1Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
2Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
3Department of Forest Ecology, P.O. Box 27, 00014 University of Helsinki, Finland
4CSIRO Marine and Atmospheric Research, GPO Box 1666, Canberra ACT 2601, Australia
Abstract. The world's forests produce atmospheric aerosol by emitting volatile organic compounds (VOC) which, after being oxidized in the atmosphere, readily condense on the omnipresent nanometer-sized nuclei and grow them to climatically relevant sizes. The cooling effect of aerosols is the greatest uncertainty in current climate models and estimates of radiative forcing. Therefore, identifying the environmental factors influencing the biogenic formation of aerosols is crucial. We show that, in addition to local meteorological factors in the forest, the magnitude of evaporation from oceans hundreds of kilometers upwind can effectively suppress or enhance new-particle formation. Our findings indicate that, unlike warm waters, the cold polar oceans provide excellent clean and dry background air that enhances aerosol formation above near-coastal forests in Fennoscandia and South-East Australia.