Three-year ground based measurements of aerosol optical depth over the Eastern Mediterranean: the urban environment of Athens

Abstract

Abstract. Three years (2006)(2007)(2008) of ground-based observations of the Aerosol Optical Depth (AOD) in the urban environment of Athens, in the Eastern Mediterranean, are analysed in this work. Measurements were acquired with a Multi-Filter Rotating Shadowband Radiometer at five wavelengths. The daily average AOD at 500 nm is 0.23, and the meanÅngström coefficient calculated between 415 and 867 nm is 1.41. The annual variability of AOD has a spring maximum dominated by coarse dust particles from the Sahara (AOD 0.34-0.42), while the diurnal pattern is typical for urban sites, with AOD steadily increasing throughout the day. The greatest contribution to the annually averaged AOD, accounting for almost 40%, comes from regional and local sources (namely the Istanbul metropolitan area, the extended areas of biomass burning around the north coast of the Black Sea, power plants spread throughout the Balkans and the industrial area in the Po valley, with average daily AOD in the range of 0.25-0.35). An additional important contribution (23%) is dust from Africa, whereas the rest of Europe contributes another 22%. The geographical distribution of the above sources in conjunction with the prevailing synoptic situation and contribution of local sources, lead to mixed types of aerosols over Athens, with highly variable contribution of fine and coarse particles to AOD in the range 10%-90%. This is the first long-term, ground based data set available for Correspondence to: E. Gerasopoulos (egera@meteo.noa.gr) Athens, and it has also been used for the validation of satellite derived AOD by MODIS, showing good agreement on an annual basis, but with an overestimation of satellite AODs in the warm period.

Retrieval of aerosol optical depth in the visible range with a Brewer spectrophotometer in Athens

“…Maximum AODs are found in summer, as already reported in the scientific literature for the Athens metropolis (e.g. Gerasopoulos et al, 2011) as well as at different midlatitude locations (e.g. Chey- (Student, 1908) and the nonparametric Mann-Kendall trend test (Mann, 1945;Kendall, 1938) all gave statistically significant results (Student's t test: t value = 4.31471, p value < 0.0001, N = 924; Mann-Kendall test: τ statistics = 0.124, 2-sided p value < 2.22 × 10 −16 ).…”

“…Aerosol conditions in Athens have been addressed in previous studies and have been found to range from relatively clean maritime conditions to long-range desert dust transport episodes, smoke advection from forest fires and small particles typical of an urban and industrialised environment (e.g. Pissimanis et al, 1994;Eleftheriadis et al, 1998;Colbeck et al, 2002;Adamopoulos et al, 2007;Tsaknakis et al, 2011;Kaskaoutis et al, 2007;Kosmopoulos et al, 2008;Liu et al, 2009;Gerasopoulos et al, 2011;Amiridis et al, 2012).…”

Source contributions to 2012 summertime aerosols in the Euro-Mediterranean region

“…These meteorological patterns govern the transport of aerosols released from various sources around the Mediterranean region and lead to a maximum aerosol load during the summer (Monks et al, 2009;Nabat et al, 2013). Moreover, a mixture of different sources is usually observed due to the long-range transport of the associated particles (Dall'Osto et al, 2010;Gerasopoulos et al, 2011;Boselli et al, 2012). Due to these complex characteristics, the specific impact of each aerosol source cannot be easily assessed.…”

Aerosol microphysical retrievals from precision filter radiometer direct solar radiation measurements and comparison with AERONET

“…According to the study of this region by Gerasopoulos et al (2011), in the summer period, the build-up of particles due to intense photochemical processes favored by high temperature and insolation contributes to elevated AOD levels in a way that reflects the geographical spread of particle precursor sources and transport processes. In comparison, the average (2006)(2007)(2008) summer AOD (at 500 nm) is 0.22 (0.27 at 440 nm), and this value is exceeded during four consecutive days during the measurement period (17-20 July) either due to the presence of dust (e.g.…”