Impact of the open biomass fires in spring and summer of 2006 on the chemical composition of background air in south-eastern Finland

In the spring and summer of 2006 the air quality in southern Finland was affected by two major biomass fire smoke episodes. At the Virolahti background station, closest to the eastern fire areas, the episodes lasted altogether several weeks. The high point in spring was 25 April and in summer 13 August. In spring the aerosol detected at Virolahti originated at distances of even hundreds of kilometres to the south and south-east, and consequently was a mixture of material from biomass burning and from other sources (both LRT and local), all of which contributed to the detected elevation of PM10 concentrations. The elevated concentrations of trace elements (Cd, Pb, Zn) during the most intense biomass fire episode were associated with other anthropogenic emissions. In contrast, during August 2006, the PM10 at Virolahti was quite exclusively impacted by close (ca. 50–100 km) biomass fire sources. The presumably organic component comprised, at its highest, as much as 90% of the total PM10. In addition to record high PM10 and PM2.5 concentrations, the concentrations of polycyclic aromatic hydrocarbons were considerably elevated, even reaching values more typical of wintertime urban environments. During the peaks of the episodes in August, the total gaseous mercury concentration in the air was more than double its background value. In general, the trace elements did not exceed their background values.