Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 2 2006 10.5194/acp-6-315-2006 http://www.atmos-chem-phys.net/6/315/2006/ http://www.atmos-chem-phys.net/6/315/2006/acp-6-315-2006.html http://www.atmos-chem-phys.net/6/315/2006/acp-6-315-2006.pdf 315 327 2006-02-06 Size and composition measurements of background aerosol and new particle growth in a Finnish forest during QUEST 2 using an Aerodyne Aerosol Mass Spectrometer J. D. Allan M. R. Alfarra K. N. Bower H. Coe J. T. Jayne D. R. Worsnop P. P. Aalto M. Kulmala T. Hyötyläinen F. Cavalli A. Laaksonen School of Earth, Atmospheric & Environmental Science, University of Manchester, UK Distributed Institute for Atmospheric Composition, NERC Centres for Atmospheric Science, UK Aerodyne Research Inc., Billerica, MA, USA Department of Physical Sciences, University of Helsinki, Finland Department of Chemistry, University of Helsinki, Finland Institute of Atmospheric Sciences and Climate, Consiglio Nazionale delle Ricerche, Bologna, Italy Department of Applied Physics, University of Kuopio, Finland now at: Laboratory for Atmospheric Chemistry, Paul Scherrer Institut, Switzerland The study of the growth of nucleation-mode particles is important, as this prevents their loss through diffusion and allows them to reach sizes where they may become effective cloud condensation nuclei. Hyytiälä, a forested site in southern Finland, frequently experiences particle nucleation events during the spring and autumn, where particles first appear during the morning and continue to grow for several hours afterwards. As part of the QUEST 2 intensive field campaign during March and April 2003, an Aerodyne Aerosol Mass Spectrometer (AMS) was deployed alongside other aerosol instrumentation to study the particulate composition and dynamics of growth events and characterise the background aerosol. Despite the small mass concentrations, the AMS was able to distinguish the grown particles in the <100 nm regime several hours after an event and confirm that the particles were principally organic in composition. The AMS was also able to derive a mass spectral fingerprint for the organic species present, and found that it was consistent between events and independent of the mean particle diameter during non-polluted cases, implying the same species were also condensing onto the accumulation mode. The results were compared with those from offline analyses such as GC-MS and were consistent with the hypothesis that the main components were alkanes from plant waxes and the oxidation products of terpenes.