseous species, and meteorological variables were
simultaneously collected on Santorini and Crete. During this short-term
campaign (15–28 July 2013) intense bursts of nucleation-mode particles were
observed at both sites. The synoptic wind flow and boundary layer dynamics,
as well as the atmospheric chemical composition that favor the enhanced NPF
events during the Etesian flow, are examined in this study. To understand how
NPF could affect cloud formation, we quantify its
impact on CCN levels, cloud droplet number concentration (CDNC), and
supersaturation formed in clouds that develop before, during, and after NPF
events at both ground sites. Complementary to this analysis, wind patterns
and atmospheric chemical composition based on WRF-Chem (Grell et al., 2005) mesoscale model
simulations are presented.
Summary of the variables and operation characteristics of
the instruments at Santorini and Finokalia stations.
Santorini
Instrument
Resolution
Period of operation
Aerosols
Aerosol number distribution
TSI 3034 SMPS
3 min
15–28 July
(10–500 nm)
Gaseous species
O3
M400E photometric
1 min
18–28 July
ozone analyzer
SO2
M100E UV fluorescence
1 min
15–28 July
analyzer
NO, NO2, NOx
M200E nitrogen
1 min
15–28 July
oxide analyzer
Finokalia
Aerosols
Aerosol number distribution
TROPOS type SMPS
5 min
16–29 July
(9–848 nm)
Chemical composition
Aerodyne Research Inc.
30 min
15–28 July
(SO42-, NO3-, NH4+, Cl-, organics)
aerosol chemical
speciation monitor (ACSM)
Gaseous species
O3
Thermo electron Model 49i
3 min
15–28 July
Meteorology
Relative humidity, temperature
MP101A humidity-temperature
5 min
15–28 July
Wind speed, direction
05103 wind monitor
5 min
15–28 July
The extended area of study where the major routes (arrows) of air
masses pass through Santorini and Finokalia on 23 July (EF – left
panel) and 26 July (MSF – right panel) is indicated. In the left panel,
the areas of NPF (black ellipse), the spatial extent of NPF event (red
line), flow entrainment into MABL (white dashed line), and the condensation
(yellow dashed ellipse) are shown. The major traffic (green) and urban
(red) emission sources are also shown. The marine traffic is shown in the
middle panel.
Methodology
Experimental observations
Ground-level measurements were conducted simultaneously at two remote
coastal areas (see Fig. 1) from 15 to 28 July 2013: on the island of
Santorini (at Ag. Artemios, 36∘26′ N, 25∘26′ E) and at
the monitoring station of Finokalia on the island of Crete (35∘20′ N, 25∘40′ E; http://finokalia.chemistry.uoc.gr;
Mihalopoulos et al., 1997). Ag. Artemios (hereafter referred to as
Santorini) is located at an elevation of 153 m above sea level (a.s.l.), while
Finokalia is located on the top of a hill at 260 m a.s.l. Both measuring sites are far
from any large city or anthropogenic activity and are close to the sea;
Finokalia faces the sea within a sector of 270 to
90∘, whereas the station on Santorini faces the sea within a sector of
340 to 120∘.
The Finokalia station houses a suite of instruments for measuring the
meteorological parameters, the concentrations of gaseous species, and
the physical properties and chemical composition of atmospheric particles.
We used an O3 analyzer (Thermo Electron model 49I), a scanning mobility
particle sizer (SMPS, TROPOS type; Wiedensohler et al., 2012) with a
TSI-3772 condensation particle counter (CPC) for measuring the size
distribution of aerosol particles with diameters from 9 to 848 nm (scanned
range), and an Aerodyne Research Inc. aerosol chemical speciation monitor
(ACSM; Ng et al., 2011) for measuring the mass and chemical composition
(SO42-, NO3-,
NH4+, Cl-, and organics) of non-refractory submicron
aerosol particles. A TSI SMPS (Model 3034) measured the size distribution of
particles with diameters from 10 to ca. 500 nm at Santorini. The
concentrations of gaseous species were also measured using an O3
analyzer (Photometric M400E), a dual channel chemiluminescence analyzer for
nitrogen oxides (NO, NO2; Photometric M200E), and a fluorescence
analyzer for sulfur dioxide (SO2; Photometric M100E). An overview of
the instruments used for the measurements is provided in Table 1.
CCN and droplet number calculations
CCN concentrations are calculated using the observations of size
distribution and chemical composition as follows. First, Köhler theory
(Köhler, 1936; Seinfeld and Pandis, 2006; Petters and
Kreidenweis, 2007) is applied to determine, based on knowledge of aerosol
composition, the minimum dry size of particles, dc, that can
activate at a given level of supersaturation, s. Then, the CCN
concentration is determined from the observed size distributions by
calculating the concentration of particles with sizes above dc
(Seinfeld and Pandis, 2006). s is either prescribed or determined
from a cloud parameterization, both of which are performed here. Chemical
composition is expressed in terms of the hygroscopicity parameter,
κ, (Petters and Kreidenweis, 2007).
Thereafter, we calculate the droplet number (Nd) and
supersaturation for clouds forming in the vicinity of both sites during all
NPF events. The droplet parameterization used is based on the “population
splitting concept” of Nenes and Seinfeld (2003), later improved by Barahona
et al. (2010) and Morales Betancourt and Nenes (2014). In the calculations
of droplet number, the size distribution is represented by the sectional
approach, derived directly from the SMPS distribution files. The updraft
velocity was calculated from high-resolution airborne measurements
performed over this region of AS under similar atmospheric conditions
(Tombrou et al., 2015). The partial sensitivity of cloud droplet number to
chemical composition and vertical velocity is derived from the finite
difference approach (Karydis et al., 2012).
Regional modeling
The WRF-Chem version 3.3 mesoscale model (Grell et al., 2005) is used to
understand the dominant meteorological regimes and the regional
characteristics of the aerosols during the sampling period. Simulations were performed by applying two-way nesting with three domains: the outermost first domain covers the
extended area of Europe (spatial resolution 0.5∘ × 0.5∘), the second domain
covers the extended area of Greece and Italy (0.167∘ × 0.167∘), and the innermost third domain is centered on
the extended area of Greece (0.056∘ × 0.056∘).
The RADM2 chemical mechanism is used to simulate the gas-phase chemistry
(Stockwell et al., 1990). Aerosol dynamics are treated with the Modal
Aerosol Dynamics Model for Europe (MADE; Ackermann et al., 1998). Aerosols
in MADE are represented by two lognormal size distributions that correspond
to the Aitken and accumulation modes. Supermicrometer particles are
represented by a coarse mode (Schell et al., 2001). NPF in MADE is treated
with the Kulmala et al. (1998) parameterization of sulfuric acid nucleation,
although it is now well-documented that other reagents (e.g., NH3 and
organics) play important roles in this process (Kulmala et al., 2004). New
particles with a diameter of 3.5 nm, are assigned to the Aitken mode and the
size distribution parameters are adjusted to retain the lognormal shape of
the distribution. Condensation rates of low-vapor-pressure gas-phase species
onto existing particles are determined by Binkowski and Shankar (1995).
The secondary organic aerosol (SOA) model (SORGAM; Schell et al.,
2001) is used to simulate organic aerosol. The aerosol species
treated by the modules are the main inorganic ions
(SO42-, NO3-,
NH4+, Na+,Cl-), elemental carbon (EC), primary organic aerosols (POA), SOA, a primary
unspeciated PM2.5 fraction covering all the unspeciated and/or unknown fine
particles (PM2.5-unsp), and three species for the coarse mode (i.e.,
anthropogenic, marine, and soil-derived aerosols). For the fine particle
fraction, each model species has an Aitken mode and an accumulation mode.
The lack of a dedicated nucleation mode in the model neglects the actual
processes of particle formation and growth towards the Aitken mode and
eventually leads to unrealistic lifetimes against deposition and coagulation
as well as to unrealistic growth rates. In the framework of this study we
use the results of WRF-Chem mainly to investigate the flow advection and
chemical composition.
HYSPLIT4 back-trajectories computed with an end point at
the Santorini station (from the heights of 100, 500, and 1000 m), on 23 (left
panel), 24 (central panel) (both during EF period), and 26 July (MSF – right
panel) 2013.
For anthropogenic emissions from Europe (first and second domains) we
use the EMEP database (http://www.ceip.at/webdab-emission-database), while for Greece (third domain) we employ the
national emission inventory (Tombrou et al., 2009). Natural (biogenic and
sea-salt) emissions are calculated online within the WRF-Chem model.
Biomass burning emissions are not considered. The chemical boundary
conditions used in this study are based on an idealized, northern
hemispheric, mid-latitude, clean environmental vertical profile from the
NOAA Aeronomy Lab Regional Oxidant Model (NALROM; Liu et al., 1996; Peckham
et al., 2011). Simulations were performed from 12 to 29 July. An extended
evaluation of the WRF-Chem model against airborne and ground observations over
the AS during the Etesians is presented in Bossioli et al. (2016). Under
long-range transport conditions, the model successfully simulates CO,
O3, sulfate, and ammonium concentrations, while it underestimates the
aerosol carbonaceous fraction, which is mostly organic matter. The biases
were mainly attributed to underestimated POA emissions and limitation of the
RADM2 mechanism regarding the treatment of monoterpene emissions (Tuccella
et al., 2012).
Air mass origin and trajectories were determined by HYSPLIT4 (Hybrid
Single-Particle Lagrangian Integrated Trajectory; www.arl.noaa.gov/ready/hysplit4.html) back-trajectory analysis
(Stein et al., 2015). The back-trajectories, initialized with meteorological
conditions from GDAS (0.5∘), were computed at several heights. All
three-dimensional trajectories were computed with an end point either at the
Santorini or Finokalia station.
Results and discussion
Prevailing atmospheric and air quality conditions
Northern winds prevailed over the AS throughout the entire campaign. Based
on the simulated wind patterns at 100 m above ground level (a.g.l.) throughout
Greece (see Fig. S1 in the Supplement) and on the sea-level
pressure fields (NCEP/NCAR; Fig. S2), 17–18 July and 22–24 July are
periods of strong Etesian winds (Brody and Nestor, 1985; Kotroni et al.,
2001; Anagnostopoulou et al., 2014). Hereafter, we refer only to the second
period, as higher aerosol number concentrations were measured at both
stations, but also because there were no O3 measurements at Santorini,
during the first period. Immediately after the second period, another
characteristic period (25–27 July) with a similar pressure
pattern to the previous two followed; the pressure gradient over the Dardanelles
was weaker. Back-trajectory analysis of the air masses sampled at both
stations indicates almost the same source regions for both periods (Figs. 2, S3). However, different conditions prevailed during these two
periods,
altering mainly the last part of the journey of the air masses over the AS.
From 22 to 24 July, strong northern wind speeds prevailed
(> 10 m s-1), with the wind direction forming the characteristic
“ring shape”
(Fig. S1) of the Etesian flow around Turkey (Tyrlis and Lelieveld, 2013).
From 25 to 27 July, moderate surface wind speeds (up to ca. 8 m s-1)
with northeasterly surface flow were present over the central and southern AS,
while stagnant conditions prevailed in the north (Figs. S1, S2). Hereafter,
the two periods will be referred to as Etesian Flow (EF) and Moderate
Surface Flow (MSF).
The measured wind speed at the Finokalia station exceeded 9 m s-1, and the
wind direction was mainly from the west-southwest during the daytime hours
(Fig. 3) owing to topographic features that steer the prevailing direction
towards the west-southwest direction. Capturing this local feature is a
known challenge for regional models (e.g., Gauss et al., 2011; Im et al.,
2011; Hodnebrog et al., 2012). At the same time, the simulated wind
direction at the Santorini station exhibited a northern direction, with wind
speeds exceeding 8 m s-1 during the daytime hours (Fig. 3).
Average (± standard deviation) of O3
concentrations and aerosol mass and number concentrations during the two
examined periods; (a) EF (22–24 July) and (b) MSF (25–27 July).
Tracer
Santorini
Finokalia
Santorini
Finokalia
EF period
MSF period
O3 (ppbv)
51.4 ± 6.2
53.8 ± 4.5
70.0 ± 8.5
71.0 ± 7.9
Sulfate (µg m-3)
N/A
3.1 ± 1.2
N/A
7.3 ± 1.5
Ammonium (µg m-3)
N/A
1.4 ± 0.6
N/A
3.1 ± 0.6
Organics (µg m-3)
N/A
4.2 ± 1.3
N/A
8.6 ± 1.2
Nitrate (µg m-3)
N/A
0.38 ± 0.12
N/A
0.8 ± 0.1
Total number conc. (cm-3)
3.6 ± 2.1 × 103
3.9 ± 1.2 × 103
2.0 ± 0.6 × 103
3.6 ± 0.5 × 103
Aitken mode (cm-3)
2.2 ± 1.4 × 103
2.5 ± 1.5 × 103
1.2 ± 0.5 × 103
1.6 ± 0.5 × 103
Accumulation mode (cm-3)
9.6 ± 3.5 × 102
1.6 ± 0.9 × 103
1.0 ± 0.5 × 103
2.1 ± 0.6 × 103
The number concentrations for the three particle modes (nucleation, Aitken,
and accumulation), together with the O3 concentrations, are shown in Fig. 4 for
both periods at the Santorini and Finokalia stations. Simultaneous
routine meteorological measurements, such as surface temperature and
relative humidity, are also provided for each station. Apart from the
region-wide differences, intense bursts in the concentration of
nucleation-mode particles with diameters smaller than 25 nm were observed
at both stations during the periods of EF (Fig. 4, shaded with yellow); it
should be noted that these events were not observed at any of the stations
during the period of MSF (Fig. 4, shaded with grey). In the subsequent
sections the different characteristics and processes prevailing under EF or
MSF are explored, aiming to elucidate the interconnection between the two
stations.
Ozone concentrations
During the EF period, O3 levels at the Santorini and Finokalia stations
ranged between 38 and 66 ppbv and 43 to 70 ppbv, respectively (Figs. 4, S4;
Table 2); these levels are consistent with previous measurements (57 ± 4 ppbv) inside the MABL for EF carried out during the Aegean-GAME campaign.
The values also agree with the climatological values recorded over the
greater area during summer (Gerasopoulos et al., 2005; Kalabokas et al.,
2007, 2013). During EF, the less pronounced diurnal cycle at the Finokalia
station (from 21 to 24 July the mean diurnal range is 8 ppbv; Fig. S4),
compared to Santorini (18 ppbv; Fig. S4), is attributed to a shallower and
more stable MABL over Santorini compared to Crete (Tombrou et al., 2015).
This favors higher primary concentrations and thus O3 scavenging at
Santorini, especially when the MABL collapses during nighttime. In the
vicinity of Crete, the MABL becomes less stable due to larger sea surface
temperatures (SST) existing south of Santorini. This fact, together with
the topography (i.e., Crete forms a mass of land that is located
perpendicular to the EF), enhances the mixing and the downward transport
from the layer above, which is rich in O3 concentrations. During the MSF, high
O3 levels (the highest concentrations of the summer in 2013) were
measured at both stations, ranging between 50 and 99 ppbv (Figs. 4, S4;
Table 2). At both stations the highest values were observed on 26 July. The
lower winds over the northern AS contributed to O3 accumulation in this
area, explaining the high O3 concentrations at both stations. The
maximum O3 concentration observed (but simulated as well) at Finokalia
had a 4 h delay compared to that observed at Santorini.
Time series of the wind speeds (ws, solid lines on left
axis) and wind directions (wd, open circles, right axis) at Santorini
(simulations from the WRF-Chem model) and at Finokalia (measurements). The
second period of the EF is shaded with yellow and the MSF with grey.
Aerosol modal number concentrations and
pollutant (O3, NO2 when available) concentrations, along with
meteorological parameters of relative humidity (RH) and surface temperature
(T) at Santorini (top panel) and Finokalia (bottom panel). Note that
SO2 is shown at Santorini, while SO4 is shown for Finokalia.
Spatial distribution of O3 concentration (ppb) and
wind speed (m s-1; shown with arrows) at 400 m a.s.l. over the extended area of Greece as simulated by
WRF-Chem at 15:00 LST for 23 July (EF – left panel) and 26 July (MSF –
right panel).
Simulations confirm that the air masses received at both stations during the
prevailing strong northern wind are of the same origin, and they are representative
of EF conditions (Fig. S3), albeit with an O3 underprediction
(average bias during afternoon hours up to -21 on 23 July and -15 % on 24
July, Figs. S4, 5). During the MSF period, simulations indicate an O3
increase, especially in the southern AS, but it was also underpredicted (average
bias during afternoon hours up to -24 % on 26 July, Figs. S4, 5). In case the
chemical boundary conditions, including stratosphere–troposphere exchange
processes, are represented realistically from a global chemical transport
model, WRF-Chem simulates a significant O3 increase inside the PBL
(up to 40 %) during Etesians (Bossioli et al., 2016). Furthermore,
inaccuracies of the emissions inventory could also have an impact on the
results.
Aerosol mass and number concentrations
Figure 6 shows the non-refractory submicron aerosol concentrations measured
at Finokalia during the whole experimental period. In general, the inorganic
and organic mass concentrations behave similarly to O3 (R2 of
O3 to organics and inorganics is 0.5 and 0.59, respectively) during
most of the experimental period (Figs. S4, 6). During EF, the PM1 mass
concentrations were reduced by roughly a factor of 2 compared to those
during the MSF period (Table 2) and were in the range of concentrations
measured in the framework of the Aegean-GAME campaign. However, despite that
the concentrations of all four species
(SO42-, NO3-,
NH4+ and organics) were
substantially decreased during EF (23–24 July), the organic fraction
exhibited a relative increase, especially at the beginning of this period.
Due to lack of data at Santorini, simulated PM2.5 mass concentrations
were used for the analysis. The modeled concentrations for sulfate are about
2 µg m-3 at both stations at 09:00 LST (Fig. 7) quite close to
the measured values at Finokalia (Fig. 6, on average during the EF
they had an underprediction of 30 for sulfates and 60 % for ammonium). Similar to
the case of O3, the two stations are located along the less-polluted
airflow over the AS.
Mass concentrations of submicron aerosol measured at the
Finokalia station from 13 to 30 of July 2013. The second period of the EF is shaded with yellow and the MSF with grey.
As in Fig. 5, but for sulfate concentration (µg m-3) at 09:00 LST.
During the MSF period, the aerosol mass concentrations at Finokalia were
substantially higher (Table 2; Fig. 6). The increased concentrations were
retained until noon of 27 July for sulfate and ammonium, while those of
organics continued to increase further until the end of the campaign. The
modeled spatial distribution of sulfate concentrations was nearly uniform
over the AS, while similarly to O3 the sulfate concentrations increased offshore of
the northeastern coast of Crete due to the aging of air masses in
combination with the strong impact of the topography (Fig. 7). The simulated
mass concentrations of secondary inorganic fine aerosols also increased
(simulated and observed concentrations correlate during both periods
R2= 0.8); however, they are lower than the measured values at
the Finokalia station (average underprediction of 50 for sulfates and 75 %
for ammonium).
In contrast to the fine-aerosol mass concentrations, their total number
concentrations were substantially increased, reaching continental levels
during Etesian flow conditions (from 1.5 × 103 to 1.5 × 104 cm-3 at Santorini and
from 2.4 × 103 to 7.5 × 103 cm-3 at Finokalia; Table 2, Fig. S5). The
Aitken-mode particles followed a similar diurnal variation at both stations,
ranging from 4.4 × 102 to 7.7 × 103 cm-3
and peaking around noon. Accumulation-mode particles were higher at
Finokalia. The total particle number concentrations measured within the MABL
of the eastern AS during the Aegean-GAME campaign under similar atmospheric
conditions were on average 8 × 103 cm-3, with almost
20 % (1.4 ± 1.2 × 103 cm-3) being in the 20–50 nm
size range (Tombrou et al., 2015). Greater differences were observed for the
nucleation-mode particles (i.e., particles with diameters smaller than 25 nm), with sudden concentration bursts observed at both stations (Fig. 4). On
23 July (EF), a nucleation-mode burst was recorded, reaching number
concentrations of up to 1.3 × 104 cm-3 at Santorini and
almost 1.4 × 103 cm-3 at Finokalia. A second event, but
of lower intensity, was recorded on 24 July (EF). It is worth mentioning
that apart from the strong winds and lower temperatures, this period is
considered humid (relative humidity values reaching up to 80 % at the
Finokalia station) in comparison to the MSF period (Fig. 4). The
nucleation-mode particles shift gradually towards larger sizes in a
banana-shape pattern at both stations, as shown in Fig. 8. The number of
particles remained high for several hours at Santorini (see Fig. 8),
indicating regional NPF (Kulmala et al., 2012).
Diurnal evolution of the aerosol size distribution on 23
and 24 July (EF) at Santorini (top panel) and Finokalia (bottom panel). The
white dots stand for nucleation, the black dots for Aitken, and the purple
dots for accumulation geometric mean diameter.
The associated growth rates (GR) for particles that increased in
size from 10 to 25 nm were estimated to be 3.06 at Santorini and
2.05 nm h-1 at Finokalia on 23 July, and 2.08 and 1.76 nm h-1, respectively, on 24 July. The average GR for particles
increasing in size from 7 to 20 nm at Finokalia was reported by Pikridas et al. (2012) to be
substantially higher (7.5 ± 5.8 nm h-1), with the highest daily GRs observed during the hottest
months of the year (May to July 2008). It should be mentioned, however, that
the nucleation events reported in that study were mainly related to air
masses spending most of the time over the island of Crete, which is not the
case for the observations reported here. The formation rates of
nucleation-mode particles, JD, were computed according to
Kulmala et al. (2012), considering both the coagulation flux and the
condensational growth as sinks. For the two consequent events at Santorini,
JD for particles with diameters from 10 to 25 nm
ranged between 4.82 (23 July, Fig. 8) and 2.77 cm-3 s-1 (24 July, Fig. 8). At the station of Finokalia,
JD was lower for particles between 9 and 25 nm, ranging
between 2.27 (23 July, Fig. 8) and 2.25 cm-3 s-1 (24 July, Fig. 8). The
similarity between the JD rates at the two sites on 24 July
indicates that a region-wide NPF event occurred, yet the rates taken a day
earlier were markedly different, thus indicating a local event. However,
we will show later (Sect. 3.4) that this was not the case and more
information needs to be taken into account.
Under MSF conditions, the total fine-aerosol number concentrations were
considerably lower than those during the EF (from 1.4 × 103
to 2.9 × 103 at Santorini and from 2.6 × 103 to 5.1 × 103 cm-3 at Finokalia, Fig. S5).
Particles in the nucleation mode were absent, while the concentrations in
the Aitken mode were substantially lower at both stations, varying from 3.2 × 102 to 4.1 × 103 cm-3 (Fig. S5). The
particle concentrations in the accumulation mode at Santorini had a
comparable variation to those of the Aitken mode, while they were apparently
always higher at Finokalia.
Spatial extent of NPF event
The synoptic wind flow and boundary layer dynamics, as well as the chemical
atmospheric background conditions that favor the enhanced NPF events during
the EF, are further examined here. This type of event could be characterized
as “type A” according to Boy and Kulmala (2002), owing to the sudden
appearance of nucleation-mode particles and their consistent growth for at
least 1 h. The horizontal scale of this event was estimated based on air
mass back-trajectory analysis (Hussein et al., 2009), taking into account
the time during which measurements at the site indicate a distinct
nucleation mode. Following Birmili et al. (2003), HYSPLIT4 back-trajectory
calculations started at the time when a nucleation mode was first
distinguishable from the Aitken mode at Santorini and were performed for
each subsequent hour until the two modes merged (nucleation duration).
Following Crippa and Pryor (2013), the duration of NPF was based on the
geometric mean diameter of particles, with sizes between 10 and 100 and
from 30 to 100 nm; an event is said to initiate when the difference between
the two geometric mean diameters becomes maximum and ends when this
difference is less than 15 % (Fig. S6). Assuming a linear GR (Lehtinen and Kulmala, 2003), this approach showed that the ca. 10 nm
particles (the smallest particles we could detect with our instrumentation)
were able to grow up to 60 nm within 4.5 h of initial detection. This
GR was then used to calculate the minimum spatial scale. On 23
July, the distance covered by the back trajectories within 4.5 h (starting
when the nucleation-mode burst was first recorded at Santorini) spanned at
least over 250 km to the northeast of Santorini in the center of AS, upwind
of the Cyclades complex (Fig. 2; red line in Fig. 1, left panel). A couple
of hours before the sunrise these back trajectories (both below and above
500 m a.g.l.) were observed over the northwestern Asian forest peninsula of
Turkey (area marked as black ellipse in Fig. 1, left panel; Fig. 2), having
previously passed (at higher altitudes > 1.5 km a.g.l.) from the
greater area of Istanbul (GAI) and the west coast of the Black Sea (from
even higher altitudes > 3 km a.g.l.). A similar spatial extent also
occurred during the less intense EF event on 24 July, although this started
with a 2 h delay (Fig. 2). Air masses were better mixed throughout the
boundary layer, covering a broader area over Asian Turkey on 24 July.
Time series of the CN and estimated CCN concentration
particles for various supersaturations at Santorini (top panel) and
Finokalia (bottom panel) on 23 and 24 July (EF). Time is in LST.
Time series of the estimated cloud droplet number
concentrations (Nd, red lines) and maximum supersaturation in the cloud
(smax, blue lines) for updraft velocities (σw) of 0.3 m s-1 and 0.6 m s-1 at Santorini (top panel) and Finokalia (bottom
panel) on 23 and 24 July (EF). Thick lines correspond to updraft
velocity (σw) equal to 0.3 m s-1, while thin lines
correspond to 0.6 m s-1.
Despite the limitations of the model (absence of a nucleation mode, only binary
homogeneous nucleation parameterization included), the conditions under
which NPF events take place and their overall impact can still be estimated
by conducting another simulation that deactivates the nucleation
parameterization (nucleation-off run). According to the simulations, a wide
stream of clean air masses of low preexisting aerosol particles (number
concentrations < 2.5 × 103 particles cm-3 not
shown) but of sufficient H2SO4 (∼ 107 molecules cm-3 from high altitudes, not shown) overpasses the urban mixing height
(at 1–2 km) over the GAI during the previous evening on 22 July (20:00 LST,
Fig. 1, left panel), avoiding mixing with the local emissions. Thereafter,
the air masses penetrated at lower levels (due to the EF structure) over northwestern
Turkey (Fig. 2, left panel). In this forested area (black ellipse in Fig. 1,
left panel) they find favorable conditions for NPF, such as low relative
humidity, H2SO4, and availability of biogenic emissions (not shown)
that further endorse the NPF efficiency. In Fig. S7, the number
concentration of new particles (number concentration differences between nucleation-on and nucleation-off runs) at 1 km at
06:00 LST for both EF (left panel) and MSF (right panel) periods is
presented. Although severely underestimated (simulated NPF contribution up
to ∼ 400 cm-3), the critical role of the EF in the NPF
event over the northern AS is revealed. Closer to the surface, the air masses
had a substantial number of primary particles (emitted by the various
activities of the GAI, Fig. S8), providing more surface available for
condensation (NPF contribution up to 200 cm-3, not shown). According to
the simulations, a plume with large particulate load in the Aitken mode
(∼ 9 × 103 particles cm-3) was transported
over northwestern Turkey (Fig. S8, left panel). Our results agree with
previous observations during an Etesian event, where number concentrations
up to ∼ 1.2 × 104 cm-3 were observed in the
northeastern AS, with the Aitken-mode particles dominating by up to 70 %
(Tombrou et al., 2015). The less intense event on 24 July is associated with
a narrow stream of low preexisting particles over the GAI (concentrations
< 2.5 × 103 particles cm-3, not shown).
The plume, after crossing the Turkish mainland overnight, was transported
over the AS, with most of the new particles above the stable MABL (Fig. S7,
top-left panel; dashed purple lines in Fig. 1, left panel). The plume
moved fast with rather negligible mixing, especially above the MABL,
thereby affecting areas located further away, such as the central AS, within
a couple of hours after sunrise on 23 July (around 09:00 LST) and around
noon on 24 July. The rapid advection above MABL, in combination with the low
number of preexisting particles there (Fig. S7, bottom-left panel), seems
to have left almost the majority of the newly formed particles intact.
Thereafter, we mainly consider that while the part of the plume above the
MABL passed over the Cyclades complex, the wakes on the lee side of the
islands enhanced vertical mixing, enabling its entrainment into the MABL
(area indicated with a white dashed line in Fig. 1, left panel). This
assumption does not reject the fact that oxidation enhanced by
photochemistry over the AS may also have contributed to the NPF process. The freshly
nucleated particles that remained constantly inside the well-mixed MABL,
suffered an early aging (i.e., growth by condensation and coagulation). The
concentrations at both nucleation and Aitken modes jump almost
simultaneously, accompanied by concurrent increases in O3, NO2, and
SO2 concentrations, at Santorini station during these two consequent
events (Fig. 4). This could be an indication that this station receives
masses simultaneously from different layers (inside and above the MABL), in
line with a number of cases where maximum rate of change of ultrafine
particle concentrations close to the surface was always preceded by
breakdown of the nocturnal inversion and enhancement of vertical mixing
(Crippa et al., 2012).
The air masses arrived 3 h later (after 13:00 LST; Fig. S3, left panel) at
Finokalia on 23 July (Fig. 4). The 3 h transit timescale is in agreement
with the prevailing wind speed (about 10 m s-1; Fig. S1) and the 120 km
distance between Santorini and Finokalia. The nucleation-mode particles were
significantly reduced as they had shifted gradually towards larger sizes
(Aitken mode) before reaching Finokalia (Fig. 4). The nucleated
concentrations measured previously at Finokalia were probably due to a local
nucleation event initiated at Heraklion (Crete). The current simultaneous
measurements along the same flow stream show that both stations are under
the influence of regional NPF events, during the Etesians.
During the MSF period on 26 July the air masses arriving at lower levels
(below 500 m a.g.l.) at the Santorini station (Figs. 1, 2, both right panel)
mainly passed from low altitudes over continental areas (< 1 km)
and were substantially enriched by anthropogenic emissions, while those
at higher levels covered longer distances over eastern Europe at the
same time (exact opposite behavior of EF). Over the GAI, the simulated
particle number concentration was much higher compared to EF conditions (5–7 × 103 particles cm-3, not shown), limiting the NPF event
(Fig. S7). These atmospheric conditions promoted the mixing of air masses
with local anthropogenic and natural emissions, favoring photochemical
production of secondary pollutants such as O3 (Fig. 5, right panel) and
higher secondary aerosols (e.g., SO4 shown in Fig. 7).
Impact of NPF events on CCN production
Understanding how NPF affects cloud formation requires quantification of its
impact on the CCN levels that develop for cloud-relevant supersaturations.
Since CCN concentrations were not measured, they were calculated using the
observations of size distribution and chemical composition as already
described in Sect. 2.2. The presentation of the results and the relevant
discussion are based on the periods before and after the NPF events.
Average (± standard deviation) of calculated
κ using the PM1 chemical composition at Finokalia, the
dc (as described in the text), and the estimated CCN
concentration particles at both stations on 23 and 24 July (EF period).
Here smax is the maximum supersaturation in the cloud,
Ntotal is the total particle number concentration, and Nd
is the potential cloud droplet number concentration calculated according to
the approach described in the main text. Two probability density functions
(PDFs) of the characteristic updraft velocity are used with σw = 0.3 m s-1
and σw= 0.6 m s-1. Time is in LST.
Santorini
Finokalia
Santorini
Finokalia
23/7
24/7
Before
After
Before
After
Before
After
Before
After
00:00–8:00
15:00–21:00
00:00–10:00
17:00–21:00
00:00–10:00
18:00–21:00
00:00–11:00
17:00–21:00
κ
0.29 ± 0.01
0.36 ± 0.03
0.28 ± 0.02
0.38 ± 0.02
0.29 ± 0.01
0.34 ± 0.01
0.30 ± 0.01
0.34 ± 0.01
dc (nm; s= 0.2 %)
104 ± 2
95 ± 2
104 ± 2
94 ± 1
101 ± 1
96 ± 1
102 ± 2
97 ± 1
dc (nm; s= 0.6 %)
50 ± 1
46 ± 1
50 ± 1
45 ± 1
49 ± 1
46 ± 1
49 ± 1
46 ± 0
CCN0.2 (cm-3)
536 ± 27
794 ± 145
1002 ± 76
1420 ± 383
682 ± 66
1028 ± 61
1062 ± 156
1822 ± 154
CCN0.6 (cm-3)
1225 ± 90
3155 ± 789
2111 ± 196
4343 ± 1119
1535 ± 66
2004 ± 224
2191 ± 270
3346 ± 399
Ntotal (cm-3)
1777 ± 421
4621 ± 1986
3506 ± 699
5710 ± 779
2306 ± 154
2557 ± 351
3198 ± 384
3921 ± 404
σw = 0.3 m s-1
smax (%)
0.25
0.22
0.11
0.10
0.23
0.19
0.11
0.07
Nd (cm-3)
110 ± 4
124 ± 8
423 ± 4
407 ± 19
121 ± 5
165 ± 9
423 ± 3
440 ± 5
Activation Fr. (%)
6.5 ± 1.5
3.1 ± 1.1
12.5 ± 2.4
7.2 ± 0.6
6.4 ± 0.4
7.9 ± 1.2
13.4 ± 1.6
11.3 ± 1.1
Contribution of κ (%)
1.4
2.7
2.6
10.2
0.7
1.9
0.9
0.3
Contribution of Naer (%)
98.6
97.3
97.4
89.8
99.3
98.1
99.1
99.7
σw = 0.6 m s-1
smax ( %)
0.32 ± 0.01
0.28 ± 0.01
0.14 ± 0.01
0.14 ± 0.01
0.29 ± 0.01
0.23 ± 0.01
0.14 ± 0.01
0.11 ± 0.01
Nd (cm-3)
192 ± 6
217 ± 15
627 ± 67
619 ± 109
213 ± 7
286 ± 15
621 ± 73
786 ± 11
Activation Fr. (%)
11.4 ± 2.6
5.4 ± 1.9
18.8 ± 5.1
10.8 ± 0.7
11.3 ± 0.7
13.7 ± 2.1
19.7 ± 3.1
20.2 ± 1.9
Contribution of κ (%)
1.2
1.9
3.8
19.0
0.6
1.6
0.7
0.2
Contribution of Naer (%)
98.8
98.1
96.2
81.0
99.4
98.4
99.3
99.8
CCN concentrations are calculated for prescribed values of s between 0.2 and 0.8 %, corresponding to supersaturations found in
relatively pristine stratiform to convective clouds (Seinfeld and Pandis,
2006). κ is calculated from the PM1 chemical
composition observed at Finokalia as follows: κ = εinorgκinorg + εorgκorg,
where κinorg= 0.6 is the value for
ammonium sulfate (Petters and Kreidenweis, 2007), and κorg= 0.16 corresponds to the organic fraction (Bougiatioti et
al., 2009), and εinorg, εorg are the volume fractions of each constituent measured at
Finokalia. The volume fractions range from 0.45 to 0.76 for inorganics and
from 0.24 to 0.55 for organics, similar to the values measured under
comparable atmospheric conditions from Bougiatioti et al. (2009, 2011) and
Bezantakos et al. (2013). Throughout the measurement period, the aerosol-exhibited predicted values of hygroscopicity from 0.20 to 0.39, which is
also consistent with the values determined by Bougiatioti et al. (2009,
2011) and Bezantakos et al. (2013). The aerosol hygroscopicity follows a
diurnal cycle, being minimum just before noon and becoming maximum late in
the afternoon, owing to a higher sulfate-to-organic-mass ratio (Fig. 6).
Consequently, average κ values were estimated to be higher
after the NPF events compared to the period before (increase by
∼ 35 % on 23 July and up to 15 % on 24 July). Given a lack
of PM1 chemical composition measurements at Santorini, the chemical
composition at Finokalia is applied instead to the Santorini
size-distribution observations. The WRF simulations support this assumption because a similar chemical behavior is simulated for both stations (Figs. 5, 7).
The model systematically underestimates the organic fraction at both
stations (organic volume fraction does not exceed 0.2) but with minimal
impact on resulting κ values since they do not differ from
measurements for more than ±6 % throughout the simulation period.
From long-term measurements in the study area, the relative contribution of
the main PM1 constituents, including ammonium, is quite consistent over
the years (Sciare et al., 2003; Koulouri et al., 2008; Bougiatioti et al.,
2013). Thus, a sensitivity test of CCN concentrations to shifts
in κ by ±20 % is also carried out at Santorini.
The resulting CCN time series during Etesian flow are shown in Fig. 9.
Average values of κ, dc, and CCN
concentrations at both stations before and after the NPF events are
provided in Table 3. The calculated CCN number concentrations follow a
diurnal cycle and tend to be at a maximum during the afternoon after the NPF
events following the increase of κ values. Most particles
are CCN-active for s ≥ 0.6 %, as they converge towards the
total CN time series. Bougiatioti et al. (2009) observed similar behavior at
Finokalia for polluted air masses with a similar origin (Balkans). For
s= 0.6 %, dc varied from 43 to 51 nm and CCN
concentrations reached up to ∼ 6 × 103 cm-3
following the Aitken-mode concentrations at both stations (Figs. 4, 9). The
higher CCN number concentrations at Finokalia, compared to those observed at
Santorini (Table 3), were the result of a higher number of accumulation-mode
particles passing previously from Santorini (that were too small to be CCN at
Santorini, but grew to CCN-relevant sizes by the time they arrived at
Finokalia, Sect. 3.3). Accordingly, the higher activation fractions
(CCN / CN) were observed at the station of Finokalia with larger and more aged
aerosol particles, while at Santorini this was observed at the end of the
events, when the smaller particles dropped in concentration because they grew
to larger sizes. On 23 July, the NPF event increased the CCN concentrations
by 157 % at Santorini and 106 % at Finokalia, compared to their
pre-event values. In some moments the increase reached up to a
factor of 6. During the second less intense event on 24 July, the CCN
increase was lower at both stations (31 % at Santorini and 53 % at
Finokalia). The lower increase was also due to the pre-event background,
characterized by higher CCN concentrations. Throughout the MSF period, the
CCN concentrations decreased by almost 48 and 23 % at Santorini and
Finokalia, respectively, compared to the levels during the NPF events.
Changes in chemical composition, as described above, exhibit a relatively low
variation in CCN concentrations (at s= 0.6 %) up to 10 %,
following the same diurnal behavior. As expected, lowering the
supersaturation at 0.2 % leads to the activation of larger particles, with
dc ranging from 91 to 106 nm, which is consistent with the
observations reported by Kalivitis et al. (2015). At s= 0.2 %,
both NPF events contribute up to 50 % to the increase of the CCN
concentrations at both stations. However, the higher CCN production at
Finokalia on 24 July is associated with the accumulation-mode particles at
the end of both events.
Impact of NPF events on droplet number
Studying the impact of NPF on CCN concentrations at prescribed levels of
supersaturation is a simple and frequently used approach for observational
studies of NPF (e.g., Kalivitis et al., 2015 and references therein). However, it
provides an incomplete description of NPF impacts on cloud droplet
number because it does not consider the feedback of CCN on cloud supersaturation
that develops in cloudy updrafts. Mechanistic cloud droplet formation
parameterizations (Ghan et al., 2011; Morales Betancourt and Nenes, 2014)
can capture this complexity by efficiently calculating the maximum
supersaturation (smax) that forms in a cloud given knowledge
of the aerosol size distribution, composition, and updraft velocity.
Observations suggest that the distribution of vertical velocities in the
boundary layer displays a spectral dispersion of σw= 0.2–0.3 m s-1 around a zero average value, which is consistent with
vertical velocities observed in marine boundary layers (e.g., Meskhidze et
al., 2005; Ghan et al., 2011). When applying the droplet parameterization,
we employed the “characteristic velocity” approach of Morales and Nenes (2010) to obtain velocity PDF-averaged values of CDNC and
smax. As a sensitivity test, we also considered calculations for
a convective boundary layer (σw= 0.6 m s-1).
The calculation of PDF-averaged values of Nd and
smax was carried out for every distribution of aerosol number
and composition measured for all NPF events. The resulting time series are
shown in Fig. 10 for Santorini (top panel) and Finokalia (bottom panel).
smax is negatively correlated with Nd at both
stations, owing to the increased competition for water vapor by the growing
droplets when CCN increase. As a result, Nd responds
sublinearly to CCN increases – the degree to which this occurs depends on the
level of aerosol concentrations before and during the NPF event. At
Santorini, the CCN levels are much lower than at Finokalia (Table 3); therefore, we
expect the relative increase in Nd from NPF to be higher
there. Assuming σw= 0.3 m s-1, the NPF events
are associated with smax decreases at both stations, compared to the period before the events. On 23 July, the decrease was on
average 12 % at Santorini and 9 % at Finokalia. As a result,
Nd concentrations during the NPF event increased by 13 % to
124 ± 8 cm-3 at Santorini, compared to the period before the
event. At Finokalia, however, aerosol levels were much higher and
Nd remained virtually the same before and after the NPF event
(Table 3). The effect of the less intense NPF event on 24 July was higher;
Nd concentration increased by 36 % at Santorini and 4 % at
Finokalia compared to pre-event values. The decrease of smax
was also higher on this day, 17 % at Santorini and 36.4 % (at
0.06–0.08 %) at Finokalia (Table 3), owing to the higher
accumulation-particle concentrations compared to the previous events. The
variance of Nd during the event period, for σw equal to 0.3 m s-1, was 475 cm-3 at
Santorini and 37 cm-3 at Finokalia, while for σw equal to 0.6 m s-1, the variance was 865 and 20 cm-3, respectively.
Altogether, this clearly shows that when NPF particles age (e.g., arrive at
Finokalia) their competition for water vapor can reduce cloud
supersaturation to very low levels.
The larger updraft velocity ends in larger values of smax,
which allow smaller particles to activate into cloud droplets. In
particular, Nd exhibits a substantial increase for
σw= 0.6 m s-1, but with a similar pattern to
that with the lower velocity, especially at Santorini. This indicates that the
impact of mean vertical velocity on the CDNC is higher at this station. In
this case, the average Nd concentration was 217 ± 15 at Santorini and 619 ± 109 cm-3 at Finokalia (increase
relative to σw= 0.3 m s-1 by 75 and
52 %) after the event on 23 July and 286 ± 15 and 786 ± 11 cm-3, respectively
(increase relative to σw= 0.3 m s-1 by ∼ 76 % for both stations) on 24 July. It
is interesting to note that for σw= 0.6 m s-1 two Nd peaks were observed at Finokalia, of which the first
is attributed to local processes since it was observed much earlier than the NPF
event at Santorini. The stronger variation in Nd at
Finokalia under the higher vertical wind compared to Santorini, indicates
that vertical velocity variations likely dominate the variance of droplet
number for clouds in the region of Finokalia. Furthermore, from the partial
sensitivity of Nd to the total aerosol number and to
κ, the relative contribution of chemical composition and
total aerosol number to the variance of Nd is attributed. We
find that in most cases the predicted Nd variability is almost
exclusively governed by the aerosol number variation (> 98 %,
Table 3) and to a lesser extent by the chemical composition (< 2 %). The relative contribution of chemical composition became more
significant at Finokalia only after the intense NPF event on 23 July (10 %
for σw= 0.3 m s-1 and 19 % for
σw= 0.6 m s-1). This can be attributed to the
more “aged” nature of the sampled aerosol at Finokalia, compared to the one
at Santorini. This is consistent with the lower smax predicted for Finokalia, leading to the activation of larger particle sizes
that were subject to longer atmospheric processing during their
transition to more unstable conditions after Santorini. Altogether, although
NPF events may strongly elevate CCN numbers, the relative impacts on cloud
droplet number (compared to pre-event levels) are eventually limited by water
vapor availability and depend on the aerosol levels associated with the
background.
Conclusions
Concentrations of chemically and size-resolved submicron aerosol particles
along with concentrations of trace gases and meteorological variables were simultaneously measured at Santorini (central AS) and Finokalia on
Crete (southern AS) from 15 to 28 July 2013. Two well-distinguished periods
are identified: the first with strong wind speeds and wind directions
forming the characteristic “ring-shape” of the Etesian flow (EF) around
Turkey, and the second with moderate surface wind speeds and northerly
direction over the AS (MSF). The two periods exhibited intense differences
in air quality levels.
During EF, the mass concentrations were reduced by roughly a factor of 2
compared to those during the MSF period. The total number concentration of
aerosol particles increased during the EF, varying from 1.5 × 103 to 1.5 × 104 at Santorini and
from 2.4 × 103 to 7.5 × 103 particles cm-3
at Finokalia. Furthermore, intense bursts of nucleation-mode particles were recorded at both stations, with more intense bursts observed at
Santorini. At Finokalia, the fragment of nucleated particles was diminished
and a higher number concentration of the Aitken-mode particles was observed,
which was attributed to atmospheric mixing, growth process, and photochemistry. The
nucleation-mode particles gradually shifted towards larger sizes at
both stations; however, at Santorini the number of particles remained high
for several hours, indicating regional NPF. During the MSF period, the total
number concentration of the particles reached lower values, while
nucleation-mode particles were not detected at any of the stations.
The observed NPF events were initiated at least 250 km (covered within
4.5 h) to the northeast of Santorini in the center of AS, upwind of
the Cyclades complex, under favorable meteorological conditions, under a
strong-channeled northeastern wind flow received by both stations. Based on
the simulations, it seems that what contributed to the NPF events was the
clean air masses of low preexisting aerosol particles with sufficient
H2SO4 from high altitudes. In contrast to the non-NPF period, the
air masses passed over the greater Istanbul area, avoiding mixing with the
local emissions. Thereafter, they penetrated at lower levels (due to the EF
structure) over northwestern Turkey, while in the case of the non-NPF period, they suffered a
strong mixing during their longer journey over the Turkish mainland. Without
excluding the role of photochemistry in NPF, we show by both
measurements and simulations that the plume over AS moved fast with
rather negligible mixing, especially above the MABL. The fast advection
above MABL and the low number of preexisting concentrations inside
the plume prevented the subsequent growth of the nucleated particles towards
the central Aegean Sea. The wakes on the lee side of the islands, however,
enhanced
vertical mixing, enabling the plume's subsequent entrainment into the MABL in the
central Aegean Sea. The freshly nucleated particles that remained constantly
inside the well-mixed MABL suffered early aging (i.e., growth by
condensation and coagulation).
To understand the impact of NPF on CCN levels, using the κ of particles in conjunction with a typical supersaturation
for the area, we calculated the number concentration of particles that act
as CCN at both stations. NPF was found to augment CCN concentrations
considerably during early afternoon (87 % on average for both stations and
both events), with concentration levels at Finokalia being higher due to
particle growth and atmospheric processing. Calculations of droplet number
generated in clouds within the observed air masses indicate that NPF augments
droplet number, but to a much lesser extent (12 %) than implied by the
variations in CCN. This behavior demonstrates that there is a limit to the amount
of droplets that NPF can contribute because the supersaturation in clouds
depresses (here, by roughly 14 %) as additional CCN are added by NPF.
The pre-NPF aerosol levels and prevailing dynamics of the clouds determine
the degree of water vapor competition and precondition cloud sensitivity – or lack thereof – to further CCN increases from NPF.