The monthly mean values of the aerosol
optical thickness in summer 2002 were considerable much than in the other years considered. A particularly high monthly check details mean AOT(500) for 2002 was recorded in August, when it reached 0.323 ± 0.237. For comparison, the monthly mean aerosol optical thicknesses in the Augusts of the other years varied from 0.065 ± 0.050 in 1999 to 0.139 ± 0.079 in 2003 (Figure 4a). The monthly mean values of < α(440, 870) > from June to September of 2002 also reached exceptionally high values ( Figure 4b). The monthly mean values of the aerosol optical thickness AOT(500) in July and August of 1999 were the lowest of all. The aerosol optical thickness above Gotland is influenced not only by periodic and incidental phenomena near the Baltic Sea shore, but also by distant continental phenomena. The origin of air masses advecting over Gotland has an impact on the aerosol optical thickness as well as the Ångström exponent. Based on a synoptic map analysis of AOT(500) measurements over five years, AOT(500) values < 0.100 were linked to the advection of maritime Arctic and maritime Polar air masses over the Baltic area. The advection of continental Polar air above the Baltic (six-day RGFP966 nmr backward trajectories leading from over central Europe) can increase the aerosol optical thickness up to 0.682 (± 0.025),
as observed on 1 April 2002. In summer 2002, fires intensified by persistent drought contributed to the high values of the aerosol optical thickness. Monthly composite satellite
images available from FIRMS (The Fire Information for Resource Management System) show the particularly numerous forest and field fires in northern Europe, Russia, Ukraine and Belarus in 2001 and 2002. Moreover, in summer 2002 the modal wind direction was different from that in the other summers considered here. For example, north-easterly winds (40°) were predominant in August 2002, whereas winds from the north-west (300° and 310°) were the most frequent in 1999, 2001 and 2003. The specific synoptic situation in 2002 favoured the transport of aerosol towards Gotland derived from the biomass burning. For example, the biomass burning aerosols transported over the Baltic Sea along with advecting air on 31 July or 12 August 2002 resulted in < AOT(500) >31072002 = 0.661 ± 0.084 and < AOT(500) >12082002 = 0.62 4 ± 0.162. The enlarged emission of aerosol and an Methisazone increase in AOT(500) in spring was presumably related to agricultural waste straw burning (Niemi 2003). It is worth noting that during the time period under scrutiny, cases of air advection from Africa at 3000 m above the Baltic region were observed in spring and summer. However, at lower altitudes the air then usually came from the burning regions of Russia, Ukraine and Belarus. In such cases the daily mean aerosol optical thicknesses for λ = 500 nm were lower (i.e. < AOT(500) >12042002 = 0.261 ± 0.055, < AOT(500) >12052002 = 0.249 ± 0.038, < AOT(500) >13082002 = 0.416 ± 0.