Importance of the organic aerosol fraction for modeling aerosol hygroscopic growth and activation: a case study in the Amazon Basin

Abstract

Abstract. 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 fail to predict both DGFs and CCN number concentration. In fact, this study shows that a good reproduction of the measured DGF and CCN concentration is obtained if the chemical composition of aerosol, especially that of WSOC, is appropriately taken into account in the calculations. New parameterizations for the computed CCN spectra are also derived which Correspondence to: M. Mircea (m.mircea@isac.cnr.it) take into account the variability caused by chemical effects (surface tension, molecular composition, solubility, degree of dissociation of WSOC).

Closure between measured and modeled cloud condensation nuclei (CCN) using size-resolved aerosol compositions in downtown Toronto

“…Aerosol solubility appears to be the driving force behind CCN activation in most cases. Mircea et al (2005) have recently proposed that the water soluble organic carbon fraction must be reasonably well characterized to accurately predict particle hygroscopic growth and activation. The impact of water soluble organic carbon in this study, however, is minimal since any soluble organic carbon will likely be found in the accumulation mode which is already dominated by sulfate.…”

Generation of Internally Mixed Insoluble and Soluble Aerosol Particles to Investigate the Impact of Atmospheric Aging and Heterogeneous Processing on the CCN Activity of Mineral Dust Aerosol

“…Atmospheric aerosols affect the Earth's climate either through the scattering and absorption of solar radiation, direct radiative forcing, or by acting as cloud condensation nuclei (CCN), indirect radiative forcing. Currently, the aerosol indirect effect has the largest amount of uncertainty when estimating the impact of aerosols on climate and as such has become a focus of a number of field and laboratory studies (Abbatt et al 2005;Dinar et al 2006;Huff Hartz et al 2006;Mircea et al 2005;Petzold et al 2005;Ramaswamy et al 2001). There has been much discussion as to what characteristics of an aerosol govern its CCN activity (size, chemical composition, etc.…”

Elemental composition of PM10 and PM2.5 for a savanna (cerrado) region of Southern Amazonia

“…11,12 Focus during the wet season has illustrated the importance of vegetation aerosol emissions, which act as cloud condensation nuclei (CCN), effectively controlling cloud formation and precipitation, and aerosol aging and humidification processes have been examined to improve regional precipitation models. 10,[13][14][15][16] Here we describe the seasonal variations in atmospheric aerosol concentration in the woodland/savannah (Cerrado) region of Southcentral Mato Grosso, Brazil. Cerrado in this region has been radically changed by human activities such as burning, cattle ranching, and agriculture.…”