The Langley Plot method can be used to retrieve the AODs from Brewer spectrophotometer onldy if the neutral density filter spectral transmittances are well known. Most of the PM10 aerosol is in the PM2.5 size fraction:the ratio between the PM2.5 and PM10 mass is around 0.6 and 0.8 during winter and spring, respectively. The PM2.5 particles originate mostly from anthropogenic sources, at least for 60%, and possibly for up to 80%. In addition to the AOD from Brewer spectrophotometer and chemical aerosol characteristics from samplers, PM2.5 mass data from a TEOM, black carbon data from an aethalometer and back trajectories from APTRA model will be used to determine the relation between the optical and chemical characteristics of the aerosol particles in a future project