Impact of large cities on the spatial distribution of aerosol optical depth in the Central Europe

The analysis of the long-term observations of aerosol optical depth (AOD) and its spatial distribution in large cities areas is presented. AOD observations were obtained from satellite and ground based measurements. In case of satellite detectors we used MODIS and SEVIRI data. As for SEVIRI measurements we created two algorithms to derive AOD from synergy of the satellite and ground-based observations, and aerosol transport model output. In mentioned methods data acquired in SEVIRI channel 1, and channels 1 and 3 was used. In both cases the surface reflectance is the most important parameter which should be calculated during retrieval process. The surface properties are estimated during days with low AOD (less than 0.1 at 500 nm) based on the radiance measured by SEVIRI detector and aerosol optical properties modeled by NAAPS aerosol transport model or measured by MODIS sensor. In case of NAAPS and MODIS data the AOD fields were corrected with the information obtained in the ground-based stations equipped with sun photometers. For this purpose the optimal interpolation method is used. Furthermore, in case of the two channels method information about land cover from Land Cover Map for Europe is essential. Finally, assuming that surface reflectance at SEVIRI resolution change slowly with time the AOD is computed. Regarding the surface observations, AOD was measured within the framework of Poland-AOD network (www.polandaod.pl), with use of MFR-7, Microtops and CIMEL instruments. Additionally, concentrations of particulate matter with an aerodynamic diameter <10 µm (PM10) at chosen stations were taken into account. Gathering and analyzing all specified data provide information about the influence of big cities emissions on the AOD. As an example study case of Warsaw (about 1.7 million of residents) extended area will be presented. The AOD was observed between 2005 and 2011 in Warsaw and in Belsk (about 45 km away from Warsaw) with hand-held Microtops (Warsaw) and CIMEL (Belsk) sun photometers. The PM10 concentrations were measured at three Warsaw stations as well as in Belsk. The estimated effect is about 0.02 (at 500 nm) based on the sun photometers' comparison and 0.03 (at 550 nm) based on the MODIS results. Relatively small impact of Warsaw emissions on the AOD (about 10-15%) is consistent with the PM10 data. The mean PM10 differences, estimated during the same time as sun photometer measurements, for Warsaw Ursynow (residential area) and Belsk is only 5.7 µg/m³ (13%), and for Warsaw Targowek (shopping, residential area) and Belsk is about 9.8 µg/m³ (20%). For the station located in the central Warsaw, near to the street with the heavy traffic, the difference in the long-term mean of the PM10 is significantly larger and reaches 22.1 µg/m³ (36%). During extreme smoke event as was observed on 4 April 2009 the differences of AOD between Warsaw and Belsk were in a range of 0.1-0.2 (at 500 nm).