Climatology and trends of aerosol optical depth over

Science of the Total Environment 551–552 (2016) 292–303

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Climatology and trends of aerosol optical depth over the Mediterranean basin during the last 12 years (2002–2014) based on Collection 006 MODIS-Aqua data A.A. Floutsi a,1, M.B. Korras-Carraca a, C. Matsoukas a, N. Hatzianastassiou b, G. Biskos c,d,⁎ a

Department of Environment, University of the Aegean, Mytilene 81100, Greece Laboratory of Meteorology, Department of Physics, University of Ioannina, 45110 Ioannina, Greece c Energy Environment and Water Research Center, The Cyprus Institute, Nicosia 2121, Cyprus d Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft 2628 CN, The Netherlands b

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• We use the newest MODIS-Aqua collection to study AOD trends over the Mediterranean. • From 2002 to 2014 the AOD exhibits an average decreasing trend of 0.0030/year. • Further analysis also shows a south-tonorth decreasing gradient. • Anthropogenic sources are dominant in the North, and desert aerosols in the South.

a r t i c l e

i n f o

Article history: Received 31 December 2015 Accepted 27 January 2016 Available online xxxx Editor: D. Barcelo Keywords: AOD AOT Aerosol fine mode Mineral dust particles

a b s t r a c t The Mediterranean basin is a region of particular interest for studying atmospheric aerosols due to the large variety of air masses it receives, and its sensitivity to climate change. In this study we use the newest collection (C006) of aerosol optical depth from MODIS-Aqua, from which we also derived the fine-mode fraction and Ångström exponent over the last 12 years (i.e., from 2002 to 2014), providing the longest analyzed dataset for this region. The long-term regional optical depth average is 0.20 ± 0.05, with the indicated uncertainty reflecting the inter-annual variability. Overall, the aerosol optical depth exhibits a south-to-north decreasing gradient and an average decreasing trend of 0.0030 per year (19% total decrease over the study period). The correlation between the reported AOD observations with measurements from the ground AERONET stations is high (R = 0.76–0.80 depending on the wavelength), with the MODIS-Aqua data being slightly overestimated. Both finefraction and Ångström exponent data highlight the dominance of anthropogenic aerosols over the northern, and of desert aerosols over the southern part of the region. Clear intrusions of desert dust over the Eastern Mediterranean are observed principally in spring, and in some cases in winter. Dust intrusions dominate the Western Mediterranean in the summer (and sometimes in autumn), whereas anthropogenic aerosols dominate the sub-

⁎ Corresponding author at: Energy Environment and Water Research Center, The Cyprus Institute, Nicosia 2121, Cyprus. E-mail addresses: [email protected], [email protected] (G. Biskos). 1 Currently at Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft 2628 CN, The Netherlands.

http://dx.doi.org/10.1016/j.scitotenv.2016.01.192 0048-9697/© 2016 Elsevier B.V. All rights reserved.

A.A. Floutsi et al. / Science of the Total Environment 551–552 (2016) 292–303

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region of the Black Sea in all seasons but especially during summer. Fine-mode optical depth is found to decrease over almost all areas of the study region during the 12-year period, marking the decreasing contribution of anthropogenic particulate matter emissions over the study area. Coarse-mode aerosol load also exhibits an overall decreasing trend. However, its decrease is smaller than that of fine aerosols and not as uniformly distributed, underlining that the overall decrease in the region arises mainly from reduced anthropogenic emissions. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Quantifying radiative forcing is key to understanding how climate is changing at local, regional and global scales. The greatest part of uncertainty in predictions of radiative forcing is attributed to limited knowledge of spatial and temporal distribution of aerosols, their physico-chemical properties, and the processes they are involved in (Boucher et al., 2013). As a result, the study of atmospheric aerosols still is a critical contribution to the field of climate change. In addition, aerosol load and composition is a major factor for air quality, with great implication on human health (Pope, 2000; McMichael et al., 2006). Extensive efforts have therefore been made in monitoring atmospheric aerosols using in situ measurements or satellite- and groundbased remote-sensing observations. In this respect, the Aerosol Robotic Network (AERONET, Holben et al., 1998), is the principal network of surface stations for aerosol observations that is commonly used as reference for satellite aerosol products. Despite its being quite widespread with data from hundreds of stations, however, its coverage is characterized as incomplete. The limitation of spatial coverage has been partly solved since the 1980s by space-borne sensors, such as the early Advanced Very High Resolution Radiometer (AVHRR) or Total Ozone Mapping Spectrometer (TOMS). The situation has been greatly improved with more sophisticated and accurate sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS), over the last decade. The MODIS instrument, on board the twin polar-orbiting satellites Terra and Aqua of the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS), performs near-global observations of atmospheric aerosols. Since February 2000, MODIS has continuously acquired measurements at 36 spectral bands having wavelengths λ between 0.415 and 14.235 μm, with spatial resolution of 250, 500 and 1000 m. Seven of these bands, between λ = 0.459 and 2.155 μm, are used to retrieve aerosol properties over cloud and surface-screened areas. The MODIS aerosol products are stored at different levels and under various versions (referred to as ‘collections’) based on algorithm updates, with collection C006 being the most recent. Changes introduced in C006 increase the global accuracy and coverage, but without major change to the basic principles of the algorithm (Levy et al., 2013). MODIS has acquired a well-respected status as one of the most reliable satellite datasets of aerosol optical depth (AOD) over ocean and land (Bréon et al., 2011; Nabat et al., 2013). The retrieval of aerosol properties is easiest over dark (in visible wavelengths) surfaces and therefore the first family of algorithms devised for MODIS were the “Dark Target”, which have been used over land (Kaufman et al., 1997) and ocean (Tanré et al., 1997). A large number of studies have been performed to assess the role of atmospheric aerosols on climate at local, regional and global scales (Mishchenko et al., 2007; Hsu et al., 2012; Itahashi et al., 2012; Mao et al., 2014; Yoon et al., 2014; Koukouli et al., 2010; Yoon et al., 2011; Ramachandran et al., 2012). Apart from climate diagnostics, regional aerosol studies for evaluating the influence of the atmospheric aerosol to visibility are of great interest for planning energy production involving renewable resources (Gueymard, 2011; Calinoiu et al., 2013) or societal policies (Rypdal et al., 2005). One of the most interesting regions globally for investigating the contribution of aerosols to climate is that of the Mediterranean Sea and the surrounding basin. This is because the region is a cross-road of air pollution (Lelieveld et al., 2002),

receiving a wide variety of aerosol types including industrial, urban, marine, desert dust, and biomass burning. Moreover, the Mediterranean is among the sea areas with the highest aerosol optical depths in the world (Husar et al., 1997), which combined with high amounts of solar radiation, some of the largest in the world (Hatzianastassiou et al., 2005), can result in large magnitudes of aerosol radiative effects (e.g., Papadimas et al., 2012). The properties of the atmospheric aerosol over the Mediterranean has been the subject of many previous studies (e.g., Moulin et al., 1997, 1998; Sciare et al., 2003, 2008; Barnaba and Gobbi, 2004; Fotiadi et al., 2006; Mona et al., 2006; Kalivitis et al., 2007; Papadimas et al., 2008, 2009; Gkikas et al., 2009, 2012, 2013; Nabat et al., 2013). Most of these studies, however, have used previous collections of MODIS and refer to time periods shorter than a decade. The objective of this study is to examine the spatial and temporal variation of the aerosol optical depth and the fraction of fine particles over the area of the Mediterranean Sea for the period 2002 to 2014, thereby providing a climatological type of dataset for the region. In doing so, we also investigate the spatial and temporal patterns and the contribution to AOD of not only the total aerosol but also its finemode, thereby enabling an estimation of the anthropogenic contribution. What's more, we examine the inter-annual changes and trends of AOD in the region, which is of primary importance in light of the changing people habits under conditions of economic crisis (e.g., Vrekoussis et al., 2013), and the actions undertaken by the European Union for Clean Air (Clean Air for Europe, CAFE, Program). Our work relies on the use of the latest available MODIS-Aqua C006 Collection. MODIS is the best performing aerosol dataset in the Mediterranean (Nabat et al., 2013) whereas C006 dataset is an improvement to previous collections (e.g., C005, Levy et al., 2013). This paper is organized as follows: Section 2 provides information on our data sources and data-handling methodology. Section 3 presents our analysis for annual, seasonal, sub-regional, and inter-annual behavior of the MODIS aerosol, along with its comparison with surface data from AERONET. Section 4 discusses the findings and highlights the most important conclusions of this study.

2. Data and methodology 2.1. MODIS data In this study we use the Collection 006 MODIS-Aqua AOD data at 470, 550, 650, and 860 nm, along with fine-mode AOD (fAOD) at 550 nm, which is a measure of the contribution of particles smaller than 1 μm to the optical depth. From these data, we calculated the Ångström exponent for 550 and 860 nm (a550–860), and the Fine Mode Fraction (FMF or FF) as fAOD/AOD. The AOD corresponding to the coarse mode is taken as the difference between AOD and fAOD. The data used are extracted from MODIS Level 3 (MYD08_D3) daily files from 4 July 2002 to 31 December 2014, reported for geographical cells with 1° × 1° spatial resolution. Our study region extends from 29.5°N to 47.5°N and from 10.5°W to 42.5°E (cf. Fig. 1). Overall, a total of 4564 daily datasets were analyzed for each of the aerosol parameters considered. The AOD, FF, and a550–860 data were used to produce 12-year (from 2002 to 2014) climatological and regional mean aerosol products, in order to assess their spatial and temporal (seasonal and year-by-year) variability over the study region.

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Fig. 1. Map of the study region and the sub-regions (Western Mediterranean, Eastern Mediterranean, and Black Sea). The locations of AERONET stations used in this study are also marked (magenta dots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2.2. AERONET data The MODIS satellite aerosol data were validated against corresponding surface-based measurements from AERONET, which involves sun/ sky radiometers to derive total column aerosol properties from solar radiance (Holben et al., 1998; Dubovik and King, 2000; Dubovik et al., 2002; O'Neill et al., 2005). The AERONET network has expanded over the last years to cover a large part of the Mediterranean basin, with 117 stations being currently within the study region. In order to ensure the overlap of AERONET and MODIS Dark Target over ocean data, however, we only chose coastal stations within the study region. This resulted in 55 AERONET stations (cf. Fig. 1) from which we obtained data for the AOD and 44 stations for the FF. AERONET data are provided on three levels (i.e. Levels 1.0, 1.5, and 2). In the present work, we use the most reliable Level 2 data, which are cloud-screened (Smirnov et al., 2000) and quality-assured. The overall uncertainty in the AOD data, under cloud-free conditions, is ±0.01 for wavelengths greater than 440 nm, and ±0.02 for shorter wavelengths (Eck et al., 1999). 2.3. GPCP combined precipitation data In order to assess the inter-annual changes of AOD, we also used precipitation data obtained from the Global Precipitation Climatology Project version 2.2 (GPCP; Adler et al., 2003). The GPCP combined precipitation data were developed and computed by the NASA/Goddard Space Flight Center's Laboratory for Atmospheres as a contribution to the GEWEX Global Precipitation Climatology Project. This dataset combines surface rain gauge measurements and precipitation observations from various satellites in monthly, 2.5° × 2.5° gridded data. 3. Results 3.1. Spatial distributions Fig. 2a shows the 12-year (2002–2014) MODIS mean spatial distributions of AOD at 550 nm, while the corresponding figures for AOD at 470 nm, 650 nm and 860 nm are included in the supplementary material (Fig. S1). We base our analysis and discussion on the results for 550 nm because they correspond to a wavelength that is in the middle of the visible range, and to the peak of the solar spectrum where the radiative effect is maximum. In addition, the AOD at this or very close to this wavelength is present in all aerosol databases. Fig. 2b and c show respectively the FF at 550 nm and a550–860 over the broader Mediterranean basin. In general, the

AOD values range from 0.15 to 0.31 at 550 nm, exhibiting a decrease as the wavelength increases (Fig. S1) in agreement with classical Mie scattering theory. The decrease of AOD with wavelength is stronger at the northern than at the southern parts of the study region, which are characterized by finer (mostly anthropogenic) and coarser (mostly natural) aerosols, respectively (cf. Fig. 2b and c and discussion below). Larger aerosol AOD and consequently aerosol load is observed mainly in the southern parts (AOD up to 0.31 at 550 nm over the Gulf of Sidra) originating from nearby great desert areas. Also large AOD values are observed over the north Adriatic Sea, the Sea of Azov and the North Aegean Sea. The MODIS C006 AOD distribution is in agreement with that reported by previous studies using older collections (e.g., Terra C005; Papadimas et al., 2009) especially in the northern areas. The agreement is poorer for the southern areas, namely the large AODs over the Gulf of Sidra, where the 6-year (2000–2006) C005 values reported by Papadimas et al. (2009) did not exceed 0.25 as compared to values ≥0.30 observed here. However, the different temporal ranges between Papadimas et al. (2009) and our study may partly account for such AOD differences, due to the large interannual variability of Mediterranean dust transport (Antoine and Nobileau, 2006). The large AOD values in the northern part of our study region are mainly associated with areas having intense anthropogenic activity (e.g., the Po Valley in North Italy) that stand out even more prominently when considering the AOD at 470 nm. The spatial long-term average value of AOD at 550 nm is 0.20 ± 0.05 for the whole area of interest, 0.20 ± 0.07 for the Mediterranean Sea, and 0.20 ± 0.05 for the Black Sea; the indicated errors correspond to the temporal heterogeneity. Papadimas et al. (2009) reported a value of AOD at 550 nm of 0.22 ± 0.07 for the region of the Mediterranean Sea, based on a shorter (2000–2006) period of Terra C005 data from Dark Target, but also including land apart from ocean areas. In an inter-comparison study for multiple satellite and model AOD climatologies (covering different periods) available for the wider region of the Mediterranean Sea, Nabat et al. (2013) reported long-term average AOD values of 0.197 (MODIS-Aqua, 2003–2010), 0.206 (MODIS-Terra, 2001–2010), 0.211 (MISR-Terra, 2001–2010). The range of their Mediterranean values for all satellite platforms examined is 0.111–0.244, with more recent datasets indicating values closer to the upper end of this range, i.e. close to 0.2. The FF values at 550 nm shown in Fig. 2b range between 0.55 and 0.80, exhibiting a significant south-to-north gradient. The north area of our study region (i.e. the Black Sea) exhibits high FF values (greater than 0.70) while moderate to low FF values (less than 0.62) are observed in the south area of the Mediterranean basin (e.g., offshore North Africa). In general, high FF values are associated with continental/urban fine


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Fig. 2. Long-term (2002–2014) annual average spatial distribution of (a) AOD at 550 nm, (b) Fine Fraction FF at 550 nm, (c) Ångström exponent a550–860, over the broader Mediterranean basin based on daily MODIS-Aqua measurements.

aerosol particles originating from anthropogenic activity or biomass burning, while low FF values are indicative of the presence of coarse particles such as desert dust or sea salt. Hence, the observed gradient of FF over the study region can be explained by the proximity of fine and coarse mode aerosol particle sources in the northern and southern parts of the Mediterranean basin, respectively. It should be noted, however, that small FF values (0.50–0.65) in the southern part of Mediterranean Sea do not necessarily imply low fAOD values, since these also depend on total AOD. Hence the south-to-north gradient of FF is mitigated when it comes to fAOD. This highlights the presence of anthropogenic aerosols not only close to their main sources (Europe) but also far from there. The Ångström exponent a550–860 in Fig. 2c correlates very well with FF values in Fig. 2b, as shown in a correlation map of their daily values (Fig. S2). The resulting correlation coefficients are higher than 0.9 over most of the study area (values up to 0.96), whereas the lower values (0.75–0.85) are observed in areas characterized by strong presence of fine anthropogenic aerosols (e.g., north Adriatic and Black Seas). The large a550–860 values (1.4–1.8) over the regions of Black Sea and Po Valley, and the quite large values along the European shores and Nile delta (1.0–1.3), verify the anthropogenic nature of the aerosols. The dominance of anthropogenic aerosols is expected around the Nile delta, since the area is characterized by a large population density (N1000 people/ km2), along with the city of Alexandria (population N 4 million people) and the nearby megacity of Cairo (largest city in Eastern Mediterranean, with a population N 16 million people). The maximum values of a550–860 (1.8) are observed over the Azov Sea, while the minimum (0.6) are found offshore Libya. The Azov Sea is an almost closed maritime area surrounded by densely populated and industrialized areas, while coastal Libya is one of the most sparsely populated areas of the Mediterranean, and in the route of frequent Saharan dust transports (Gkikas et al., 2013, 2014). It should be noted that the difference between the Azov and the rest of the Black Sea, and especially the northern Adriatic Sea, is more pronounced in a550–860 than FF (Fig. 2c and b). This highlights the slightly different nature of these two parameters, according to their definition, with a550–860 being basically a function of aerosol size while FF being an indicator of the relative magnitude of the fine and coarse mode contributions to the total extinction AOD. Such differences between these two parameters have been reported in the literature (e.g. Eck et al., 2010). It should also be pointed out that notably higher a550–860 (and FF) values are observed over

the Black Sea as compared to the rest of the region. This is attributed to the fact that the Mediterranean is an open region, having smaller influences from polluted areas to the north and stronger from deserts to the south. A clear seasonal variation of the aerosol load over the Mediterranean is depicted in Fig. 3. The regional AOD mean is highest in April and July. Moderate AOD values are observed in October, while the lowest of the year appear in January (largest precipitation amounts in winter). More specifically, at 550 nm the AOD values range between 0.08 and 0.31 in January, 0.15 and 0.59 in April, 0.17 and 0.42 in July, 0.15 and 0.27 in October. Fig. 4 shows the spatial distribution of FF at 550 nm. Higher FF values are observed in the northern Mediterranean basin during all seasons due to the presence of anthropogenic particles (urban and industrial). In general, FF values range between 0.49 and 0.81 in January, 0.53 and 0.85 in April, 0.51 and 0.84 in July, and 0.53 and 0.82 in October. The highest FF values are observed over the Black Sea (Azov Sea, Crimea) and North Greece. All these areas are characterized by intense anthropogenic activity and biomass burning processes, and therefore the aerosol load consists mainly of fine particles. During April (spring period), the greatest AOD values are observed in the South and Southeastern Mediterranean and particularly offshore Libya (Gulf of Sidra). At the same time, low FF values are observed in most of the Eastern Mediterranean basin indicating the presence of coarse aerosol particles, mainly in the southern parts. The high aerosol load in the Eastern Mediterranean, consisting mainly of coarse mode particles, can be attributed to the transfer of desert dust by the prevailing synoptic patterns from Sahara (Fotiadi et al., 2006; Kalivitis et al., 2007; Papadimas et al., 2008; Gkikas et al., 2009; Hatzianastassiou et al., 2009; Gkikas et al., 2013, 2014). The Saharan mineral dust particles are transported towards the Mediterranean and Europe with south-southwesterly winds induced by thermal Saharan lows developed south of the Atlas Mountains and moving eastwards across the North African coast (Moulin et al., 1997, 1998; Gkikas et al., 2014). The central and the eastern parts of the Mediterranean Sea are commonly affected by dust transports under such synoptic conditions (Barnaba and Gobbi, 2004; Papadimas et al., 2008; Hatzianastassiou et al., 2009; Gkikas et al., 2013). The highest AOD values over the region of Black Sea are observed during spring, which has been attributed to solitary Saharan dust transport and infrequent agricultural fires,

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Fig. 3. Long-term (2002–2014) average spatial distribution of AOD at 550 nm for the four mid-seasonal months: (a) January, (b) April, (c) July and (d) October.

based on other satellite products (POLDER/PARASOL, POLDER-2/ADEOS; Bovchaliuk et al., 2013). During July (summer period), high AOD and low FF values are observed above the westernmost part of the Mediterranean Sea and more specifically along the coasts of Algeria and Morocco. The aerosol load is generally greater in the Western than in the Eastern Mediterranean in July, but the opposite is observed during the other midseasonal months and especially in April (spring). This can be explained by the fact that dust transport begins over the Eastern Mediterranean in spring and that over the summer it spreads over the central and western basins (Moulin et al., 1998) at altitudes between 1.8 and 9 km (Mona et al., 2006). The Saharan dust transport takes place in the Western

Mediterranean basin, as also reported in previous studies (e.g., Barnaba and Gobbi, 2004; Toledano et al., 2007; Papadimas et al., 2008; Querol et al., 2009; Gkikas et al., 2013; Salvador et al., 2014) because of thermal lows located either over northwestern Africa or over the Iberian Peninsula, and an upper level high located in northern Africa. Salvador et al. (2014) report that during summer the intense surface heating results in convective injection of dust particles from source areas, the transport of which towards the Iberian Peninsula and the Balearic Islands, is driven by the North African high, alone or in combination with a relative low pressure system placed west of the Iberian Peninsula coast. During summer the Azores anticyclone is strengthened and extended eastwards, contributing to dust transport over the Western

Fig. 4. Long-term (2002–2014) average spatial distribution of FF at 550 nm for the four mid-seasonal months: (a) January, (b) April, (c) July and (d) October.


Mediterranean (Gkikas et al., 2012, 2013). More specifically, a synoptic situation that consists of stable anticyclonic conditions near the surface and a southwestern airflow at 700 hPa over the Western Mediterranean (created by a trough over the eastern Atlantic Ocean and a ridge over the Western Mediterranean, see Fig. 5 in Gkikas et al., 2014) prevail, leading to strong dust transport into the Western Mediterranean Sea. During summer, the aerosol load over the Eastern Mediterranean (especially over the Aegean and Libyan Seas) and the Black Sea is much lower compared to that in the Western part of the region, while FF values are higher. This can also be justified by the prevailing synoptic conditions. At this time of the year, the Eastern Mediterranean region is under the influence of the thermal low of southwest Asia (Pakistan's thermal low) from the East, and of the Azores high to the West. This results in strong dry North winds (known as the Etesian winds; Tombrou et al., 2015) over the Aegean Sea and the surrounding area. The Etesians transfer the fine aerosol particles from urban and industrialized regions of Europe, all over southern Balkans and as far as Crete. The Etesians also carry biomass-burning aerosols from regions such as the northern Balkan Peninsula, the western shores of the Black Sea and Ukraine (Metaxas and Bartzokas, 1994; van der Werf et al., 2006; Sciare et al., 2003, 2008; Gkikas et al., 2012; Tombrou et al., 2015). Slightly lower AOD values are observed over the Black Sea in July than in April. This AOD seasonality, exhibiting higher values in spring and summer, is in agreement with that reported by Bovchaliuk et al. (2013), who attributed summer aerosols to forest and peat wildfires, as well as harvesting activities. The high FF values over Black Sea in summer corroborate the strong presence of fine aerosols that originate either from massive fires in the western coasts of Black Sea (van der Werf et al., 2006) or from intense photochemical reactions under large amounts of solar radiation (Gkikas et al., 2012). During January (winter season) the aerosol load is much lower compared to that in spring and summer, especially over the Western Mediterranean basin, due to the efficient removal by wet deposition. Larger AOD values are also observed over the Southeastern Mediterranean and especially over the Gulf of Sidra and offshore Egypt during winter. In October (autumn) the aerosol load is generally low, although it is larger than that in winter. The higher AOD values during this season are observed again over the southern parts of central Mediterranean basin, over the northern Adriatic Sea, the northern Aegean Sea and the Sea of Marmara in the eastern Black Sea, as well as the eastern Levantine Sea. In those regions (with the exception of the Middle-eastern shores) the aerosol load consists mainly of fine particles, most likely produced by anthropogenic activities. The relatively large AOD and small FF values (b0.55) over the Gulf of Sidra for both winter and autumn, indicate northward Saharan dust transport (caused by cold winter cyclones; cf. Nastos, 2012). Over northern Egypt, and especially over the Nile delta, the aerosol load consists mainly of fine particles as indicated by relatively elevated FF values (up to about 0.70) originating from industrial activity and combustion processes (e.g., domestic heating and traffic; Favez et al., 2008; El-Metwally et al., 2008). In general, the lowest FF values in winter and autumn are observed North of Libya and in some remote regions in the Western Mediterranean. The highest FF values are observed over the European and Anatolian shore of the Mediterranean Sea (maximum values over the northern Adriatic), as well as over the Black and Azov Seas. The seasonality of AOD at 470, 650 and 860 nm, as well as of a550– 860 , is shown in Figs. S3–S6 in the supplementary material. AOD at these wavelengths (Figs. S3–S5) exhibit a similar seasonal behavior with that of the AOD at 550 nm (Fig. 3) especially if one takes into account the dependence of AOD on wavelength for the various a550–860. Figs. 4 and S6 show the aforementioned high correlation of a550–860 with FF to hold for all mid-seasonal months and all regions, except for Azov and Black Seas where the contrast between the two regions is larger for a550–860 than for FF. The results shown in Figs. 3 and 4

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confirm the main findings of previous MODIS-based AOD climatologies (e.g., Papadimas et al., 2008) but also underline new features, e.g., the higher values over the Black Sea and the spatial features of FF over a more extended time period. Our data also provide additional details (seasonal) to those from similar MODIS-Aqua based climatology (Nabat et al., 2013), while they highlight differences with other contemporary (e.g., Terra/MISR, NOAA/AVHRR, ENVISAT/ MERIS in Nabat et al., 2013) or older (e.g., NIMBUS7/TOMS, METEOSAT/MVIRI in Nabat et al., 2013) climatologies. 3.2. Regional analysis In order to provide an easier assessment of the seasonal cycle of the AOD and its changes from one region to another, but also among the different wavelengths (470, 550, 650 and 860 nm), the study region was divided in 3 smaller sub-regions (Western Mediterranean, Eastern Mediterranean and Black Sea; cf. Fig. 1). Fig. 5 provides the average values of monthly mean AOD (for every wavelength) together with estimations of the FF for each sub-region. It appears that the seasonal cycle differs between the sub-regions, as has already been shown (with coarser temporal resolution) by the geographical map distributions discussed in the previous section. For the Black Sea region (Fig. 5a), the intra-annual variation of AOD is characterized by a double maximum: one observed in spring (April) and the other at the end of summer (August), for all examined wavelengths. The intra-annual variation of the AOD over the Black Sea region can be explained by the transport of biomass burning and desert dust aerosol particles. Sciare et al. (2008) report that long-range transport of agricultural waste burning from European countries surrounding the Black Sea occurs during two periods (March–April and July–September). During spring there is also a Saharan dust transport that contributes to the high aerosol load (Gkikas et al., 2012; Bovchaliuk et al., 2013). The primary AOD minimum is observed in winter (December) whereas a secondary minimum is also observed in June. The FF values are large, ranging from 0.71 to 0.80 year-round, being relatively larger during the warm compared to the cold period, due to biomass burning fine aerosols as explained in Section 3.1. Over the Eastern Mediterranean Sea (Fig. 5b), minimum AOD values are observed in December and maximum in April, while AOD remains constantly at higher values (N0.20 for λ = 550 nm) from July to September. The AOD for this sub-region ranges between 0.14 and 0.32 at 550 nm, while the range between the maximum and minimum AODs is decreasing as the wavelength increases. This decrease, however, is not as big as that observed over the Black Sea (compare Fig. 5a and b), which is indicative of the presence of coarser aerosols over the Eastern Mediterranean than over Black Sea. The FF values for this sub-region range between 0.57 in December and 0.73 in August, while the average is quite smaller than that observed over the Black Sea, highlighting the smaller anthropogenic contribution in the Eastern Mediterranean. The distinct FF maximum in August (0.72) is associated with fine aerosol particles originating from anthropogenic or biomass-burning sources (both local and remote), as opposed to the significantly lower FF values observed in spring caused by the transported dust aerosols from northern Africa. Over the Western Mediterranean (cf. Fig. 5c), the aerosol load exhibits a plateau of high AOD values in spring and summer (AOD at λ = 550 nm is higher than 0.25 from April to August). The FF is characterized by an intra-annual variation with two maxima, one in April and one during August and September, and in both cases is equal to 0.68. The primary minimum FF values are observed in December while the secondary minimum is in May. Although the timings of the AOD maxima and minima observed for the Black Sea region and the Eastern Mediterranean basin are similar (with the spring maximum being more prominent for the Eastern Mediterranean), the seasonal variation in the Western Mediterranean

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Fig. 5. Mean intra-annual (2002–2014) variation of the AOD at 470 nm (blue line), 550 nm (green line), 650 nm (orange line), 860 nm (red line) and of the FF at 550 nm (gray line) for (a) Black Sea, (b) Eastern Mediterranean and (c) Western Mediterranean. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

exhibits a different pattern. The drivers of these observations are to a large extent the behavior of dust transport from North Africa and fine particles from northern Balkans, as well as the stronger contribution of desert dust in the Eastern Mediterranean and of anthropogenic particles in the Black Sea. More specifically, the different behavior of synoptic systems between the Eastern and Western parts of the Mediterranean, differentiate the timing of the dust AOD maximum from April to July, respectively. A secondary explanation for the smaller AOD values observed in spring and autumn (October–November) for the Western compared to the Eastern Mediterranean may be the larger spring and autumn precipitation over the west.

3.3. Inter-annual variability To generate the time series of deseasonalized anomalies for each 1° × 1° cell, we take its monthly AOD and FF values and from these we subtract the corresponding long-term (2002–2014) average for the specific month. We then apply linear regression for both variables with time and compute the values of the slope (cf. Fig. 6), which is a measure of the sign and magnitude of AOD trend. The greater part of our study region clearly experiences a decrease in the AOD (greenish and blueish colors) with only a few spots exhibiting increasing slopes (reddish colors). The decreasing tendencies at most cells were found

Fig. 6. Slopes for deseasonalized a) AOD and b) FF at 550 nm (units are decade−1), over the broader Mediterranean basin for the period 2002–2014. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)


to be statistically significant at a 99% confidence level (cf. Fig. S7a for 95% confidence level), as determined by the student t-test. The absolute change of AOD during these 12 years can be derived by multiplying the values of Fig. 6a by the temporal extent of our study (1.2 decade). On average, the AOD decreased by 0.0030 per year over the entire region. Similar spatially extended decreasing AOD trends have been reported in the past either based on MODIS-Terra data (on average 0.0067 per year over 2000–2006 as reported by Papadimas et al., 2008) or from other databases (cf. Nabat et al., 2013, 2014; AOD decrease equal to 0.0023 per year over the Mediterranean Sea, but not statistically significant at 95% level). In fact, it has been documented that this decreasing trend has started even before, since the early 1980s (cf. Mishchenko et al., 2007) and could be associated with decreasing emissions of anthropogenic sulfate aerosols resulting from actions aiming to reduce aerosol emissions, such as the Air Framework Directive of the Clean Air for Europe. Nabat et al. (2013) also reported that dust over North Africa has also decreased by 0.0045 per year (statistically significant at 95% level, student's test), which may also account for the overall AOD decrease. The largest decreasing tendencies are observed over the Libyan Sea (slope down to −0.10 decade−1) with large slope values also over the Algerian shores and the Gulf of Sidra. There are also a few regions with slightly increasing tendencies in the aerosol load (Cyprus and Eastern Black Sea), although the temporal AOD differences are not statistically significant. The FF decreases (trend down to −0.06 decade−1; Fig. 6b) over the greater part of the Mediterranean basin, indicating that not only the overall AOD declines but also the part attributed to fine aerosols does so even more, on average, as explained in the following paragraphs. Nonetheless, in the Western part of the Levantine Sea and the Alboran Sea, the FF values have a minor increasing trend (up to 0.02 decade−1). Areas exhibiting increasing FF and decreasing AOD indicate that simultaneous decrease of fine and coarse (dust) aerosols takes place, but to quantify these changes requires further analysis (provided below). With the exception of only a few pixels that exhibit negative slope (cf. Fig. S7b), FF trends over the entire region are not statistically significant (at 95% confidence level). The Ångström coefficient a550–860, however, shows a more robust decrease with practically all cells in the region exhibiting a negative slope (Fig. S8), which is statistically significant at a 95% confidence level for 41% of the cases. The a550–860 decreasing trend provides evidence for the size increase of particulate matter. Fig. 7 shows the inter-annual AOD mean over the whole area of interest between July 2002 and December 2014, where double maxima in spring and autumn can be seen in some years. In others the second

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maximum is in summer instead of autumn. Overall, the annual AOD has decreased with a slope of 0.030 per decade during the last 12 years, resulting in an absolute AOD decrease of 0.04, or 19%. The decrease would be more easily discernible had we plotted the deseasonalized values. In order to understand the changes of the aerosol load over the Mediterranean basin, we also investigated the trends of precipitation since wet deposition is the major aerosol removal process. Fig. 7 also shows the monthly precipitation data between July 2002 and December 2014 from the GPCP database. The negative correlation of AOD and precipitation is clear in the figure (R = −0.58). Interestingly, no significant trend was identified during the aforementioned period. Precipitation changed (statistically insignificantly) by −0.56 mm/month per decade, or −1.3% during these 12 years. Therefore, wet deposition can be excluded as the main process affecting the observed decreasing trends on aerosol load, suggesting that they are mostly affected by changes in the aerosol emissions (Yoon et al., 2014). The aerosol load over the broader Mediterranean basin originates from (a) urban/industrial activity and biomass burning (mainly in the European continent), (b) desert dust transport (mostly from the Saharan desert) and (c) the marine environment. Because anthropogenic particles (category a) are mostly fine while the natural aerosols (desert dust and sea salt particles) are coarse, we investigated separately their inter-annual trends in order to identify possible sources of detected/computed AOD trends. As shown in Fig 8a, the fine mode AOD in most parts of the Mediterranean decreased during the period from 2002 to 2014. These trends were found to be statistically significant at a 95% confidence level over 77% of the cells (Fig. S9a). In a few cells in the Levantine Sea and the Eastern Black Sea the fine-mode AOD exhibits a small increase, albeit not statistically significant. The coarse mode AOD trends shown in Fig. 8b also indicate a decrease, although to a smaller extent than fine AOD, with the largest (negative) slope observed over the Algerian coast and south of Crete (Libyan Sea). The calculated trends over these areas are statistically significant at a 95% confidence level for 28% of the cells, and rather clustered over the Western Mediterranean basin (Fig. S9b). A few regions mainly in the Eastern parts of the study region exhibit positive trends, but those are not statistically significant. Overall, the aforementioned trends indicate that during our study period there is a nearly general decrease of the transported anthropogenic particles and a decrease of the Saharan desert dust particles, mainly in the Western Mediterranean. The desert dust transport over the latter sub-region occurs mainly during summer and therefore a decrease of the summer AOD values is expected. The seasonal analysis of the changes in the AOD (results not shown here) confirms that

Fig. 7. Time series of monthly AOD values averaged over Mediterranean at 550 nm (black line) and precipitation (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. Deseasonalized AOD slope (units are decade−1) of (a) fine mode particles, (b) coarse mode particles at 550 nm, over the broader Mediterranean basin for the period 2002–2014.

expectation. The decrease of the desert dust particles over the Mediterranean, results in an increase of the FF values, while the decrease of the anthropogenic pollution induces a decrease in the FF. It appears that both fine and coarse particles are being removed from the atmosphere between 2002 and 2014, with the removal rate of the fine particles being slightly faster. This could explain the small but not statistical significant decrease of the FF and the larger, statistically significant decrease of a550–860 during our study period. Previous studies also reported that the anthropogenic emissions in Europe and more specifically in the industrialized world and over the oceans have decreased (Hsu et al., 2012, Streets et al., 2006; Mishchenko and Geogdzhayev, 2007; Wild et al., 2007; Nabat et al., 2013; Yoon et al., 2012, 2014) possibly after the introduction of environmental regulations. The weakening of desert dust transport reported here is also consistent with recent studies showing a decrease on dust activity and dust export from the Saharan desert. More specifically, Hsu et al. (2012), based on SeaWiFS data, report a negative tendency in the emission and export of Saharan dust over the western North Africa and the North Atlantic between 1998 and 2010. Moreover, Pey et al. (2013) found decreasing trends of African dust episodes between 2006 and 2011 in the NW Mediterranean because of alterations in the atmospheric circulation. Opposite trends were observed in the dust activity of the Arabian Peninsula, in agreement with Notaro et al. (2015) who report increased dust activity after 2006 and attribute it to synergy between persistent La Niña and negative phase of the Pacific Decadal Oscillation. This increase might justify our observations of increasing coarse mode AOD over the nearby Levantine Sea, although the predominant winds over the aforementioned middle eastern areas carry the dust mainly towards Arabia (Notaro et al., 2015). The generally decreasing coarse mode AOD observed in the present study is also in agreement with the findings of Gkikas et al. (2013) who reported that desert dust episodes over the Mediterranean decreased between 2000 and 2007. A general decreasing trend in the aerosol load over the region was also reported by Papadimas et al. (2008), associated with increasing trends for the precipitation. This is contradictory to our findings concerning the role of precipitation, but it should be noted that Papadimas et al. (2008) results refer to a different and shorter period (2000–2006) than ours, highlighting the varying role of different physical processes affecting aerosol trends over time. Our observations suggest that the previously reported decreases apparently continue into 2014.

3.4. Validation with ground-based aerosol data In order to validate the MODIS observations, we compare them with optical measurements from the global network of AERONET surface stations. The locations of all coastal AERONET stations used in this study are shown in Fig. 1. The stations are quite uniformly distributed all over the study regions, with slightly better coverage in the Western Mediterranean than in the Eastern Mediterranean and Black Sea. Table 1 provides statistical metrics for all wavelengths (Pearson correlation coefficient, bias and root mean square error) of the comparison between daily surface AOD and FF derived from AERONET and from MODIS-Aqua, which corresponds to the 1° × 1° cell within which each station is located. The bias is calculated as the difference between the overall average of both datasets, i.e. data(MODIS) − data(AERONET). On an annual level, the MODIS-Aqua daily AOD values are in good agreement with the respective data from AERONET (Figs. 9a–d and S10). The calculated correlation coefficients range between 0.77 (at 550/500 nm, where the first number indicates the λMODIS and the second one the λAERONET) and 0.80 (at 650/675 nm and 860/870 nm). However, the satellite data are slightly overestimated against the ground-based measurements. Due to the fact that the wavelengths of MODIS and AERONET are not identical and that AOD decreases with wavelength, MODIS biases may be underestimated for the 470/440 and 550/500 nm comparisons. On the other hand, for pairs 650/675 and 860/875 nm the biases may be slightly overestimated. Papadimas et al. (2009) showed that the MODIS C005 AOD comparison with AERONET improved when compared with the C004 at the 550 nm. We also observe improvement going from C005 to C006. For the 550/500 nm comparison (cf. Fig. 9b), we have a slope that is closer to unity, a smaller RMSE and a larger R compared to the ones presented in Papadimas et al. (2009) for C005. The bias has not changed significantly between C005 and C006. Papadimas et al. (2009) however used also a few non-coastal stations with data from the Dark Target over land algorithm. Better agreement between satellite and ground-based data is observed in autumn for all wavelengths (R increase up to 0.84 at 860/ 870 nm and bias decreases down to −0.002 and 0.004 at 550/500 nm and 470/440 nm, respectively). During spring, the agreement between MODIS and AERONET AOD data was slightly worse than in the other seasons, but the comparison statistical metrics remain satisfactory.

Table 1 Statistical metrics for the comparison of 2002–2014 daily AOD between MODIS-Aqua and AERONET for the Mediterranean basin for three wavelength pairs. The comparison is performed through the calculation of bias (MODIS minus AERONET), correlation coefficient R, and root mean square error (RMSE). Both annual and seasonal results are shown. λ (nm)

AOD

FF

470–440 550–500 650–675 860–870 550–500

Total

Winter

Spring

Summer

Autumn

R

Bias

RMSE

R

Bias

RMSE

R

Bias

RMSE

R

Bias

RMSE

R

Bias

RMSE

0.782 0.766 0.798 0.795 0.658

0.012 0.004 0.033 0.021 0.043

0.102 0.105 0.083 0.074 0.127

0.789 0.793 0.801 0.804 0.631

0.004 0.002 0.018 0.011 0.024

0.067 0.068 0.052 0.044 0.136

0.730 0.732 0.752 0.748 0.656

0.031 0.025 0.043 0.027 0.087

0.1149 0.172 0.132 0.123 0.129

0.773 0.781 0.815 0.829 0.692

0.013 -0.001 0.041 0.027 0.033

0.099 0.086 0.075 0.064 0.115

0.823 0.812 0.833 0.835 0.658

0.004 −0.002 0.027 0.018 0.038

0.074 0.068 0.057 0.048 0.123


The MODIS-Aqua Fine Fraction values at 550 nm (Fig. 9e) are also in satisfactory agreement with the respective data from AERONET at 500 nm (R = 0.66, bias = 0.043 at annual level). The calculated statistical metrics are slightly better during summer and autumn compared to spring and winter. 4. Discussion and conclusions Using daily satellite data from the latest available collection (i.e., C006) of MODIS-Aqua dataset, we examined the spatiotemporal variations of

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the aerosol optical depth, Fine Fraction and Ångström exponent over the Mediterranean basin during the period 2002–2014. To our knowledge, this is the first time the latest available release of MODIS data is used in a study focusing on the Mediterranean. Generally, higher AOD values were observed offshore North Africa, with the maximum values being over the Gulf of Sidra. Relatively large AOD values were also observed over the north Adriatic Sea, the Sea of Azov, the North Aegean Sea and the Sea of Marmara. Our findings generally confirm previous MODIS AOD distributions of shorter temporal coverage, but they also exhibit differences.

Fig. 9. Scatterplot comparison between daily MODIS and AERONET AOD values at (a) 470/440 nm, (b) 550/500 nm, (c) 650/675 nm, (d) 860/870 nm and (e) for FF values at 550/500 nm. The correlation coefficients (R), mean bias and root mean squared error between MODIS and AERONET data are also provided.

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The FF values were found to be greater over the northern than the southern part of the region, exhibiting a north-to-south gradient. The largest FF values (indicating strong presence of fine aerosol particles) appeared over the European shores of the Mediterranean Sea, and mainly over the Adriatic and Aegean Seas, as well as above the broader Black Sea region. The smallest FF (indication of small presence of the fine particles) was observed in the southernmost part of our study region. The combination of high aerosol load in the latter case with the predominance of coarse particles indicates the enhanced presence of transported Saharan dust. Over the northern Mediterranean basin, on the other hand, the large amounts of fine-mode particles indicate the dominance of anthropogenic and biomass burning particles from both local and remote sources (e.g., industrialized regions in central Europe). Analysis of the AOD shows a decreasing trend with wavelength, especially when the aerosol load consists of fine particles. The spectral decrease of AOD is weaker when the aerosol particles are coarser, which is consistent with Mie theory for scattering. The aerosol optical properties over our study region exhibited a clear seasonal variation. In general, greater aerosol load was observed during spring and summer. Desert dust transport was observed from the Sahara desert towards the Eastern Mediterranean during spring and towards the Western Mediterranean during summer. In the same period, a predominance of fine anthropogenic particles transported from the European continent, was identified over the Eastern parts of the study region. During winter the aerosol load over the Mediterranean basin was low due to the efficient removal process of wet deposition. The inter-annual trends of the AOD and FF were also investigated during the study period. The AOD and Ångström exponent exhibited a statistically significant decrease, while the trends of the FF were also negative, although not statistically significant at the 95% confidence level. Further analyzing the fine and coarse AOD inter-annual trends separately revealed that aerosol particles in both modes decreased during the study period. We can therefore assume that there is a decrease of transported anthropogenic pollution over the Mediterranean basin and a decrease also of the transported desert dust particles mainly in the western sub-basin. Such a decrease has been noted in other studies (Mishchenko and Geogdzhayev, 2007; Papadimas et al., 2008; Gkikas et al., 2009, 2013; Yoon et al., 2012, 2014; Nabat et al., 2013), and here we show that the trends persist until 2014 using the recent MODIS collection (C006), taking into account the contribution of both fine and coarse aerosols (previous studies referred to total aerosol only). Finally, by comparing daily satellite data with corresponding collocated surface data from AERONET, we validate the reliability of the former. Although both AOD and FF data derived from MODIS-Aqua (C006) are slightly overestimated compared to the ground-based measurements, the correlation was good with coefficients being up to 0.80 for AOD and 0.66 for FF. The good performance of MODIS retrieval strengthens the conclusions of this study. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2016.01.192. References Adler, R.F., Huffman, G.J., Chang, A., Ferraro, R., Xie, P.P., Janowiak, J., Nelkin, E., 2003. The version-2 global precipitation climatology project (GPCP) monthly precipitation analysis (1979–present). J. Hydrometeorol. 4, 1147–1167. Antoine, D., Nobileau, D., 2006. Recent increase of Saharan dust transport over the Mediterranean Sea, as revealed from ocean color satellite (SeaWiFS) observations. J. Geophys. Res. 111, D12214. Barnaba, F., Gobbi, G.P., 2004. Aerosol seasonal variability over the Mediterranean region and relative impact of maritime, continental and Saharan dust particles over the basin from MODIS data in the year 2001. Atmos. Chem. Phys. 4, 2367–2391. Boucher, O., Randall, D., Artaxo, P., Bretherton, C., Feingold, G., Forster, P., Kerminen, V.-M., Kondo, Y., Liao, H., Lohmann, U., Rasch, P., Satheesh, S.K., Sherwood, S.,

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