Using explicit mechanisms of Secondary Organic Aerosol (SOA) formation and evolution to extrapolate chamber studies to the atmosphere

The applicability of chamber-derived Secondary Organic Aerosol (SOA) yields to the atmosphere remains a key uncertainty in modeling SOA. The chemical and environmental conditions achieved in chambers are narrower than and often significantly biased from those experienced in the atmosphere. We present results from applying explicit chemical mechanisms in a dynamic gas-particle partitioning model (FOAM-WAM) to simulate SOA formation and evolution from a range of chamber experiments involving isoprene and monoterpenes. We focus on how such comparisons can highlight the applicability, or the lack thereof, of derived SOA yields, extrapolate measured SOA yields to more complex chemical or environmental conditions, and identify key gaps in chemical or physical mechanisms and thus feedback on chamber experiment design and earth system model parameterizations. In particular, we show that current mechanisms of low-NOx isoprene and a-pinene oxidation that incorporate RO2 H-shift reactions can adequately explain corresponding fresh SOA without the need for substantial vapor-pressure lowering accretion chemistry, while substantial particle-phase photo-chemistry is required to explain the dynamic evolution of SOA characteristics (volatility, O/C ratios, etc)observed in chambers at longer aging times. We find that chemical conditions, such as absolute concentrations, are as important as vapor wall loss, or even more so, at perturbing SOA yields from realistic values. Consistent with recent field studies but in contrast to previous chamber studies, our modeling predicts that low-NOx isoprene oxidation is unlikely to produce significant SOA in warm boundary layers, except through isoprene epoxy-diol multi-phase chemistry. Current mechanisms are unable to reproduce the non-linear response of isoprene-derived photochemical SOA with NOx observed in multiple chambers, suggesting a potentially important missing mechanism of volatility reduction at intermediate NOx concentrations in that system.