Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 4 2005 10.5194/acpd-5-5253-2005 http://www.atmos-chem-phys-discuss.net/5/5253/2005/ http://www.atmos-chem-phys-discuss.net/5/5253/2005/acpd-5-5253-2005.html http://www.atmos-chem-phys-discuss.net/5/5253/2005/acpd-5-5253-2005.pdf 5253 5298 2005-07-25 Importance of the organic aerosol fraction for modeling aerosol hygroscopic growth and activation: a case study in the Amazon Basin M. Mircea M. C. Facchini S. Decesari F. Cavalli L. Emblico S. Fuzzi A. Vestin J. Rissler E. Swietlicki G. Frank M. O. Andreae W. Maenhaut Y. Rudich P. Artaxo Istituto di Scienze dell’Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, I-40129 Bologna, Italy Division of Nuclear Physics, Lund University, S-221 00 Lund, Sweden Biogeochemistry Department, Max Planck Institute for Chemistry, D-55020, Mainz, Germany Department of Analytical Chemistry, Institute for Nuclear Sciences, Ghent University, B-9000 Gent, Belgium Department of Environmental Sciences, Weizmann Institute, 76100 Rehovot, Israel Instituto de Fisica, Universidade de Sao Paulo, CEP 05508-900 Sao Paulo, Brazil The aerosol in the Amazon basin is dominated throughout the year by organic matter, for the most part soluble in water. In this modeling study, we show how the knowledge of water-soluble organic compounds (WSOC) and the associated physical and chemical properties (e.g. solubility, surface tension, dissociation into ions) affect the hygroscopic growth and activation of the aerosol in this area. The study is based on data obtained during the SMOCC field experiment carried out in Rondônia, Brazil, over a period encompassing the dry (biomass burning) season to the onset of the wet season (September to mid-November, 2002). The comparison of predicted and measured cloud condensation nuclei (CCN) number concentration shows that the knowledge of aerosol WSOC composition in terms of classes of compounds and of their relative molecular weights and acidic properties may be sufficient to predict aerosol activation, without any information on solubility. Conversely, the lack of knowledge on WSOC solubility leads to a high overestimation of the observed diameter growth factors (DGF) by the theory. Moreover, the aerosol water soluble inorganic species describe reasonably well the aerosol hygroscopic behavior at 90% relative humidity, but fail to predict CCN number concentration. In fact, this study shows that a good reproduction of the measured DGF does not result in realistic estimations of CCN concentration if the chemical composition of aerosol, especially that of WSOC, is not appropriately taken into account in the calculations. New parameterizations for the computed CCN spectra are also derived which take into account the variability caused by chemical effects (surface tension, molecular composition, solubility, degree of dissociation of WSOC).