Aerosol samples from a boreal forest in Finland (Hyytiälä) and an anthropogenically influenced grassland in Germany (Melpitz) were analyzed for carbonaceous content by a thermal-optical transmission method. In order to assess the adsorption of semi-volatile compounds on the quartz fiber filters at both locations, a backup quartz filter was placed beneath the filter used for the particulate matter collection. The fraction of particulate organic carbon with a secondary origin in gas/particle conversion of volatile organic compounds was estimated from the minimum ratio between particulate organic and black carbon, which occurred during periods of reduced photochemical activity. Values calculated by this method were 71% for the Finnish aerosol, whilst in Melpitz only 56% could be observed. Especially in Hyytiälä, the fraction of secondary organic carbon showed a maximum during daytime and minimum during night time, indicating a local or regional formation of secondary aerosol. It was verified that the mass of carbon adsorbed in most backup filters is not significantly different from the mass adsorbed in a blank filter.