Introduction
Atmospheric aerosol particles play an important role in the atmosphere
because they can affect the Earth's radiation budget directly
by the scattering and absorption of solar and terrestrial radiation (e.g.,
Haywood and Shine, 1997), and indirectly by modifying cloud properties
(e.g., Kaufman et al., 2005), and hence have important climate implications.
Understanding the influence of atmospheric aerosols on radiative transfer in
the atmosphere requires accurate knowledge of their columnar properties,
such as the spectral aerosol optical depth, a property related to aerosol
amount in atmospheric column (Haywood and Boucher, 2000; Dubovik et al.,
2002). Global measurements of columnar aerosol properties including spectral
aerosol optical depth can be assessed from satellite platforms (e.g.,
Kaufman et al., 1997). However, satellite aerosol retrievals suffer from
large errors due to uncertainties in surface reflectivity. Currently, the
ground sun photometric technique is considered the most accurate one for the
retrieval of aerosol properties in the atmospheric column (e.g., Estellés
et al., 2012). Thus, many ground-based observation networks have been
established in order to understand the optical and radiative properties of
aerosols and indirectly evaluate their effect on the radiation budget and
climate (e.g., AERONET (Aerosol Robotic Network)). However, the quantification of aerosol effects is
very difficult because of the high spatial and temporal variability of physical and optical properties of aerosol (Forster et al., 2007). This high
aerosol variability is due to their short atmospheric lifetime, aerosol
transformations, aerosol dynamics, different meteorological characteristics,
and the wide variety of aerosol sources (Haywood and Boucher, 2000; Dubovik
et al., 2002). In this sense, Forster et al. (2007) highlighted the large
uncertainties on the aerosol impact on radiation budget. Therefore,
monitoring of aerosol properties at different areas in the world can
contribute to reduce these uncertainties.
Most of the planet is covered by oceans and seas, and thus the study of
marine aerosol is a topic of ongoing interest (e.g., Smirnov et al., 2002).
Particularly, many efforts are being made to characterize this aerosol type
from ground-based measurements, leading to the creation of the Maritime
Aerosol Network (MAN) as part of the AERONET network (Smirnov et al., 2009).
However, MAN lacks of continuous temporal measurements, and thus
measurements from remote islands in the oceans and seas are required.
Particularly, in the Mediterranean basin aerosol properties are
characterized by a great complexity, due to the presence of different types
of aerosols such as maritime aerosols from the Mediterranean Sea itself,
biomass burning aerosols from forest fires, anthropogenic aerosols
transported from European and North African urban areas, mineral dust
originated from north African arid areas, and anthropogenic particles
emitted from the intense ship traffic in the Mediterranean Sea (e.g.,
Lelieveld et al., 2002; Barnaba and Gobbi, 2004; Lyamani et al., 2005, 2006a, b; Papadimas et al., 2008; Viana et al., 2009;
Pandolfi et al., 2011; Alados-Arboledas et al., 2011; Becagli et al., 2012;
Valenzuela et al., 2012a, b; Mallet et al., 2013). Past studies revealed
that the aerosol load and the aerosol direct radiative effect over the
Mediterranean are among the highest in the world, especially in summer
(e.g., Lelieveld et al., 2002; Markowicz et al., 2002; Papadimas et al.,
2012; Antón et al., 2012).
In this framework, the characterization of aerosol over the Mediterranean
has received great scientific interest. To date, a large number of studies
has been done focusing on the eastern and central regions (e.g., Formenti et
al., 1998; Balis et al., 2003; Gerasopoulos et al., 2003; Di Iorio et al.,
2003; Kubilay et al., 2003; Pace et al., 2005, 2006; Fotiadi et al., 2006;
Meloni et al., 2007, 2008; Di Sarra et al., 2008; Di Biagio
et al., 2010; Boselli et al., 2012). However, few studies have been done in
the western Mediterranean Basin (Horvath et al., 2002; Alados-Arboledas et
al., 2003; Mallet et al., 2003; Estellés et al., 2007; Saha et al.,
2008; Pérez-Ramírez et al., 2012, Foyo-Moreno et al., 2014). The
majority of these studies have been performed in coastal Mediterranean urban
sites largely influenced by local pollution emissions, except those carried
out at Crete and Lampedusa islands in the eastern and central Mediterranean
Sea regions. In general, columnar aerosol data are scarce over the
Mediterranean Sea and almost absent over the western Mediterranean Sea.
Thus, measurements of the aerosol properties over the western Mediterranean
Sea are needed in order to evaluate the aerosol regimes over this scarcely
explored region (Smirnov et al., 2009). In order to fill this gap and
provide columnar aerosol properties over the western Mediterranean Sea, the
Atmospheric Physic Group of the University of Granada, Spain, in
collaboration with Royal Institute and Observatory of the Spanish Navy
(ROA), has installed a sun photometer at Alborán, a very small island in
the westernmost part of Mediterranean Sea located midway between the African
and European continents. Currently, this station is part of AERONET network
(http://aeronet.gsfc.nasa.gov).
This study focuses on the characterization of aerosol load and aerosol types
as well as on their temporal variability over Alborán Island in the
western Mediterranean from 1 July 2011 to 23 January 2012. In addition,
special attention is given to the conditions responsible for large aerosol
loads over this island, and much attention is paid to identify the potential
aerosol sources affecting Alborán. Furthermore, additional aerosol
properties from three AERONET stations (Málaga, Oujda and Palma de
Mallorca) surrounding Alborán Island and from a MAN cruise over the
Mediterranean Sea, Black Sea and Atlantic Ocean (Fig. 1) are analyzed here
to investigate the spatial aerosol variation over the Mediterranean basin.
The work is structured as follows. In Sect. 2 we describe the
instrumentation used and the experimental sites. Section 3 is devoted to the
main results, where we analyze the aerosol optical properties at Alborán
Island and the spatial variability of aerosol properties in the
Mediterranean. Finally, in Sect. 4 we present the summary and conclusions.
Map of Mediterranean basin showing the location of
Alborán Island, Málaga, Oujda and Palma de Mallorca and a
MAN cruise track over the Mediterranean Sea, Black Sea and Atlantic Ocean
during 26 July–13 November 2011.
Instrumentation and study sites
AERONET measurements
Columnar aerosol properties were measured by a CIMEL CE-318-4 sun
photometer, which is the standard automated sun photometer used in the
AERONET network (Holben et al., 1998). This instrument has a full view angle
of 1.2∘ and makes direct sun measurements at 340, 380, 440,
500, 670, 870, 940 and 1020 nm (nominal wavelengths). The direct sun
measurements are then used to retrieve the aerosol optical depth at each
wavelength, δa(λ), except for 940 nm which is used to
compute precipitable water vapor (Holben et al.,1998). Detailed information
about the CIMEL sun photometer can be found in Holben et al., 1998. The
total estimated uncertainty in δa(λ) provided by
AERONET is of ±0.01 for λ > 440 nm and ±0.02 for shorter wavelengths (Holben et al., 1998). Furthermore the spectral
dependency of the δa(λ) has been considered through
the Ångström exponent, α(440–870), calculated in the range
440–870 nm. The Ångström exponent provides an indication of the
particle size (e.g., Dubovik et al., 2002). Small values of the
Ångström coefficient (α(440–870) < 0.5) suggest a
predominance of coarse particles, such as sea salt or dust, while α(440–870) > 1.5 indicates a predominance of small
particles such as sulphate, nitrate and biomass burning particles. Also
included in the analysis are aerosol optical depths at 500 nm for fine mode
(δF(500 nm)) and for coarse mode (δC(500 nm)) as
well as the fine mode fraction (FMF) (ratio of δF(500 nm) to
δa(500 nm)), determined using the spectral de-convolution
algorithm method developed by O'Neill et al. (2003). In this study, the
level 2 AERONET aerosol data are used.
AERONET stations
This study focuses on the AERONET sun photometer measurements acquired at
the Alborán Island (35.90∘ N, 3.03∘ W,
15 m a.s.l.), in the western Mediterranean Sea, from 1 July 2011 to 23
January 2012. Alborán is a small island with an approximate surface of 7 ha, located ∼ 50 km north of the Moroccan coast and 90 km south of the Spanish coast (Fig. 1). Currently, only 12 members of a
small Spanish Army garrison live on the island. The island and its
surrounding area are declared a natural park and marine reserve. There is no
significant local anthropogenic emission source at Alborán; however, the
island is just south of an important shipping route (www.marinetraffic.com). Due to its location, Alborán Island
is expected to be affected, depending on regional circulation, by
anthropogenic pollutants originated in urban and industrial European areas,
anthropogenic particles emitted from the ship traffic, desert dust
transported from North African arid regions and maritime aerosols from the
Mediterranean Sea. The climate of the region depends strongly on the Azores
anticyclone. Winter is mainly characterized by low pressure systems passing
over the Iberian Peninsula, resulting in the prevalence of westerly winds
and enhanced rainfall. In this season, the weather is unstable, wet and
windy. In summer, the well-established Azores high pressure produces dry and
mild weather with easterly winds that combine with sea/land breezes created
by the aridity of the coastal mountains (Sumner et al., 2001).
In addition, to investigate the spatial variation of aerosol properties over
the western Mediterranean, we used AERONET data obtained from 1 July 2011 to
23 January 2012 over three AERONET stations surrounding Alborán Island;
Oujda, Málaga and Palma de Mallorca (see Fig. 1). These sites cover
different environments including, urban, coastal and island sites,
respectively, and have different background aerosol characteristics. Palma
de Mallorca (39.35∘ N, 2.39∘ E, 13 m a.s.l.),
the capital of the Balearic Islands, is the largest city in the Mallorca
Island with a population of around 400 000. It is located in the western
Mediterranean Sea, about 250 km from the African continent and 190 km from
the Spanish coast. Málaga (36.72∘ N,
4.5∘ W, 40 m a.s.l.), with a population of around 600 000 is
the major coastal city in southeast Spain on the Mediterranean coast. Oujda
city (34.65∘ N, 1.89∘ W, 450 m a.g.l.) is
located in eastern Morocco, 60 km south of the Mediterranean Sea, with an
estimated population of 450 000.
Maritime Aerosol Network measurements
Furthermore, we used shipborne sun photometer measurements collected
onboard the Nautilus 11 on the Mediterranean Sea, Atlantic
Ocean and Black Sea during the period 26 July–13 November 2011. These
measurements were made in the framework of the Maritime Aerosol Network
(MAN), a component of AERONET (Smirnov et al., 2011). More detailed
information about the Nautilus 11 cruise track can be found at http://aeronet.gsfc.nasa.gov/new_web/cruises_new/Nautilus_11.html. MAN uses
Microtops II hand-held sun photometers and utilizes calibrations and data
processing procedures of AERONET network. The Microtops II sun photometer
used in this cruise acquires direct sun measurements at 440, 500, 675 and
870 nm. The estimated uncertainty of the optical depth in each channel is
around ±0.02 (Knobelspiesse et al., 2004). Level 2 MAN data are used
in this study.
Air mass trajectories
To characterize the transport pathways and the origins of air masses
arriving at our studied AERONET sites, 5-day backward trajectories ending at
12:00 UTC at these sites for 500, 1500, 2500, 3500, 4500 and 5000 m above
ground level were calculated using the HYSPLIT model for days with AERONET
measurements (Draxler and Rolph, 2003). In addition, backward trajectories
ending at the different points of MAN cruise for 500, 1500, 2500, 3500, 4500
and 5000 m above ground level were also performed for days with MAN
observations. The HYSPLIT model version employed uses GDAS meteorological
data and includes vertical wind.
Results and discussion
Temporal evolution of aerosol properties over Alborán Island
Figure 2 shows the temporal evolutions of daily mean values of aerosol optical
depths at 500 and 1020 nm and α(440–870) measured at Alborán
Island in the western Mediterranean from 1 July 2011 to 23 January 2012.
There are some gaps in δa(λ) and α(440–870)
data series due to some technical problems and the presence of clouds
(invalid data). Table 1 presents a statistical summary of daily average
values of all the analyzed aerosol properties. One of the main features
observed is the large variability of δa(λ) (for
example, δa(500 nm) ranged from 0.03 to 0.54) that is primarily
related to changes in the air masses affecting the study area, as can be
seen hereafter. The coefficient of variation (COV), defined as the standard
deviation divided by the mean value, can be used to compare the variability
of different data sets. As shown in Table 1, the δa(λ)
at 1020 nm (with COV of 91 %) showed much greater variability than at 340
nm (with COV of 60 %). It is well known that δa(λ)
at higher wavelengths is more affected by naturally produced coarse
particles (radius above 0.5 µm) like dust and sea salt particles,
while δa(λ) at smaller wavelengths is more sensitive
to the fine particles (radius below 0.5 µm) such as those from
anthropogenic activities or biomass-burning. Thus, the higher variability of
δa(λ) for larger wavelengths indicates strong
variability in the coarse particle load (dust or sea salt) over Alborán
Island. This result is also supported by the larger COV of δC(500 nm) as compared to δF(500 nm) (Table 1). Aerosol
salt emission variations due to the wind speed variation and the changes in
the frequency and intensity of dust intrusions over the island may explain
the large variability in the coarse particle component and hence the large
δa(λ) variability for large wavelengths. Moreover,
coarse particles have shorter residence time in the atmosphere in comparison
with fine particles, which could explain also the large δC(λ) variability. On the other hand, α(440–870)
values also show large variability and vary from 0.2 to 1.7 with mean value
of 0.8 ± 0.5, indicating different atmospheric conditions dominated by
different aerosol types (coarse particles, fine aerosols and/or different
mixtures of both coarse and fine particles). It is noted that on 70 % of
the analyzed days, the values of α(400–870) were lower than 1,
suggesting that coarse particles dominated the aerosol population over the
Alborán Island for most of the analyzed days. This is further supported
by the analysis of fine mode fraction which ranged from 0.20 to 0.90 (mean
value of 0.47 ± 0.15), with daily mean values less than 0.5 on 65 %
of the analyzed days.
Temporal evolution of the daily mean values of (a) aerosol optical
depth at 500 and 1020 nm and (b) the Ångström exponent calculated in
the range 440–870 nm, measured at Alborán Island in the western
Mediterranean from 1 July 2011 to 23 January 2012. The error bars are
standard deviations.
The observed mean δa(500 nm) value over Alborán Island was
significantly higher (by factor of 2) than that reported by Smirnov et al. (2002) (δa(500 nm) in the range 0.06–0.08) for open oceanic
areas in the absence of long-range transport influences. Moreover, the mean
δa(500 nm) and α(440–870) values obtained in this study
were larger than the global mean δa(500 nm) value of 0.11 and
α(440–870) of 0.6 reported for maritime aerosols by Smirnov et al. (2009). On the other hand, average aerosol optical depths at 495.7 nm of
0.24 ± 0.14 and α(415–868) of 0.86 ± 0.63 were obtained
from multi filter rotating shadowband radiometer at Lampedusa Island (in the
central Mediterranean Sea) during July 2001–September 2003 (Pace et al.,
2006). Using AERONET data measured in Crete Island (eastern Mediterranean
Sea) during 2003–2004, Fotiadi et al. (2006) reported mean δa(500 nm) value of 0.21 and α(440–870) of 1.1. The differences
between aerosol properties observed over the islands of Alborán, Lampedusa and Crete could be explained in terms of differences in the period and
duration of the measurements, in air mass circulation and in the
methodologies employed. Later we compare the results obtained over
Alborán to those observed over three nearby AERONET stations (during the
same period and using the same type of instruments).
Statistical summary of daily mean values of spectral aerosol optical
depth at 1020, 500 and 340 nm, Ångström exponent, α(440–870), fine and coarse mode aerosol optical depths at 500 nm, δF(500 nm) and δC(500 nm), and fine mode fraction, FMF,
observed over Alborán Island in the western Mediterranean during 1 July
2011–23 January 2012; SD is the standard deviation and COV is the
coefficient of variation.
Mean
SD
Minimum
Maximum
COV( %)
δa(1020 nm)
0.11
0.10
0.01
0.46
91
δa(500 nm)
0.17
0.12
0.03
0.54
70
δa(340 nm)
0.25
0.15
0.05
0.65
60
α(440–870)
0.8
0.4
0.2
1.7
50
δF(500 nm)
0.08
0.05
0.01
0.30
63
δC(500 nm)
0.10
0.09
0.01
0.4
90
FMF
0.47
0.15
0.20
0.94
32
According to the Smirnov et al. (2003) criterion, pure maritime situations
can be generally found when δa(500 nm) < 0.15 and
α(440–870) is less than 1. Considering this criterion, pure maritime
situations were observed over Alborán Island on 40 % of the analyzed
days. According to back trajectory analysis, almost all these days were
characterized by advection of clean Atlantic air masses over the study area.
In addition, the majority of these pure maritime cases were observed during
the wet season from November to January. This result is in agreement with
the study performed at the island of Lampedusa, in the central
Mediterranean, showing that pure maritime situations are usually observed
during Atlantic air advection (Pace et al., 2006). However, clean maritime
conditions observed over Alborán Island during the analyzed period are
more frequent than those observed over Lampedusa. Pace et al. (2006) showed
that clean maritime conditions are rather rare over the central
Mediterranean due to the large impact of natural and anthropogenic sources.
The difference in the occurrences of clean maritime conditions at these two
sites can be explained by their different locations. Alborán is closer
to the Atlantic Ocean than Lampedusa is, and the Atlantic air masses
reaching Lampedusa may be influenced more by anthropogenic aerosol during
their passage over Mediterranean Sea and continents.
Threshold values for δa(500 nm) and α(440–870) have
been widely used in remote sensing to identify marine aerosol type. For
example, Smirnov et al. (2003) used δa(500 nm) ≤0.15 and
α(440–870) ≤1 and Sayer et al. (2012a, b) proposed δa(500 nm) ≤0.2 and 0.2≤α(440–870)≤1 while
Toledano et al. (2007) used δa(500 nm) ≤0.15 and α(440–870) ≤0.6 for identifying pure maritime situations. However,
the proposed threshold values for δa(500 nm) and α(440–870) to identify maritime aerosol type are purely empirical.
Therefore, not all observations that meet these thresholds will represent
the pure maritime aerosol. In fact, in Alboran Island we found
measurements that fulfill these criteria but that are not associated with
pure maritime conditions. In this sense, in Fig. 3 we show the δa(500 nm) and α(440–870) observed on 26 August, 2011. During
this day, the δa(500 nm) values ranged from 0.06 to 0.13 with
mean daily value of 0.09 ± 0.01 and α(440–870) was in the range
0.3–0.6, indicating clean atmospheric condition dominated by coarse
particles. Thus, according to the above criteria this day is classified as
pure maritime case. However, the back trajectory analysis and Meteosat Second Generation (MSG) satellite
image (Thieuleux et al., 2005) for 26 August revealed the presence of dust
over Alborán Island (Fig. 3). Therefore, care must be taken when using
δa(500 nm) and α(440–870) thresholds for discriminating
the pure maritime cases since dusty situations with low dust loads can be
confused with pure maritime conditions. Additional information such as air
mass back trajectory or satellite images is needed for better identifying
the pure maritime cases.
As can be seen in Fig. 2, there were several days strongly influenced by
aerosols, with δa(500 nm) values exceeding 0.3. High aerosol
loads (δa(500 nm) > 0.3) over Alborán Island were
observed on 30 of the 160 analyzed days. All these events were observed from
July to October. In 27 of these cases, the mean daily α(440–870)
values were lower than 0.8 and fine mode fraction (FMF) lower than 0.5; suggesting predominance
of coarse particles as either sea salt or dust transported from desert
areas. According to the analyses of back trajectories and MODIS satellite
images (not shown), all these 27 cases were related to dust intrusions from
North Africa. It is important to note that in these dust events, the δF(500 nm) values were also relatively high (for this remote site) and
ranged from 0.07 to 0.20 with mean value of 0.12 ± 0.03. These results
highlight a considerable contribution of fine mode particles (either dust or
anthropogenic or both) to the aerosol population (FMF ranged from 20 to
52 %) during these dust events. Back trajectory analysis for dusty days
with highest fine aerosol load revealed that the air masses reaching the
study area at low levels (at 500 or 1500 m level) have originated over
Europe and the Mediterranean Sea. However, during desert dust events with
lowest fine aerosol loads, none of the air masses affecting the study area
come from Europe or Mediterranean Sea, which points out significant
contribution of anthropogenic particles to the fine mode fraction of δa(500 nm) during desert dust events associated with large loads of
fine aerosol particles.
(a) Aerosol optical depth at 500 nm and Ångstrom exponent in
the range 440–870 nm, (b) backward trajectories ending at 12 UTC over
Alborán Island at height altitudes of 500, 1500, 3000 and 4000 m,
(c)
MSG satellite image for 26 August 2011 (http://www.icare.univ-lille1.fr).
(a) Total, fine and coarse aerosol optical depths at 500 nm and
(b) Ångstrom exponent in the range 440–870 nm obtained at Alborán
Island during 29 September–5 October 2011. (c) Backward trajectories ending
at 12:00 UTC on 4 October 2011 over Alborán Island at altitudes of 500,
1500 and 3000 m. (d) and (f) NAAPS maps for sulfate surface concentrations for 2
and 3 October 2011 at 12:00 UTC
The remaining high aerosol load events were observed from 30 September to 4
October 2011 (Fig. 4). During these days, the high aerosol loads were
associated with relatively high α(440–870) values that reached the
highest α(440–870) value (about 1.6) during the entire study on 4
October. During these days, the δF(500 nm) values were also
high (> 0.19) and reached the highest mean daily value of 0.33 on
4 October. This behavior suggests a predominance of fine particles
transported from continental industrial/urban areas as there is no local
anthropogenic activity in Alborán. The high δa(λ)
values observed in this event were associated with persistent intense high
pressure systems centered over the Azores, which favor transport of
anthropogenic particles emitted in Europe to Alborán Island. Indeed, on
this day the air mass ending at 1500 m a.g.l. (Fig. 4c) came from central
Europe and traveled at low altitude on the last 3 days before its arrival at
Alborán Island, over an area with a great sulfate surface concentration
according to Navy Aerosol Analysis and Prediction System (NAAPS) model (Fig. 4d, f). Therefore, these air masses might pick
up fine anthropogenic particles in their way to Alborán Island, which
may explain the high values of both δa(500 nm) and α(440–870) parameters observed during this event. Thus, the desert dust
transport appears to be a main cause of high aerosol loads while transport
from central European urban areas is associated with occasional large
aerosol loads over Alboran Island. These results are in accordance with
those reported by Fotiadi et al. (2006) for Crete, who found the highest
values of δa(λ) primarily during southeasterly winds,
associated with coarse dust aerosols, and to a lesser extent to
northwesterly winds associated with fine aerosols originated in urban
industrial European areas.
Monthly variation of aerosol properties over Alborán Island
Figure 5 shows the monthly mean values of δa(500 nm), δF(500 nm) and δC(500 nm) as well as α(440–870)
and FMF with the corresponding standard deviations for the analyzed period.
The monthly average data are calculated from daily averaged data. The
largest values of δa(500 nm), reflecting high aerosol load,
were observed during July–October while the lowest values (0.06–0.08) were
measured from November to January (Fig. 5a). On the other hand, the monthly mean
values of α(440–870) and FMF were lower than 1.0 and 0.5
respectively, indicating a relatively high abundance of coarse particles in
each month of the analyzed period, except in October (Fig. 5b). For October,
the mean α(440–870) was 1.1 ± 0.4 and the FMF 0.63 ± 0.20,
indicating an increase in fine particle contribution during this month (Fig. 5b). It is also worth noting that both δF(500 nm) and δC(500 nm) showed a pronounced increase during July–October, suggesting
increased loads of both fine and coarse particles during these months (Fig. 5a). Moreover, δC(500 nm) reached its maximum in August while
δF(500 nm) peaked in October (Fig. 5a).
Monthly variations of (a) total, coarse and fine mode optical
depths at 500 nm and (b) fine mode fraction and Ångstrom exponent in the
range 440–870 nm obtained at Alborán Island from July 2011 to January
2012. The error bars are standard deviations. (c) Monthly relative frequency
of Saharan dust intrusions and Atlantic air mass advections over Alborán
Island from July 2011 to January 2012.
This pronounced change in aerosol loads from summer to winter in 2011 is
primarily due to the seasonal change in atmospheric circulation over the
Mediterranean (Fig. 5c). The increased coarse aerosol load observed during
July–October was associated with the high frequency of desert dust
intrusions in summer in comparison to November–January (Fig. 5c). In fact,
40, 70, 41 and 14 % of measurement days in July, August,
September and October were associated with Saharan dust intrusions, while in
November–January there was no Saharan dust intrusion (Fig. 5c). Moreover,
the air mass recirculation over the western Mediterranean especially in
summer (Millan et al., 1997) along with the increased photochemical activity
due to the high insolation during this season may favor the accumulation of
fine aerosols that can explain the high fine particle loads during
July–October in comparison with November–January. In addition, the presence
of these fine aerosol particles may be favored by pollution transport from
Europe and coastal urban industrial areas in northeast Africa. In this
sense, the highest fine mode aerosol optical depth observed in October was
associated with the increase in the frequency of air masses coming from
European urban areas (see for example Fig. 4). The low aerosol loads
registered in November–January can be explained by the high frequency of
clean Atlantic air advection (70–100 %) and the absence of Saharan dust
intrusions (Fig. 5c) as well as efficient wet removal aerosol processes due
to cloudy conditions and precipitation in this period. These results
highlight the important role of the large scale circulation on monthly
aerosol variation over Alborán Island.
Spatial variability of aerosol properties over western Mediterranean region
AERONET data of level 2 from Alborán and three AERONET stations
surrounding the island (see Fig. 1) obtained from 1 July 2011 to 23 January
2012 are considered in this study to investigate the spatial variation of
aerosol optical properties over the western Mediterranean region. For
analyzing the spatial aerosol variability we compared the aerosol data
obtained over Alborán during 1 July 2011–23 January 2012 with those
observed over these nearby sites using only time coincident measurements.
Temporal evolutions of daily mean values of δa(500 nm) from 1
July 2011 to 23 January 2012 obtained over Alborán Island and Málaga
stations are shown in Fig. 6a. Daily mean data were calculated only from
time coincident measurements for direct comparison. Málaga is located
approximately 150 km northwest of Alborán. The temporal variations of
daily mean values of δa(500 nm) were similar for both sites on
most days of the analyzed period, indicating similarities in the processes
that control the aerosol load over both sites. In fact, high correlation in
δa(500 nm) with correlation coefficient, R, of 0.75 between
these two sites was found. Similar results were obtained when comparing
δa(500 nm) over Alborán with those in Oujda (R=0.8) and
Palma de Mallorca (R=0.6), Fig. 6b, c. However, large differences are also
present on some days (e.g., on 8 August 2011 at Alboran Island we
registered δa(500 nm) above 0.5 while at Málaga the values
were below 0.1). These differences are due, in large part, to the
differences in the times of occurrence and intensity of Saharan dust
intrusions over these sites. In fact, the correlation in δF(500 nm) between Alborán Island and Málaga, R=0.86, was higher than the
correlation in δC(500 nm), R=0.65. Similar results were
obtained when comparing the aerosol properties over Alborán with those
in Oujda (R=0.82 for δF(500 nm) and R=0.70 for δC(500 nm)) and Palma de Mallorca (R=0.67 for δF(500 nm)
and R=0.32 for δC(500 nm)).
Temporal evolutions of daily mean values of δa(500 nm) from 1 July 2011 to 23 January 2012 obtained over (a) Alborán Island
and Málaga, (b) Alborán Island and Oujda and (c) Alborán Island
and Palma de Mallorca. Daily mean data were calculated only from time
coincident measurements.
Average values and standard deviations of δa(λ),
α(440–870), δF(500 nm) and FMF from 1 July 2011 to 23
January 2012 for Alborán Island, Málaga, Oujda and Palma de Mallorca.
Only days with coincident measurements at Alborán and at each one of the
additional AERONET stations are used for direct comparison.
Alborán
Málaga
Alborán
Palma de
Alborán
Oujda
Mallorca
δa(1020 nm)
0.09 ± 0.09
0.06 ± 0.05
0.13 ± 0.10
0.06 ± 0.04
0.13 ± 0.11
0.16 ± 0.17
δa(870 nm)
0.10 ± 0.09
0.08 ± 0.06
0.14 ± 0.11
0.08 ± 0.05
0.14 ± 0.11
0.18 ± 0.18
δa(670 nm)
0.12 ± 0.10
0.09 ± 0.07
0.16 ± 0.12
0.10 ± 0.06
0.16 ± 0.12
0.19 ± 0.18
δa(500 nm)
0.16 ± 0.11
0.14 ± 0.09
0.20 ± 0.13
0.14 ± 0.07
0.20 ± 0.13
0.23 ± 0.19
δa(440 nm)
0.18 ± 0.12
0.16 ± 0.10
0.23 ± 0.14
0.18 ± 0.09
0.22 ± 0.14
0.25 ± 0.19
δa(380 nm)
0.21 ± 0.13
0.20 ± 0.12
0.26 ± 0.15
0.21 ± 0.10
0.25 ± 0.15
0.29 ± 0.20
δa(340 nm)
0.23 ± 0.14
0.23 ± 0.13
0.29 ± 0.16
0.24 ± 0.11
0.28 ± 0.16
0.30 ± 0.20
α(440–870)
0.9 ± 0.4
1.0 ± 0.3
0.8 ± 0.4
1.2 ± 0.4
0.8 ± 0.4
0.8 ± 0.4
δF(500 nm)
0.09 ± 0.06
0.09 ± 0.06
0.09 ± 0.07
0.09 ± 0.06
0.09 ± 0.07
0.09 ± 0.06
FMF
0.50 ± 0.15
0.53 ± 0.13
0.47 ± 0.18
0.60 ± 0.14
0.47 ± 0.19
0.47 ± 0.19
Number of
141
141
93
93
101
101
coincident days
Table 2 shows average values of δa(λ), α(440–870), δF(500 nm) and FMF as well as the number of
measurement days for each comparison (Alborán–Málaga,
Alborán–Oujda and Alborán–Palma de Mallorca). Only days with
coincident measurements obtained at Alborán and at each one of the
additional AERONET stations from 1 July 2011 to 23 January 2012 were used
for direct comparisons. For λ > 500 nm, values of δa(λ) were slightly larger over Alborán than over
Málaga (Table 2). Indeed, the mean δa(1020 nm) value
obtained at Alborán was 35 % larger than that observed over
Málaga. This indicates that the coarse particles levels were
significantly larger over Alborán in comparison with Málaga during
the analyzed period. In fact, the mean δC(500 nm) for the
entire analyzed period was slightly higher (0.09 ± 0.08) at Alborán
in comparison with Málaga (0.06 ± 0.05). The lower coarse particles
load over Málaga as compared to Alborán is likely due to the higher
frequency of Saharan dust outbreaks over Alborán as compared to
Málaga and also to dust deposition in its way from Alborán to
Málaga. On the other hand, for λ < 500 nm, the mean value of
δa(λ) over Alborán was almost similar to that over
Málaga (Table 2). It is interesting to note that the mean δF(500 nm) value for the entire studied period observed over
Alborán (0.09 ± 0.06) was similar to that obtained (0.09 ± 0.06) over the Málaga urban coastal site, suggesting similar
concentrations of fine particles over both sites. This result is quite
surprising because Málaga is a coastal city with significant local
anthropogenic emissions in comparison to Alborán where there are no
local anthropogenic activities. As we commented before, Alborán Island
is located near an important shipping route and hence it is expected to be
highly influenced by ship emissions. Thus, these results suggest that
emissions from ships and/or from urban-industrial areas in Mediterranean
countries could play in Alborán a similar role to that played by
anthropogenic particles in Málaga. Further studies using chemical
analysis of particles sampled in situ are needed to evaluate this
hypothesis.
The comparison of the aerosol properties obtained at Oujda and Alborán
Island is also shown in Table 2. In this case, the δa(λ) at all wavelengths were lower at Alborán than at Oujda, indicating
lower aerosol concentrations over Alborán. However, δF(500
nm) was similar over Oujda and Alborán (Table 2), indicating similar
fine particle loading over both sites. This result is again surprising
because Oujda is an urban site with significant local anthropogenic
emissions in comparison to Alborán Island where there is no local
anthropogenic activities. These results also point to the significant role
that anthropogenic emissions from traffic ships and/or Mediterranean
countries may play over Alborán. On the other hand, δC(500
nm) obtained over Oujda (0.14 ± 0.15) was higher than that observed
over Alborán (0.11 ± 0.10), indicating higher coarse particle
concentrations over Oujda. The large coarse particle load over Oujda may
result from its proximity to dust sources and local dust resuspension.
The mean δa(λ) values at all wavelengths over
Alborán were higher than those observed over Mallorca, especially at the
larger wavelengths which are more influenced by coarse particles (Table 2).
However, as in the other cases, δF(500 nm) was very similar
over both sites (Table 2) in spite of the large distance (about 650 km)
separating the sites and site characteristic differences. These results
suggest homogeneous spatial distribution of fine particle loads over the
four studied sites in spite of the large differences in local sources. On
the other hand, the observed decrease in δa(500 nm) from south
(Alborán) to north (Mallorca) may be attributed to the proximity of
Alborán Island to the dust sources in north Africa as compared to
Mallorca. A gradient in dust load from south to north in western
Mediterranean has also been reported by other authors (e.g., Moulin et al.,
1998; Barnaba and Gobbi, 2004). Overall, based on the above comparisons it
may be concluded that δC(λ) showed a south-to-north
decrease in this region of western Mediterranean, while the fine mode
aerosol optical depth was fairly similar over these sites.
Variability of aerosol properties during a MAN cruise
From 26 July to 13 November 2011 the Maritime Aerosol Network acquired
measurements over the whole Mediterranean Sea, Black Sea and Atlantic Ocean
from the ship Nautilus 11. Figure 7 shows δa(500
nm), δF(500 nm), δC(500 nm) and FMF obtained
during this cruise. The measurements made over the Mediterranean Sea were
divided (on the basis of the differences in the aerosol sources and air
masses affecting each area) into three regions: western, central and eastern
Mediterranean. As can be seen from Fig. 7, all the analyzed aerosol
properties showed large variability with no evident pattern during the
cruise period. This large variability in aerosol properties during this
cruise can be explained by the different aerosol sources and air masses that
affected each region during the measurement period (see below). For the
entire cruise period, the δa(500 nm) varied from 0.08 to 0.70
with a mean value of 0.22 ± 0.12. On the other hand, δF(500 nm) also showed large variability and ranged between 0.04 and
0.60 with a mean value of 0.16 ± 0.10 while δC(500 nm)
fluctuated within the range 0.01–0.30 with mean value of 0.06 ± 0.04.
For 85 % of the measurements, the fine mode fraction was in the range
0.52–0.96, indicating the predominance of situations dominated by fine mode
particles during this cruise.
Temporal evolutions of δa(500 nm), δF(500 nm), δC(500 nm), and FMF obtained onboard the
Nautilus ship. The data belong to the Maritime Aerosol Network (MAN) and
were acquired between 26 July and 13 November 2011.
The highest δa(500 nm) values ranging from 0.20 to 0.46 with a
mean value of 0.35 ± 0.09 were observed over the western Mediterranean
Sea during the cruise period 28 September–08 October (Table 3). Also,
δF(500 nm) values were highest (varying in the range of
0.14–0.40 with a mean value of 0.29 ± 0.09) over the western
Mediterranean. These high aerosol loads were associated with high FMF values
in the range 0.70–0.87, which show the predominance of fine anthropogenic
particles over this area during this period (Fig. 7b). According to the back
trajectory analyses, the air masses that affected the western Mediterranean
region during this period come from European urban-industrial areas which
explains the observed large values of δa(500 nm) and the
predominance of fine mode particles (see for example Fig. 4c). The aerosol
loads were also relatively high over the Black Sea (δa(500 nm),
ranging from 0.08 to 0.68 with a mean value of 0.25 ± 0.16 during 26
July–15 August cruise period) and were strongly dominated by fine particles
as showed by FMF values ranging from 0.64 to 0.94. The large values of
δa(500 nm) and those of δF(500 nm) (δF(500 nm) in the range 0.07–0.60) and the predominance of the fine
mode over the Black Sea during this cruise period was associated, according
to the HYSPLIT back trajectory analyses, with air masses coming from
northeastern Europe (Figure not shown); this region has been identified as
a strong source of pollutants and biomass burning particles during summer
(e.g., Barnaba and Gobbi, 2004). In contrast, the lowest δa(500
nm) values (varying in the range 0.08–0.26 with mean value of 0.14 ± 0.06) were observed over the eastern Mediterranean at the end of the cruise
(5–13 November). These low δa(500 nm) values were associated
with FMF ranging between 0.30 and 0.64, showing a predominance of coarse
aerosol over this area during this period. It is worth noting that the
aerosol loads over the eastern Mediterranean during 5–13 November decreased
drastically in comparison with the aerosol levels observed in the same
region during the cruise period from 18 August to 13 September (Table 3).
The decrease was more pronounced for the fine particle load; δF(500 nm) decreased from 0.16 ± 0.07 in the first measurements
over the eastern Mediterranean to 0.07 ± 0.02 in the last ones. In
contrast, δC(500 nm) showed an increase from 0.04 ± 0.02
during 18 August–12 September to 0.08 ± 0.04 during 5–13 November. This
drastic change may be explained by the seasonal changes in the
meteorological conditions. In this sense, the last measurements over the
eastern Mediterranean Sea were obtained during the end of autumn when
aerosol wet deposition is more effective and secondary aerosol formation is
less important than in summer, which may explain the lower aerosol loads
observed at the end of the expedition.
Mean values of δa(500 nm), δF(500 nm),
δC(500 nm), α(440–870) and FMF obtained over the Black Sea; the western,
central and eastern Mediterranean Sea; and the Atlantic Ocean during the Nautilus ship cruise from 26 July to 13 November 2011.
Region
δa(500 nm)
δF(500 nm)
δC(500 nm)
α(440–870)
FMF
Black Sea
0.25 ± 0.16
0.21 ± 0.14
0.04 ± 0.03
1.76 ± 0.30
0.82 ± 0.08
(26 July–15 August)
Eastern Mediterranean I
0.20 ± 0.08
0.16 ± 0.07
0.04 ± 0.02
1.74 ± 0.20
0.81 ± 0.08
(18 August–12 September)
Central Mediterranean I
0.18 ± 010
0.12 ± 0.09
0.06 ± 0.03
1.27 ± 0.40
0.66 ± 0.08
(13–28 September)
Western Mediterranean
0.35 ± 0.09
0.29 ± 0.09
0.07 ± 0.02
1.50 ± 0.13
0.80 ± 0.07
(28 September–8 October)
Atlantic Ocean
0.19 ± 0.10
0.11 ± 0.05
0.09 ± 0.06
1.08 ± 0.25
0.56 ± 0.09
(9–19 October)
Central Mediterranean II
0.22 ± 0.10
0.13 ± 0.07
0.09 ± 0.04
1.05 ± 0.30
0.57 ± 0.13
(25 October–5 November)
Eastern Mediterranean II
0.14 ± 0.06
0.07 ± 0.02
0.08 ± 0.04
0.90 ± 0.35
0.49 ± 0.12
(5–13 November)
Conclusions
AERONET sun photometer measurements obtained over Alborán Island and
three adjacent sites in the western Mediterranean were analyzed in order to
investigate the temporal and spatial variations of columnar aerosol
properties over this poorly explored region.
Within the analyzed period the daily average values of δa(500 nm) over Alborán Island ranged from 0.03 to 0.54 with a mean and
standard deviation of 0.17 ± 0.12, indicating high aerosol load
variation. The observed mean δa(500 nm) value over Alborán
Island was significantly higher than reported for open oceanic areas not
affected by long range aerosol transport (0.06–0.08). The α(440–870)
values were lower than 1 for 70 % of the measurement days, suggesting that
coarse particles dominated the aerosol population over the Alborán
Island for the majority of the measurement days.
High aerosol loads over Alborán were mainly associated with desert dust
transport from arid areas in North Africa and occasional advection of
anthropogenic fine particles from central European urban-industrial areas.
The aerosol optical depth values of fine mode during dust events were also
relatively high (for this remote site), suggesting that the fine mode
particles also have considerable influence on optical properties during
these dust events. Background maritime conditions over Alborán
characterized by low aerosol load and Ångström exponent (δa(500 nm) < 0.15 and α(440–870) < 1) were
observed on about 40 % of the measurement days during the analyzed period;
almost all of these days were characterized by advection of clean Atlantic
air masses over the study area.
The mean value of δF(500 nm) over Alborán Island was
comparable to the observations over the other three nearby AERONET stations,
suggesting homogeneous spatial distribution of fine particle loads over the
four studied sites in spite of the large differences in local sources. A
northward decreases in δC(λ) was found which was
probably associated with increased desert dust deposition from south to
north or decreased dust frequency from south to north.
Aerosol properties acquired on board the ship Nautilus 11
within Maritime Aerosol Network over the whole Mediterranean Sea, Black Sea
and Atlantic Ocean from July to November 2011 showed large variability and
no evident pattern was found. In 85 % of the measurements, the fine mode
fraction was in the range 0.52–0.96, indicating the predominance of fine
mode particles over the cruise areas during the monitoring period. The
highest δa(500 nm) and δF(500 nm) mean values of
0.35 ± 0.09 and 0.29 ± 0.09 during the cruise period were observed
over the western Mediterranean Sea, which were related to polluted air
masses coming from European urban-industrial areas. In contrast, the lowest
δa(500 nm) values (mean value of 0.14 ± 0.06) during this
cruise were observed over the eastern Mediterranean Sea on the final days of
the cruise in autumn, when aerosol wet deposition is more effective and
secondary aerosol formation is less important than in summer.