Biogenic SOA formation through gas-phase oxidation and gas-to-particle partitioning – comparison between process models of varying complexity

Abstract

Abstract. Biogenic volatile organic compounds (BVOCs) emitted by the vegetation play an important role for the aerosol mass loadings since the oxidation products of these compounds can take part in the formation and growth of secondary organic aerosols (SOA). The concentrations and properties of BVOCs and their oxidation products in the atmosphere are poorly characterized, which leads to high uncertainties in modeled SOA mass and properties. In this study the formation of SOA has been modeled along an air mass trajectory over the northern European boreal forest using two aerosol dynamics box models where the prediction of the condensable organics from the gas-phase oxidation of BVOC is handled with schemes of varying complexity. The use of box model simulations along an air mass trajectory allows us to, under atmospheric relevant conditions, compare different model parameterizations and their effect on SOA formation. The result of the study shows that the modeled mass concentration of SOA is highly dependent on the organic oxidation scheme used to predict the oxidation products. A near-explicit treatment of organic gas-phase oxidation (Master Chemical Mechanism version 3.2) was compared to oxidation schemes that use the volatility basis set (VBS) approach. The resulting SOA mass modeled with different VBS-schemes varies by a factor of about 7 depending on how the first generation oxidation products are parameterized and how they subsequently age (e.g. how fast the gas-phase oxidation products react with the OH-radical, how they respond to temperature changes and if they are allowed to fragment during the aging process). Since the VBS approach is frequently used in regional and global climate models due to its relatively simple treatment of the oxidation products compared to near-explicit oxidation schemes; better understanding of the abovementioned processes are needed. Compared to the most commonly used VBS-schemes, the near-explicit method produces less – but more oxidized – SOA.