Cold oceans enhance terrestrial new-particle formation in near-coastal forests 1Department of Physics, University of Helsinki, P.O. Box 48, 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
Received: 03 April 2009 – Published in Atmos. Chem. Phys. Discuss.: 11 June 2009 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. In this paper, we connected biogenic
aerosol formation events observed in a Eucalypt forest in South-East
Australia during July 2005–December 2006 to air mass history using 96-h
back trajectories. Formation of new particles was most frequent in the dry
westerly and south-westerly air masses. According to NDVI (Normalized
Difference Vegetation Index) measurements, photosynthesis was not
significantly higher in this direction compared to the north-east direction.
It is unlikely, therefore, that differences in photosynthesis-derived
organic precursor emissions would have been significant enough to lead to
the clear difference in NPF frequency between these two directions. Instead,
the high evaporation rates above the Pacific Ocean resulted in humid winds
from the north-east that effectively suppressed new-particle formation in
the forest hundreds of kilometers inland. No other factor varied as
significantly in tune with new-particle formation as humidity and we
concluded 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.
Revised: 14 October 2009 – Accepted: 03 November 2009 – Published: 16 November 2009
Citation: Suni, T., Sogacheva, L., Lauros, J., Hakola, H., Bäck, J., Kurtén, T., Cleugh, H., van Gorsel, E., Briggs, P., Sevanto, S., and Kulmala, M.: Cold oceans enhance terrestrial new-particle formation in near-coastal forests, Atmos. Chem. Phys., 9, 8639-8650, doi:10.5194/acp-9-8639-2009, 2009.