Signs of a negative trend in the MODIS aerosol optical depth over the Southern Balkans

Atmospheric Environment 44 (2010) 1219e1228

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Signs of a negative trend in the MODIS aerosol optical depth over the Southern Balkans M.E. Koukouli a, *, S. Kazadzis b, V. Amiridis c, C. Ichoku d, D.S. Balis a, A.F. Bais a a

Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Institute for Environmental Research & Sustainable Development, National observatory of Athens, Greece c Institute for Space Applications and Remote Sensing, National Observatory of Athens, Athens, Greece d Climate & Radiation Branch, NASA Goddard Space Flight Centre, Greenbelt, MD, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 February 2009 Received in revised form 5 November 2009 Accepted 11 November 2009

A negative trend is being revealed in the MODIS aerosol optical depth [AOD] observed over the Southern Balkan/Eastern Mediterranean region. Collection 005 MODIS/Terra and MODIS/Aqua AOD at 470 nm measurements were evaluated against Brewer ground-based measurements over Thessaloniki, Greece and CIMEL ground-based measurements of AOD over Heraklion, Crete. A detailed study of the monthly, seasonal and inter-annual variability of the MODIS/Terra and MODIS/Aqua AOD values over selected locations around the Balkan Peninsula showed that the higher mean AOD values occurred in the spring and summer months, whereas the lowest were found in the winter-time. For all seasons, the highest AODs were observed for the northern-most latitudes with a marked decrease towards the southern-most sites. A statistically significant decreasing trend in aerosol load in the region over all sites as derived from the MODIS/Terra measurements gave the highest per annum change seen for the summer months to be
4.09  2.34%, and the lowest for the winter months as
2.55  4.36%, which also shows the higher variability. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Urban aerosol Aerosol optical depth MODIS Brewer spectrophotometer CIMEL

1. Introduction The main aim of this work is to assess the temporal and spatial variability of the atmospheric aerosol load in the Southern Balkan region using the new collection 005 (i.e. C005) Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth together with coincident ground-based measurements. The two MODIS instruments, flying on board NASA's Terra and Aqua satellites, have been providing a steady long term monitoring of the atmospheric conditions since December 1999 and May 2002, respectively, and therefore enable the in-depth analysis and statistical study of the variability and seasonality of the aerosol optical depth in the region. The importance of aerosols in the Earth's climate has long been discussed in the scientific literature (Satheesh and Krishna, 2005; IPCC, 2007, and references within) which highlight the part they play in the radiation budget, air quality issues, and cloud microphysics. Any changes in the atmospheric aerosol load result in associated altering of the global climate forcing by aerosols, the dominant uncertainty in the climate radiative forcing (IPCC, 2007). The driving force behind this study is the fact that the Eastern Mediterranean region, which encompasses the Southern Balkans and is often * Corresponding author. Tel.: þ30 2310998049; fax: þ30 2310998090. E-mail address: [email protected] (M.E. Koukouli). 1352-2310/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.11.024

affected by continental aerosols of surrounding regions, is also known for its high atmospheric aerosol content (for e.g. Mihalopoulos et al., 1997; Papayannis et al., 2005; Ichoku et al., 1999; Formenti et al., 2001; Lelieveld et al., 2002; Zerefos et al., 2002; Gerasopoulos et al., 2003; Balis et al., 2004). Recent satellite measurement studies have shown a likely decrease in the global optical thickness of tropospheric aerosols. In the local scale, Papadimas et al. (2008), revealed a statistically significant decreasing tendency from 2000 to 2006 equal to
0.04 over the Mediterranean basin whereas Mishchenko et al. (2007) showed a decrease of 0.03 during the period from 1991 to 2005 as observed by Advanced Very High Resolution Radiometer, AVHRR, radiances. Barnaba and Gobbi (2004), who analyzed one year of AOD measurements from MODIS/Terra over ocean and highlighted an evident AOD seasonal cycle over the whole Mediterranean region, with AOD values mainly below 0.15 in winter and above 0.2 in summer. They also postulate a North-to-South gradient of continental aerosol optical thickness and an opposite South-to-North AOT gradient associated with Saharan dust export, the latter showing a marked seasonal cycle. Kishcha et al. (2007), by examining six years of level 3, Collection 004, MODIS/Terra data have also found a highly pronounced AOD decrease. In the present study, in Section 3, we first validate the new collection 005 level 2 MODIS/Terra and MODIS/Aqua aerosol optical

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depth measurements at 470 nm with coincident ground-based Brewer spectrophotometer aerosol optical depth measurements at 355 nm for Thessaloniki and CIMEL aerosol optical depth at 470 nm for Heraklion, Crete. The temporal and spatial variability of the satellite AOD over a wide range of locations around the Balkan Peninsula were then examined in Section 4. The trend analysis discussed afterwards in Section 5 reveals a widespread decrease in the aerosol load in the region where the aid of air-mass provenance back-trajectory calculations discuss the possibility of discerning the aerosol loading between two nearby sites. Finally, in Section 6, the ability of satellite AOD measurements to provide insightful and usable air quality assessments is expanded. 2. MODIS/Aqua and MODIS/Terra measurements in the Southern Balkans MODIS/Terra and MODIS/Aqua aerosol products were used in order to analyze the temporal and spatial variability of aerosols over a wide area of interest. In this study, we use daily level-2 collection 005 MODIS/Terra and MODIS/Aqua AOD at 470 nm, from February 2000 to December 2006 and from November 2003 to December 2006 respectively (King et al., 2006). Details of the MODIS aerosol products and their extensive validation can be found in Chu et al. (2003) and Remer et al. (2005). A complete 7-year and 3-year data AOD dataset from MODIS/Terra and MODIS/Aqua, respectively, were analyzed in order to evaluate the aerosol seasonal, intra-seasonal and geographical variability over the Southern Balkan Peninsula. Daily overpass data for specific study areas were extracted at two spatial resolutions: 10  10 km2 and 25  25 km2. In a previous study by Koukouli et al. (2007), the choice between the two spatial resolutions was investigated for the purposes of this work and it was found that the 25  25 km2 product ensures sufficient daily measurements without loosing out to the higher spatial resolution and hence a better opportunity of correctly viewing the atmospheric aerosol load. Further details and findings on the use of different spatial resolutions for comparing MODIS and ground-based aerosol data can be found in Ichoku et al. (2002). Seven locations around the Southern Balkan Peninsula, in Greece and Bulgaria, were chosen in order to study the regional and temporal variability of the MODIS/Terra and MODIS/Aqua AOD measurements. These locations are shown in Fig. 1 and their characteristics summarised in Table 1. The selection of the seven geolocations was made to cover a wide range of atmospheric conditions: a heavily polluted city: Thessaloniki, a medium-sized town: Heraklion, two power plant complexes: Ptolemaida and Maritsa, and three locally clean areas: Mount Athos, Aegean Sea and Lake Kerkini. Heraklion, the biggest town on the island of Crete, is plagued throughout the year by extreme Saharan dust events, apart from the local aerosol sources, and has also been of notable scientific focus (see for e.g. Gerasopoulos et al., 2006). Ptolemaida and Maritsa, a Greek and a Bulgarian lignite burning power plants, are located respectively to the West and to the North-North-East of Thessaloniki and are often the cause of polluted air being transported to the city (Amiridis et al., 2005; Koukouli et al., 2006). The Mount Athos site is situated at a big peninsula to the south-east of Thessaloniki which due to historical and religious reasons is sparsely inhabited. It is surrounded by the sea and has no significant local anthropogenic aerosol sources, hence the aerosols over this area are expected to originate from the free tropospheric layer or transported from neighbouring regions or counties (Gerasopoulos et al., 2003). The Aegean Sea site is in the middle of the Greek archipelagos where we expect to observe mostly sea spray and transported aerosols. Lake Kerkini is situated to the North-East of Thessaloniki, near the border with Bulgaria and has few local anthropogenic aerosol emissions.

Fig. 1. The seven geolocations chosen for this study.

3. Validation of MODIS data using ground-based measurements The first step in this study was to validate the MODIS Collection 005 AOD measurements over two sites with different aerosol load characteristics, one metropolis in the North of Greece, Thessaloniki and one medium-sized town on the island of Crete, Heraklion. Long-term daily ground-based measurements are available at those two sites that have been validated and studied extensively in the past; a Brewer spectroradiometer at Thessaloniki (Kazadzis et al., 2007) and a Cimel sunphotometer at Heraklion (Fotiadi et al., 2006). Details concerning the comparison of the MODIS and ground-based instruments are summarized in Table 2. The validation study first examines the improvement of the new MODIS Collection 005 compared to Collection 004. Levy et al. (2007) provide a detailed documentation of the new algorithm and the two major differences which are the treatment of the surface reflectance and the dominant aerosol type governing and in particular, aerosol absorption. The new algorithm is based on a true inversion that uses the apparent reflectances at 470 nm, 660 nm and 2130 nm which are in turn used to derive three products: AOD, fraction of AOD attributed to non-dust aerosol and the surface reflectance at 2130 nm. Hence the AODs at individual wavelengths are no longer derived independently since all wavelengths are linked through the Table 1 The seven geolocations chosen for this study and their characteristics. Location

Longitude

Latitude

Cite characteristics

Local aerosol characteristics

Maritsa

25.83

42.03

Local pollution

Ptolemaida

21.83

40.42

Aegean Sea Lake Kerkini Mount Athos Thessaloniki

25.00 23.2 24.25 22.956

39.25 41.1 40.25 40.634

Industrial complex Industrial complex Rural Rural Rural Urban [large]

Heraklion

25.13

35.33

Urban [small]

Local pollution Maritime No local pollution Maritime Maritime, Saharan Dust, Local pollution Maritime, Saharan Dust

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Table 2 The characteristics of the satellite and ground-based measurements used in this study. Instrument

Measured value

Measurement time

Time period studied

MODIS/Terra MODIS/Aqua Brewer Cimel

AOD AOD AOD AOD

9.35  0.50 UT 11.54  0.53 UT All day All day

02.2000e12.2006 11.2003e12.2006 02.2000e12.2006 02.2000e12.2006

at at at at

470 470 355 470

nm nm nm nm

choice of the aerosol model. Other changes to the land algorithm include updated aerosol models with new geographic distribution, the inclusion of negative AODs, more sophisticated relationships between surface reflectance in the various wavelengths and improved snow and cloud masking. The net effect of these major changes to the over land algorithm is to reduce the biases previously noted in various validation studies (Ichoku et al., 2005; Levy et al., 2005; Schaap et al., 2008), and also to produce more realistic aerosol particle size information, both in terms of fine model fraction and fine model AOD which is a new parameter introduced for Collection 005. The report on the improvements between the two collections can be found in the following official MODIS Aerosol Algorithm document-link: http://modis-atmos.gsfc.nasa.gov/C005_Changes/ C005_Aerosol_5.2.pdf. 3.1. Brewer AOD measurements over Thessaloniki Measurements of the aerosol optical depth have been conducted in Thessaloniki (40 380 N, 22 570 E), a city with a population of approximately 1.2 million. The measuring station is located at the city centre at 60 m a.s.l. The site is facing the Aegean Sea to the south and is situated along pathways through which pollution from central and Eastern Europe influences aerosol loading over the Eastern Mediterranean. For the columnar AOD retrieval direct spectral irradiance measurements from a Brewer MKIII double monochromator were used. The methodology of the AOD retrieval is described in Kazadzis et al. (2005) and references therein. The Brewer spectroradiometer has been operational since 1997 and comparisons of its long term AOD measurements with other instruments showed very good agreement (Kazadzis et al., 2007). AOD measurements of the specific instrument have been used for aerosol climatology studies over the area and also in a number of intensive aerosol related campaigns, e.g. Balis et al. (2004), Amiridis et al. (2005) and Kazadzis et al. (2007). All Brewer direct irradiance spectra that were used for the AOD retrieval in this analysis were acquired under cloud-free conditions. For the selection of the cloud free spectra we used the methodology described in Vasaras et al. (2001), which is based on the variability of the measurements of a collocated pyranometer. For the comparison of ground-based Brewer AOD measurements and MODIS AOD products, we have used Brewer AOD values averaged 30 min around the MODIS overpass time, which corresponds to between one and four measurements per day, depending on the cloud conditions. In Fig. 2 we show the comparison between the previous Collection 004 of the MODIS AOD product [open circles], downloaded from the official NASA download centre http://modis-atmos.gsfc.nasa. gov/MOD04_L2/index.html, and the Collection used throughout in this study, Collection 005 [crosses]. The aim of this first task was to examine the effect of the algorithm improvements discussed previously. The scatter plot depicts the MODIS/Aqua overpasses over Thessaloniki collocated in space and time with the Brewer spectroradiometer. For the extrapolation of MODIS AOD from 470 nm to 355 nm, the Ångstrom coefficient for each day included in our analysis was calculated from the AOD Brewer spectral retrievals (range: 330e365 nm, 0.5 nm step). Even though the Ångstrom

Fig. 2. Comparison of the MODIS/Aqua AOD (converted to 355 nm through the Ångstrom exponent) to the Brewer AOD measurements over Thessaloniki averaged over 30 min around the satellite overpass time. The circles represent the collection 004 MODIS data and the crosses the new 005 collection.

exponent used in this work was calculated from a small range of wavelengths its use has been verified by the significantly large dataset utilised and hence is applicable to this work. The linear extrapolation from 470 nm to 355 nm using an Ångstrom exponent derived in the UV range could have an effect in the comparison of absolute values between MODIS and Brewer AOD. Eck et al. (1998) reported such Ångstrom exponent wavelength dependencies especially for biomass burning aerosol cases. To quantify this effect we have used AERONET AOD measurements at Thessaloniki for the year 2006 (8140 level-2 cases). Using the CIMEL-AERONET measured wavelengths (340, 380, 500 nm) we have compared measured AOD at 340 nm, with extrapolated AOD (at 340 nm) using AOD measurements at 500 nm and an Ångstrom exponent that was derived from the UV (340e380 nm) range. Results showed mean values of 0.446 and 0.445 and correlation coefficient of 0.968 for the two AOD's. Taking into account this result, we believe that the error related with the wavelength dependence of the Ångstrom exponent is negligible compared with other sources of errors related with the different techniques of AOD retrievals from the ground based and satellite sensors. The MODIS/Aqua data were chosen for this validation exercise since, even though they cover less of a time span than MODIS/Terra, they provided more coincident data with the Brewer measurements, allowing the comparison to be performed with high statistical significance. Furthermore, we have only allowed MODIS AOD 25  25 km values that have resulted from 5 or more individual 10  10 km measurements as suggested by the MODIS team. The results, illustrated in Fig. 2, show that the new algorithm used to retrieve the MODIS AOD over land has indeed provided significant improvements to the data. The mean AOD and associated standard deviation are 0.484  0.269 [0.636  0.407] for the 005 [004] MODIS/Aqua and 0.395  0.254 [0.336  0.220] for the co-located Brewer AOD measurements. The positive bias of the satellite data is obviously reduced, with a number of large AOD values seen in the previous dataset missing from the current one, and the linear correlation between the satellite and the ground-based data is improved. We hence deduce that the known surface and cloud

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contamination effects existent in the Collection 004 measurements have been significantly decreased, although they may not be totally eliminated. The number of common points is slightly reduced from 233 in Collection 004 to 204 in Collection 005. This improvement was also seen in the work of Jethva et al. (2007), for Kanpur, India, who found that about 70% of the total retrievals at 470 nm in the new Collection fall within the pre-launch uncertainty (Ds ¼ 005  0.15s, where s is the AOD) with an R2 of 0.83. They also concluded that MODIS still tends to over-estimate AOD for a few retrievals and attributed it to days with a significant presence of dust aerosols. Remer et al. (2008) used 205 AERONET stations globally in order to validate the collection 005 of MODIS/Terra and Aqua AOD at 550 with very similar findings; in Collection 004 there was a 41% overall positive mean bias indicating that the mean MODIS AOD is 41% higher than mean AERONET AOD in the collocation dataset. In Collection 005 they found almost no mean bias for the Terra comparisons and w7% mean bias for Aqua. In Fig. 3, we compare both MODIS/Aqua and MODIS/Terra Collection 005 data against the Brewer measurements. This second task aims at assessing the compatibility between the datasets of the two satellites and the possible effect of their different crossing time. The mean crossing overpass time over Thessaloniki for MODIS/Terra is 9.35  0.50 UT and for MODIS/Aqua 11.54  0.53 UT. Allowing for a half hour time collocation to ascertain that the same atmospheric state is observed between the two instruments it was found that the mean satellite AOD ranges between 0.484  0.268 [0.471  0.281] for the Aqua [Terra] compared to the 0.375  0.262 [0.365  0.260] for the Brewer spectroradiometer. Although the new MODIS algorithm shows a vast improvement over the previous data Collection, it continues to over-estimate the Brewer measurements by approximately 0.105 for both satellite overpasses. The number of common days of data is rather low due to the fact that only totally clear sky days, as these were defined by each of the instruments, were selected. The main differences between the satellite and groundbased measurements for Collection 005 cannot be attributed to the few remaining high satellite estimates [AODs higher than 1.2] since the statistics produced when these data points are excluded improved only marginally. When investigating the possibility of

a seasonally-introduced effect in the differences and similarities seen in the estimates of the two types of measurement no clear dependence on the day of year or solar zenith angle was found.

Fig. 3. Comparison of the MODIS/Aqua [circles] and MODIS/Terra [crosses] collection 005 AOD at 355 nm to co-located Brewer measurements.

Fig. 4. Comparison of the MODIS/Aqua [circles] and MODIS/Terra [crosses] collection 005 AOD at 470 nm to co-located Cimel measurements in Crete.

3.2. CIMEL AOD measurements over Heraklion In addition to the Brewer measurements, we have used CIMEL measurements (AERONET level 2 data collection, http://aeronet. gsfc.nasa.gov/) carried out at FORTH-CRETE AERONET station, which is located on the northern coast of Crete and has different aerosol load characteristics. The station is located 15 km east from Heraklion, which is a city with about 200,000 inhabitants and about 5.5 South of Thessaloniki. The CIMEL sunphotometer is an automatic sun-sky scanning spectral radiometer located on the roof of the Cretan National Center for Marine Research building, which is 20 m high and 100 m from the coast. The instrumentation, data acquisition, retrieval algorithms and calibration procedure conform to the standards of the AERONET global network, and are described in detail in Fotiadi et al. (2006). The overall uncertainty in AOD data, under cloud-free conditions, is 0.01 for wavelengths larger than 440 nm, and 0.02 for shorter ones (Dubovik and King, 2000). The main findings of the analysis of two years of CIMEL data, discussed in Fotiadi et al. (2006), and references therein, showed that low concentrations of sea-salt provide the site's background aerosol load, which is periodically enforced by the transport of dust particles from the Sarahan desert region. The satellite-derived AOD overestimation with respect to the CIMEL data is shown in Fig. 4. The CIMEL Ångstrom exponent was also utilised here in order to transform the CIMEL AOD at 440 nm to a wavelength of 470 nm. No morning-noon variability in the aerosol load over the city of Heraklion is revealed, with a mean MODIS AOD at 470 nm of around 0.300  0.150 for both satellite measurements and an under-estimation of the CIMEL measurements of 0.085 for MODIS/Aqua and 0.072 for MODIS/Terra, well within the statistical deviation. The amount of common data is also increased to between 461 and 623. The slope approaches unity in both cases, also seen in the linear regression lines, both of which show a positive offset indicating the overestimation by both satellite sensors.

M.E. Koukouli et al. / Atmospheric Environment 44 (2010) 1219e1228

The linear correlation coefficient is quite satisfactory with an R of 0.805 and 0.801 for the two cases. Note that the axes only reach the value of 1.2 for the AOD compared to what was seen for Thessaloniki, which is due to the different wavelength studied and by no means implies a lesser aerosol load over the island. In Table 3, selected statistical results discussed in this section are presented for both the Thessaloniki/Brewer and the Heraklion/Cimel comparisons with the satellite overpass data. The mean difference between the Brewer spectroradiometer AOD measurements over Thessaloniki and the MODIS AOD estimates differs between the two satellites, with MODIS/Terra performing better than the late afternoon MODIS/Aqua overpass with a mean difference of 0.073  0.185 against 0.105  0.217. The mean difference between the Cimel measurements and the two MODIS instruments is quite stable at 0.085  0.083 for MODIS/Terra and 0.072  0.082 for MODIS/Aqua. The high standard deviation of the Thessaloniki comparisons and the difference between noon and afternoon [in local time] aerosol loads points to a highly variable and varying environment compared to the more stable atmospheric state of Heraklion whose main sources of natural aerosols, namely Saharan dust and marine biogenic particles, as well as anthropogenic aerosols at different layers (Kouvarakis et al., 2002), have a high day-to-day and seasonal variability but not a pronounced diurnal change. 4. Aerosol climatology from MODIS/Terra In order to examine the climatological features of the region as observed by the MODIS instruments, the MODIS/Terra dataset was investigated further for all sites reported in the introduction to better understand their seasonal and long term AOD variability. The seasonality of the aerosol loading over the region is investigated through the study of the spatial variability of the AOD per season, the monthly mean AODs observed which also provide information about the different seasonalities of the different regions chosen and the trend analysis of the entire MODIS/Terra database. 4.1. Seasonal and spatial variability The mean aerosol load for each of the four seasons of the year for selected geolocations and associated standard deviation per season is shown in Table 4. From here onwards, and unless stated otherwise, the satellite AOD presented refers to the wavelength of 470 nm. Examining Table 4 in more detail a number of initial deductions may be made. The lowest mean AOD values are observed, as can also be seen in Fig. 5, during the winter months and the highest during the spring and summer-time. The standard deviation is slightly higher for the spring and summer AOD values than for the winter, in the same proportions as the mean values are higher. As a general rule it has been found that for all seasons the highest AOD values are observed for the northern-most latitudes with a good decrease towards the southern-most sites, as seen also in Barnaba and Gobbi (2004). This is a quite a generalised statement, of course, but the North-South gradient is quite apparent in this dataset as all six northern-most locations show a decline in the Table 3 Statistics of the comparisons between the MODIS/Aqua and MODIS/Terra collection 005 data with ground-based measurements over Thessaloniki and Crete. Mean difference  std

No of points

Thessaloniki [Brewer] MODIS/Terra 0.760 MODIS/Aqua 0.654

0.074  0.185 0.100  0.209

303 204

Heraklion [Cimel] MODIS/Terra 0.805 MODIS/Aqua 0.801

0.085  0.083 0.072  0.082

461 623

Correlation R

1223

observed aerosol load. Of the southern-most locations, the city of Heraklion shows an increase in optical depth for all seasons compared to the northern locations, especially during the spring and summer months which agrees with various studies that have shown that Greece is mostly affected by Saharan dust storms during the spring-time months (Moulin et al., 1998, Kubilay et al., 2003, Kalivitis et al., 2007). In Fig. 5 we present the seasonal variability of the monthly mean MODIS/Terra AOD for 470 nm for seven of the chosen geolocations for this study. The standard deviation bars of the Heraklion site [diamonds] are shown as indicator of the typical daily variability of the dataset. The rest of the scatter bars are omitted for clarity purposes. The cleanest two sites are, predictably, the Mount Athos [crosses] and the Aegean Sea [dots] locations. For Mount Athos we observe the lowest aerosol loads for the winter months with the same steady seasonal increase observed for the entire region, with maximum in August were the values appear as high as those of Thessaloniki and Maritsa and a steady drop during the autumn months. This behaviour points rather strongly to the fact that transported aerosol load affects the peninsula, a fact discussed in more detail below. This very clear and distinct annual cycle has also been observed in the measurements of aerosol optical properties between 1999 and 2002 presented in Gerasopoulos et al. (2003), made in the near vicinity of the chosen location we assigned as Mount Athos. For the Aegean Sea, seasonality is not as pronounced, with two small peaks in the spring and in autumn, which however remain close to the background conditions observed in that region. The Aegean Sea geolocation was chosen to fall on a very small island named Agios Efstratios, where in the past a large experimental campaign took place with the purpose of studying the interconnectivity between changes in total ozone, tropospheric aerosols, UV radiation and photochemical activity in the boundary layer and free troposphere (Marenco et al., 1997). In the framework of that campaign, it was found that at the regional level the Eastern Mediterranean in summer-time is governed by two diverse outflow regimes, a Northern flow and a South-Western episode flow which might be representative of a Sarahan dust transport (Zerefos et al., 2002). The former results in a highly variable aerosol load with low events of AOD around 0.1e0.2 in the cases of strong Northern winds. The spring-time peak and subsequent drop in aerosol load with the slight increase in the summer months observed in the Aegean Sea site corresponds nicely to the pattern seen in the Heraklion [open diamonds] seasonal aerosol load. As mentioned above, Crete has high aerosol loading in spring-time [and often in the month of October, as well] which explains the large scatter shown in the standard deviation bars compared to the more stable air patterns observed in the summer-time where a small secondary peak is apparent. Fotiadi et al. (2006), using AERONET measurements in Heraklion of the years 2003e2004 show winter mean values around 0.1e0.15, spring-time highs of 0.3 and summer-time peak of around 0.2e0.25 which agrees well with the variability shown in the seven years of MODIS data seen in Fig. 5. Discussing the two Mediterranean basin sites together, we refer to the work of Barnaba and Gobbi (2004), where the Mediterranean basin was separated into ten sectors and our Aegean Sea cite corresponds to sector 5 of their Fig. 10 and also our Heraklion site to sector 8. In contrast to the Heraklion site which is mostly affected by dust loading in all seasons apart from winter-time, Barnaba and Gobbi (2004) corroborate our finding that even though the South-to-North Saharan dust events often reach the North Aegean sea, the region remains mostly affected during all seasons by the presence of continental particles. This conclusion is also reached by Bryant et al. (2006), who found that the highest AODs were associated with air arriving to the Aegean Sea from Turkey and Central/Eastern Europe and the lowest with maritime air.

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Table 4 The seasonal mean MODIS/Terra AOD at 470 nm and standard deviation for seven geolocations sorted by decreasing latitude. Latitude ( N)

Maritsa Lake Kerkini Thessaloniki Ptolemaida Mount Athos Aegean Sea Heraklion

42.03 41.1 40.634 40.42 40.25 39.25 35.33

Spring

Summer

Autumn

Winter

Mean AOD

Std

Mean AOD

Std

Mean AOD

Std

Mean AOD

Std

0.436 0.412 0.413 0.360 0.299 0.307 0.386

0.205 0.251 0.268 0.229 0.209 0.166 0.201

0.469 0.506 0.445 0.406 0.424 0.28 0.347

0.232 0.29 0.263 0.240 0.204 0.128 0.124

0.331 0.335 0.343 0.246 0.264 0.266 0.265

0.285 0.212 0.231 0.181 0.206 0.176 0.126

0.250 0.209 0.209 0.183 0.114 0.177 0.197

0.167 0.124 0.146 0.117 0.087 0.132 0.128

For Thessaloniki [closed diamonds] the spring-time and summer-time double peak is more pronounced with generally higher AOD values in all seasons due to the high background levels of aerosol over the city from local pollution sources, in addition to the maritime suspended particles. Mean values around 0.4e0.42 for the spring peak and 0.48e0.50 for the summer peak are well in agreement with the work of Kazadzis et al. (2007) who observed the aerosol load seasonality using ground-based spectroradiometric measurements. Findings from that work showed that part of the summer peak is due to the transport of forest fire biomass burning aerosol from the North-Easterly direction, a finding also corroborated by Balis et al. (2003) and Koukouli et al. (2006). EARLINET Lidar AOD measurements of the vertical distribution of the aerosol layer showed that the mean summer-time aerosol vertical profiles indicate higher aerosol loads than any other season for heights above 1500 m, i.e. in the free troposphere (Amiridis et al., 2005). Holben et al. (2001) also attributed high August and September aerosol loading in Northern Hemisphere midlatitudes to wildfire biomass burning and showed that the extend depends highly on local meteorological and fuel-loading conditions. The Ptolemaida Power Plant Complex [filled triangles] is situated in the North of Greece, in the Prefecture of West Macedonia. Ptolemaida is a highly industrialized area and its four power plants produce 70% of Greece's needs in electrical power, using large local deposits of lignite as fuel. MODIS/Terra sees enhanced AOD values from April to August of around 0.40, following the summer loading peak pattern of the Balkan region which includes the wet removal of aerosols during the winter months. Recent studies on the characterization of the airborne particles in the region of the power plant presented in Iordanidis et al. (2008), showed that directly in the power plant vicinity mining activities alongside the lignite

burning itself contribute to the heavy air pollution whereas further afield this effect quickly diminishes. The Maritsa Iztok Power Plant Complex [open squares] is the largest energy complex in South Eastern Europe. It is located in the Stara Zagora Province in South-Central Bulgaria. It consists of three lignite-fired thermal power stations and is located in a large lignite coal basin, which includes several mines, enrichment plants, a briquette plant and its own railway system. In the MODIS/Terra dataset it is the location that shows the highest AOD values of the entire period, with a steady high monthly mean above 0.4 for five of the spring and summer-time months, and also is the highest loaded atmosphere for the winter months with mean values more than 0.20e0.25 in some cases. A very interesting surprise in this work was the finding that a site that was selected as rural and hence locally clean, Lake Kerkini [green open triangles], showed high values of aerosol load during the entire year, with spring and summer-time highs around 0.45e0.48. Lake Kerkini lies close to the Bulgarian border in Northern Greece and is renowned as one of the finest wetland sites in Europe. As such, it was expected to show minimal atmospheric particulate load. However, the high correlation between the seasonal pattern of the AOD of Lake Kerkini and that of the Maritsa-Itzok Compound, alongside a high statistical correlation of the common days of data [not shown here], points directly to the fact that the transboundary transport of pollutants does not solely apply to the gaseous constituents (Zerefos et al., 2000) but also to particulate matter. In the work of Zerefos et al. (2004), they showed that the percentage of the observed SO2 column amount over Thessaloniki attributed to lignite burning power plants in Bulgaria, Romania and former Yugoslavia states may rise to 70% at periods with NE flow, with 60% contributions in winter and 75% during summer. Lake Kerkini falls directly under that same wind pattern and it stands to reason that the same plumes probably also transport particulate matter downwind and into Greece near the borders. 4.2. Trend analysis

Fig. 5. The seasonal variability of the monthly mean MODIS/Terra AOD at 470 nm for seven of the geolocations for this study.

In the following, we examine the evidence of a decrease of the aerosol loading in the region shown by the measurements chosen. In a similar study, Papadimas et al. (2008), using collection 005 level-3 MODIS AOD estimates for the years 2000e2006, report that in the broader Mediterranean basin the aerosol loading has decreased by 20% in statistically significant relative percentage terms. They attributed the 14% decrease in the summer-time to decreased emission rates of anthropogenic pollution and the 19% increase in the winter-time to decreased precipitation in the region. In this work, since level 2 MODIS/Terra overpass data were used for the trend analysis, we are expecting the changes to be even more prominent since these data were not subjected to as much smoothing as the gridded level 3 data. The seasonal trends discussed in this section were calculated as follows: the monthly mean AOD value for each of the months of the year was found, and then the seasonal mean for each year was

M.E. Koukouli et al. / Atmospheric Environment 44 (2010) 1219e1228

calculated for the four seasons. The linear fit to the seasonal variability for the seven years of MODIS/Terra data divided with the mean of the seasonal AODs provided the trend per annum, using the word trend rather loosely since more than a few decades worth of data is typically required to be able to talk of trends per se. The annual trends discussed were calculated in the same fashion from the monthly mean AOD values per year. In Table 5 the seasonal and the annual trends for seven geolocations are given as percentage per annum values. The values in italics in the table denote the trends with a statistical significance of more than 90%. The mean annual trend is
4.05  1.85% per annum. The largest negative trends those are observed for the locally clean sites under the influence of transported pollution, namely Lake Kerkini and Mount Athos which show declining trends larger than w5% per annum for three out of four seasons. The smallest negative trends are seen over the Maritsa site [one of the most polluted locations chosen] and the Aegean Sea site, one of the cleanest. Maritsa shows the highest variability with a high positive and a high negative trend for the months of autumn and winter, with low statistical significance. The mean trend for Thessaloniki across all seasons is w4% which coincides quite well with the findings of Papadimas et al. (2008), who used a larger area of MODIS coverage to calculate their trend estimates. Most of the spring and summer trends belong to the 90th percentile and half the autumnewinter months. In Fig. 6 the percentage departures of the monthly mean values are shown as a function of time for the entire seven year MODIS/Terra dataset for four of the geolocations. After the seasonality has been extracted from the data, Thessaloniki, Heraklion and Mount Athos all share an approximate 3e4% reduction in aerosol load per annum and the Aegean Sea site an 1e2 % decrease. The fact that the Aegean Sea site is the cleanest site, with no local polluting sources and with mostly a background sea-spray aerosol load, would suggest that the transported aerosol load has decreased as well as the local sources. The findings of Kazadzis et al. (2007) from ground-based measurements of the double Brewer monochromator over Thessaloniki for the years 1996e2007 agree very well with these satellitederived percentages at
3% and
1.95% respectively at the 99% statistical significance level.

5. Investigation of the possible cause of the observed decrease in aerosol load In order to discern whether the observed negative trend in the aerosol load over the Southern Balkan sites considered in this work is due to the decrease in the local sources and/or transported aerosols and also possibly to a change in the wind patterns in the past decade (Maheras et al., 2004), a first endeavour was made to separate the free tropospheric load from the boundary layer load.

1225

Fig. 6. The de-seasonalised monthly mean values for MODIS/Terra for four locations. The straight line shows the downward trend of the atmospheric load over Thessaloniki which is in sync with Heraklion and Mount Athos sites as well.

In Fig. 7, this separation is attempted for the air over Thessaloniki and two nearby locations, Mt Athos [left] and Aegean Sea [right]. For the almost 600 common days of MODIS/Terra measurements between Thessaloniki and Mt Athos the slope of the curve reaches unity, as shown in Fig. 7, left, and the difference between the mean values of the AOD for the two cites is 0.06. This is a rather low estimate considering that the difference between the two sites was expected to represent the contribution of the boundary layer over the city to the total aerosol load. The high correlation between the two sites with coefficient of 0.80 points to the fact the air over Mt Athos can be affected by the same air circulation patterns as Thessaloniki and that enough of the local pollution from the city rises to the free troposphere and is transported to the areas nearby. The high discrepancy between the atmospheric loads of Thessaloniki and the Aegean Sea site [Fig. 7, right] can be verified by the statistical analysis of the common days, which show almost no correlation [coefficient of 0.50] and a higher mean difference of almost 0.10, which is higher than the standard deviation. A similar picture is also seen when viewing the common days of the Mt Athos and Aegean Sea sites [not shown here]. Even though both have no significant local sources of pollution and mainly sea spray and continental aerosols as local aerosol sources, the effects of the transported pollution that the Mt Athos cite suffers from is evident from the difference in mean levels of AOD between these two sea sites. In order to identify the postulated common regional and intercontinental sources of atmospheric load between the two sites, Thessaloniki and Mt Athos, analytical backward trajectories were calculated. Backward trajectories provide some information about

Table 5 The seasonal trend in percentage value for the MODIS/Terra AOD at 470 nm for the seven geolocations sorted by decreasing latitude. The trends in italics show the trends with more than 90% statistical significance. Latitude

Maritsa Lake Kerkini Thessaloniki Ptolemaida Mount Athos Aegean Sea Heraklion Seasonal Trend

42.03 41.1 40.634 40.42 40.25 39.25 35.33

Spring

Summer

Autumn

Winter

Annual

Trend [%]

Trend [%]

Trend [%]

Trend [%]

Trend [%]

0.20
6.34
3.58
4.75
5.51
1.90
3.97
3.69  2.23


0.65
8.30
4.66
4.73
4.09
2.58
3.59
4.09  2.34


7.65
5.05
3.03
2.85
4.66
1.61
4.33
4.17  1.95

5.79
1.61
6.64
7.75
2.29
2.75
2.61
2.55  4.36


0.98
6.28
4.28
5.05
5.50
2.28
4.00
4.05  1.86

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M.E. Koukouli et al. / Atmospheric Environment 44 (2010) 1219e1228

Fig. 7. Scatter plots of the observed MODIS/Terra AOD over different locations. Left: Thessaloniki vs Mt Athos. Right: Thessaloniki vs Aegean Sea.

the possible origin of the observed aerosols and about the synoptic patterns corresponding to the measurements. In our study, fourday back trajectories were computed for the days of interest, using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and Hess, 1998), which uses the meteorological data produced by the National Weather Service ETA Model to compute advection and dispersion of air parcels. HYSPLIT was used in order to generate 4-day back trajectories for air-parcels arriving over the sites of interest at the altitude of 1500 m for all the available days of AOD measurements. The atmospheric back trajectories were calculated for the arrival time of 12:00 UTC. A cluster analysis algorithm was then used to merge the computed trajectories into clusters, using the clustering algorithm for atmospheric trajectories recommended by Dorling et al. (1992). In this method, the optimum number of clusters follows from the algorithm itself and need not be assumed. The cluster analysis was used only for trajectories which end at 1500 m arrival height over Thessaloniki and Mt Athos. These trajectories are believed to be most representative for the main air transport in the upper part of the atmospheric boundary layer and the lower troposphere. The optimum number of clusters was found to be six for almost all cases. The results of the previously described clustering algorithm were used to classify MODIS/Terra AOD values in relation to the large-scale weather regime. Each AOD observation was assigned to the cluster of its corresponding trajectory to derive groups of AOD under similar large-scale synoptic conditions. For each cluster, the statistics of the measured AOD at 470 nm over Thessaloniki and Mt Athos are given in Table 6, showing the air trajectory direction, the mean AOD values and associated standard deviation. Discussing the clusters from the downwind perspective between the two cites, the lowest mean AOD values correspond to clusters 2 and 5, which represent fast transport from North-Western Europe and the Atlantic Ocean respectively and are about 0.21  0.17 for Thessaloniki. For Mt Athos the AOD values are slightly smaller, namely 0.21  0.24 for cluster 2 and 0.07  0.02 for cluster 5. The free tropospheric contribution for these clusters is relatively small due to the lack of aerosol sources in these directions (Amiridis et al., 2005). The air masses that descend from the North bringing clean air from the Atlantic Ocean [cluster 5] are well represented by the low mean AOD over Mt Athos compared to the typical mean AOD for Thessaloniki. The highest AOD values of 0.561  0.296 for Thessaloniki and 0.456  0.264 for Mt Athos correspond to cluster 6. Cluster 6 is mostly representative of the local circulation patterns including an Easterly component. We can assume that the observed aerosol loading can

be attributed mainly to local pollution with some enhancement in the free troposphere from possible transportation of pollution from an Easterly direction. Analogous to the local circulation patterns with a highly probable dust aerosol component is cluster 3, where high values of AOD, of the order of 0.420  0.350 at both sites are again observed. Clusters 3 and 6 are associated with the higher mean AOD values and the higher standard deviations showing the great range of aerosol loads for the associated days. AOD for Thessaloniki, reached the maximum value of 1.786 for a day residing in cluster 6, corresponding to a typical Saharan dust intrusion over the city. Cluster 1 corresponds mainly to transport from the Balkans and Central/Eastern Europe. AOD values for this cluster are of the order of 0.340 [0.260]  0.165 [0.160] for each site. There are many power plants along this NE axis in the countries of Eastern Europe northwards of Greece. Zerefos et al. (2000) showed enhanced columnar SO2 values that corresponded to NE directions mainly during summer-time, providing further evidence for the pollution transport. In addition the air represented by this cluster is often affected by severe biomass burning episodes depending on the year in question (Balis et al., 2003). Finally, cluster 4 shows AOD values of the order of 0.314  0.175 for Thessaloniki and 0.157  0.115 for Mt Athos. This cluster is associated with North-Westerly flow and is characterized by high wind speeds with a katabatic motion in the vertical, associated with Table 6 The statistics of the MODIS Terra AOD at 470 nm over Thessaloniki [upper line] and Mt Athos [lower line] for the six main clusters. Shown are the minimum, maximum, mean values and standard deviation [from left to right.]. Cluster no Air provenance

Percentage of total air

Location

Mean AOD & Standard deviation

1

North-East

20%

Thessaloniki 0.34  017 Mount Athos 0.26  0.16

2

West

15%

0.21  0.17 0.21  0.24

3

Local & Sahara 10%

0.43  0.31 0.42  0.38

4

North-West

25%

0.31  0.18 0.16  0.11

5

North-Far

4e4.5%

0.24  0.19 0.07  0.02

6

Local & East

20%

0.56  0.30 0.46  0.26

M.E. Koukouli et al. / Atmospheric Environment 44 (2010) 1219e1228

anti-cyclonic synoptic features. Lower AOD values associated with this cluster, indicate clean air arriving from high altitudes in the troposphere. Amiridis et al. (2005) have shown that this katabatic vertical motion of these air masses often indicates that these were not modified significantly by anthropogenic activities while travelling across Europe at that height.

6. Conclusions Seven years of MODIS/Terra and three years of MODIS/Aqua level 2 aerosol optical depth measurements were analyzed in order to examine the spatial and temporal evolution of the atmospheric aerosol load over the Southern Balkan region. The previous, 004, and current, 005, collection of the MODIS data have been validated against ground-based measurements in two diverse sites, a Brewer spectroradiometer in Thessaloniki and a Cimel sunphotometer on the island of Crete. The use of the latest collection 005 AOD with the one extracted from the Brewer improves the correlation coefficient between satellite and ground estimates from 0.487, found for collection 004, to 0.647 for the MODIS/Aqua afternoon measurements. The positive bias of the satellite data and the unreasonably high AOD values encountered in the previous Collection are reduced in the correct Collection, indicating that the cloud contamination effect seen in Collection 004 measurements is much reduced. The comparison of the aerosol loading over Crete revealed a mean MODIS AOD at 470 nm of around 0.300  0.150 for both satellite overpass measurements and an under-estimation of the CIMEL measurements of 0.085 for MODIS/Aqua and 0.072 for MODIS/Terra. The associated linear correlation coefficient is quite satisfactory with an R of 0.805 and 0.801 for the two cases. The absence of morning-noon variability in the aerosol load is not surprising as Remer et al. (2008), found that statistically significant diurnal aerosol differences cannot be discerned above the products' uncertainties at this time. The annual AOD pattern over the region shows high values for spring and summer and low loading for the winter months. From a meteorological point of view, this behaviour can be attributed to spring-time high desert dust loads and summer-time lows in precipitation. In addition, the different geolocations around the Balkan peninsula chosen for this study reveal some information about local sources of pollution, about the different seasonal variabilities of these local sources and also about transported aerosol loading. The cleanest sites are the Aegean Sea and Mount Athos sites, with a mean AOD at 470 nm of 0.26  0.03 and 0.32  005 respectively and the most polluted are the Maritsa site, with a mean of 0.39  0.06 and Thessaloniki with 0.37  0.06. The Heraklion cite shows a pronounced spring-time maximum which agrees well with the expected increase in dust storms reaching the island during that time of year, and the Aegean Sea also seems to follow the same statistically significant annual pattern. The mean annual trend in AOD for the region chosen rises to
4.05  1.85% per annum, and is well in agreement with the works of Kazadzis et al. (2007) and Papadimas et al. (2009). This decreasing tendency is consistent with declining trends over much of the globe and over the same period presented in similar works based on level 3 MODIS data (Papadimas et al., 2008). The highest variability is found for the winter-time months with a decrease of
2.55  4.36% per annum and the strongest statistical significance in the calculated trends is found for the summer-time, with a mean of
4.09  2.34%. These findings corroborate assessments made using emission inventories and radiative transfer code which showed that the two-decadal rate of decline in aerosol loading resulting from these emission changes, 0.13% per annum, can be compared with the reported

1227

increase in solar radiation of 0.10% per annum in 1983e2001 which has led to global brightening conditions (Streets et al., 2006). Differences and similarities of level 2 MODIS data between two nearby sites with different expected atmospheric aerosol properties were examined. Of great interest is to what extend this negative trend is due to the decrease in the local sources of aerosols and/or the transported aerosols and/or to a change in the wind patterns of the area in the past decade. It was found that in some cases of clean air trajectory direction, the common seasonality and load can be explained with the common air masses that feed the air with particles. However, more detailed study of the meteorological conditions and the emission inventories over the region is necessary before a conclusion can be reached as to the exact causes of the negative trend in aerosol loading over the Southern Balkans. Acknowledgments The authors would like to acknowledge the MODIS aerosol team for providing the satellite aerosol products and the AERONET network for making the Cimel measurements over Heraklion, Crete, publicly available. SK would like to acknowledge the Research Committee of the Aristotle University of Thessaloniki for the PostDoctoral Scholarship 2008: “Aerosol optical depth measurements with ground based and satellite sensors for Thessaloniki, Greece area”. References Amiridis, V., Balis, D.S., Kazadzis, S., et al., 2005. Four-year aerosol observations with a Raman lidar at Thessaloniki, Greece, in the framework of European Aerosol Research Lidar Network (EARLINET). Journal of Geophysical Research 110, D21203. doi:10.1029/2005JD006190. Balis, D.S., Amiridis, V., Zerefos, C., et al., 2003. Raman lidar and sunphotometric measurements of aerosol optical properties over Thessaloniki, Greece during a biomass burning episode. Atmospheric Environment. doi:10.1016/S1352-2310 (03)00581-8. Balis, D.S., Amiridis, V., Zerefos, C., et al., 2004. Study of the effect of different type of aerosols on UV-B radiation from measurements during EARLINET. Atmospheric Chemistry and Physics 4, 307e321. 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. Atmospheric Chemistry and Physics 4, 2367e2391. Bryant, C., Eleftheriadis, K., Smolik, J., et al., 2006. Optical properties of aerosols over the eastern Mediterranean. Atmospheric Environment 40 (32), 6229e6244. Chu, D.A., Kaufman, Y.J., Zibordi, G., et al., 2003. Global monitoring of air pollution over land from the earth observing system-terra moderate resolution imaging spectroradiometer (MODIS). Journal of Geophysical Research 108 (D21), 4661. doi:10.1029/2002JD003179. Draxler, R.R., Hess, G.D., 1998. An overview of the HYSPLIT 4 modelling system for trajectories, dispersion and deposition. Australian Meteorological Magazine 47 (4), 295e308. Dorling, S.R., Davies, T.D., Pierce, C.E., Oct 1992. Cluster analysis: A technique for estimating the synoptic meteorological controls on air and precipitation chemistrydMethod and applications. Atmospheric Environment. Part A. General Topics 26 (14), 2575e2581. Dubovik, O., King, M.D., 2000. A flexible inversion algorithm for retrieval of aerosol optical properties from sun and sky radiance measurements. Journal of Geophysical Research 105, 20673e20696. Eck, T.F., Holben, B.N., Slutsker, I., et al., 1998. Measurements of irradiance attenuation and estimation of aerosol single scattering albedo for biomass burning aerosols in Amazonia. Journal of Geophysical Research-Atmospheres 103 (D24), 31865e31878. Fotiadi, A., Hatzianastassiou, N., Drakakis, E., et al., 2006. Aerosol physical and optical properties in the eastern Mediterranean Basin, Crete, from Aerosol Robotic Network data. Atmospheric Chemistry and Physics 6, 5399e5413. Formenti, P., Andreae, M.O., Andreae, T.W., et al., 2001. Aerosol optical properties and large scale transport of air masses: observations at a coastal and a semiarid site in the eastern Mediterranean during summer 1998. Journal of Geophysical Research 106, 9807e9826. Gerasopoulos, E., Andreae, M.O., Zerefos, C.S., et al., 2003. Climatological aspects of aerosol optical properties in Northern Greece. Atmospheric Chemistry and Physics 3, 2025e2041. Gerasopoulos, E., KouvarakisBabasakalis, P., et al., 2006. Origin and variability of particulate matter (PM10) mass concentrations over the eastern Mediterranean. Atmospheric Environment. doi:10.1016/j.atmosenv.2006.04.020.

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