Key parameters controlling OH‐initiated formation of secondary organic aerosol in the aqueous phase (aqSOA)

Abstract

Secondary organic aerosol formation in the aqueous phase of cloud droplets and aerosol particles (aqSOA) might contribute substantially to the total SOA burden and help to explain discrepancies between observed and predicted SOA properties. In order to implement aqSOA formation in models, key processes controlling formation within the multiphase system have to be identified. We explore parameters affecting phase transfer and OH(aq)-initiated aqSOA formation as a function of OH(aq) availability. Box model results suggest OH(aq)-limited photochemical aqSOA formation in cloud water even if aqueous OH(aq) sources are present. This limitation manifests itself as an apparent surface dependence of aqSOA formation. We estimate chemical OH(aq) production fluxes, necessary to establish thermodynamic equilibrium between the phases (based on Henry's law constants) for both cloud and aqueous particles. Estimates show that no (currently known) OH(aq) source in cloud water can remove this limitation, whereas in aerosol water, it might be feasible. Ambient organic mass (oxalate) measurements in stratocumulus clouds as a function of cloud drop surface area and liquid water content exhibit trends similar to model results. These findings support the use of parameterizations of cloud-aqSOA using effective droplet radius rather than liquid water volume or drop surface area. Sensitivity studies suggest that future laboratory studies should explore aqSOA yields in multiphase systems as a function of these parameters and at atmospherically relevant OH(aq) levels. Since aerosol-aqSOA formation significantly depends on OH(aq) availability, parameterizations might be less straightforward, and oxidant (OH) sources within aerosol water emerge as one of the major uncertainties in aerosol-aqSOA formation.

Quantifying sources and sinks of reactive gases in the lower atmosphere using airborne flux observations

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A better understanding of hydroxyl radical photochemical sources in cloud waters collected at the puy de Dôme station – experimental versus modelled formation rates

“…For this estimation, models describing the multiphase cloud chemistry have been developed and have considered the reactivity in the gas and aqueous phases along with the mass transfer between the two phases (Ervens et al, 2014;Long et al, 2013;Tilgner and Herrmann, 2010). These numerical tools allow for the estimation of the steady-state concentration of HO q ([HO q ] ss ), which is a crucial quantity to understand the fate of atmospheric pollutants (Arakaki et al, 2013).…”

Mixing state of oxalic acid containing particles in the rural area of Pearl River Delta, China: implications for the formation mechanism of oxalic acid

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