Secondary organic aerosol (SOA) formation through cloud processing is a potentially important source of atmospheric particulate matter (PM) that has been unrecognized until quite recently. Aromatics and alkenes (e.g., isoprene) can oxidize in the gas phase and particularly in the interstitial spaces of clouds where hydroxyl radical concentrations are high. Their water-soluble products (e.g., glyoxal, methylglyoxal) partition into cloud droplets where they oxidize further to form low volatility species (e.g., oxalic acid). Upon cloud droplet evaporation these compounds remain, at least in part, in the particle phase, yielding new (secondary) particulate matter. In this work we present photochemical aqueous phase batch experiments and initial modeling efforts that demonstrate that in cloud SOA formation occurs and provide an initial assessment of the potential importance of this process. Isoprene does not appear to be an important precursor for SOA formation through homogeneous (gas-phase) reactions except possibly at high NOx concentrations. However, our work suggests isoprene is an important precursor of SOA through the aqueous phase (in-cloud or in-aerosol) pathway and that oligomer formation enhances this process.

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