Spatial and temporal variability in aerosol properties over the Mediterranean basin based on 6-year (2000-2006) MODIS data

The temporal variability of aerosol optical properties is investigated over the broader Mediterranean basin, with emphasis on aerosol optical depth ( AOD) that is an effective measure of aerosol load. The study is performed using Collection 005 Level-3 mean daily spectral aerosol data from the Moderate Resolution Imaging Spectroradiometer ( MODIS) instrument on board the Terra and Aqua satellites, which cover the 6-year period from 2000 to 2006. The results of our analysis reveal a significant interannual variability of AOD in the study region. Specifically, the regional mean visible AOD over land and ocean has decreased over the period 2000-2006 by 20% in relative percentage terms ( or by 0.04 in absolute terms). This tendency is statistically significant according to the Man-Kendall test. However, the decreasing tendency of AOD is not uniform over the whole basin. It appears mainly in the western parts of Iberian, Italian, and Balkan peninsulas ( and coastal areas), as well as in the southern Anatolian peninsula. The analysis for summer ( June to September) and winter ( November to March) seasons revealed different tendencies in both AOD and precipitation. The summer-period AOD has decreased by 0.04 ( or by 14%) probably due to decreased emission rates of anthropogenic pollution. In contrast, the winter AOD has increased by 0.03 ( or 19%) mainly related to decreased precipitation ( associated with an increasing tendency in the NAO index). The decreasing tendency in MODIS AOD is in good agreement with corresponding AOD tendencies based on data from Aerobot Robotic Network ( AERONET) stations in the study region and ground based PM10 measurements at selected stations.