1Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
2Finnish Meteorological Institute, Erik Palménin aukio 1, 00560 Helsinki, Finland
3Institute for Energy and Climate Research (IEK-8), Forschungszentrum Jülich, 52425 Jülich, Germany
4Aerodyne Research, Inc., Billerica, Massachusetts, USA
5Department of Applied Environmental Science and Bert Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden
Abstract. The volatility of atmospheric 20–500 nm aerosol particles was investigated at a boreal forest site in Hyytiälä, Finland. Measurements were performed continuously between January 2008 and May 2010. The ambient aerosol sample was heated step-wise to six temperatures ranging from 80 °C to 280 °C and the total mass concentration of aerosol particles was determined from the measured particle number size distributions before and after heating assuming particle density of 1.6 g cm−3. On average 19% of the total aerosol mass stayed in the condensed phase even after heating to 280 °C. The observed non-volatile residual at 280 °C had a seasonal pattern; during winter the aerosol mass fraction remaining after heating was the highest and during summer the lowest. Black carbon concentrations correlated positively with the non-volatile fraction of the aerosol, but could not explain the presence of the non-volatile material completely: most of the time a notable fraction of the non-volatile residual was something else than black carbon. Using additional information on ambient meteorological conditions and trajectories, and results from an Aerodyne aerosol mass spectrometer (AMS), the chemical composition of the non-volatile residual and its seasonal behavior was further examined. During winter and spring months the non-volatile mass fraction had a marked positive linear correlation with pollutant trace gases, such as CO, SO2 and NOx. This suggests an anthropogenic influence on the non-volatile fraction of the aerosol in winter and spring. The anthropogenic effect on the formation of the low-volatility material was furthermore supported by observed correlation between the non-volatile residual and the mass fractions of poly-aromatic hydrocarbons (PAHs) sampled simultaneously at the site. During the fall the aerosol particles had relatively more non-volatile material in them when the aerosol mass fractions of organic nitrate and organics in the AMS data were high, and when the measurement site was influenced by clean air masses passing over the forest. Thus, the existence of very low volatile organic nitrates in the aerosol phase can be speculated.