Abstract
Secondary organic aerosol (SOA) constitutes a large fraction of atmospheric aerosol. To assess its impacts on cli-mate and air pollution, knowledge of the number of phases in internal mixtures of different SOA types is required. Atmospheric models often assumed that different SOA types form a single phase when mixed. Here, we present visual observations of the number of phases formed after mixing different anthropogenic and biogenic SOA types. Mixing SOA types generated in environmental chambers with oxygen-to-carbon (O / C) ratios between 0.34 to 1.05, we found six out of fifteen mixtures of two SOA types to result in two phase particles. We demonstrate that the number of phases depends on the difference in the average O / C ratio between the two SOA types (Δ(O / C)). Using a threshold Δ(O / C) of 0.47, we can predict the phase behavior of over 90 % of our mixtures, with one- and two-phase particles predicted for Δ(O / C) < 0.47 and Δ(O / C) ≥ 0.47, respectively. This Δ(O / C) threshold further allows to predict if mixtures of fresh and aged SOA form one- or two-phase particles in the atmos-phere. In addition, we show that phase separated SOA particles form when mixtures of volatile organic compounds emitted from real trees are oxidized.
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