Retrieval and monitoring of aerosol optical thickness over an urban area by spaceborne and ground-based remote sensing

We used an instrumental synergy of both ground-based (sunphotometer) and sp aceborne [POLDER (polarization and directionality of the Earth's reflectanc es) and Meteosat] passive remote-sensing devices to determine the aerosol o ptical thickness over the suburban area of Thessaloniki, Greece, from April 1996 to June 1997. The POLDER spaceborne instrument measures the degree of polarization of the solar radiance reflected by the Earth-atmosphere syste m. Aerosol optical thickness (AOT) retrieval needs an accurate estimate of the contribution of the ground surface to the top of atmosphere's polarized radiance. We tested existing surface reflectance models and fitted their p arameters to find the best model for the Thessaloniki area. The model was c onstrained and validated by use of independent data sets of coincident sunp hotometer and POLDER measurements. The comparison indicated that the urban AOT over Thessaloniki was retrieved by the FOLDER instrument with an accura cy of +/-0.05. From analysis of Meteosat data we found that similar to 40% of the days with high AOT (>0.18) are associated with African dust transpor t events, all observed in the period March-July. Excluding dust events, the 15-month AOT averages 0.12 +/- 0.04. During the 15-month period that the s tudy was conducted, we observed aerosol pollution peaks with an AOT of >0.2 4 on 15 of the 164 days on which measurements were possible. (C) 1999 Optic al Society of America OCIS codes. 010.1100, 010.1110, 010.1120, 280.1100, 2 80.1120, 290.1310.