Oxidant supply and aqueous photochemical SOA formation in cloud droplets and aqueous aerosol
Abstract
Many recent laboratory, field and model studies point to significant contributions to the total secondary organic aerosol (SOA) budget from aqueous phase reactions in cloud droplets and aqueous aerosol particles. Laboratory studies of the photochemical oxidation of glyoxal and methylglyoxal in the aqueous phase show a strong dependence on the initial concentration of dissolved organics, with preferential formation of large molecules (dimers, oligomers) at the high concentrations found in ambient deliquesced aerosol particles. In such experimental studies OH radicals are produced in the aqueous phase (via hydrogen peroxide photolysis) and OH radical is assumed to be the major oxidant. An explicit aqueous photooxidation mechanism has been validated, in part, based on the observed temporal evolution of organic intermediates and products in these experiments. In this work, this mechanism was incorporated into multiphase process models (box, cloud parcel) in order to further explore aqueous SOA formation in dilute cloud droplets and concentrated aerosol particles. We found that the predicted SOA mass in both aqueous phases can be comparable despite the much lower liquid water content in aerosols, where oligomer formation is favored. Direct uptake from the gas phase was the largest source of OH radicals in the aqueous phase. In-situ production through the Fenton reaction (Fe), hydrogen peroxide and nitrate photolysis were minor sources. Since phase transfer is slower than the OH(aq) consumption by organics, modeled OH(aq) concentrations were smaller by 1-2 orders of magnitude than predicted based on thermodynamic equilibrium. Our model studies suggest that, unless there are substantial additional sources of OH radical in the aqueous phase, aqueous SOA formation will be oxidant limited. Since the phase transfer rate is a function of the drop (or particle) surface area, aqueous SOA formation may occur preferentially at or near the drop/particle surface (e.g., be surface-limited) and thus add SOA mass preferentially to particle populations with higher surface/volume ratios. These model results will be compared and contrasted with aqueous sulfate formation. We will discuss our findings in the context of ambient data that might suggest a preferential aqueous SOA formation in smaller (high surface/volume) particles. We will also highlight current gaps in our understanding of oxidant production and cycling in the aqueous phase that might influence our results and change dependencies on bulk water volume versus surface area.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A34C..06T
- Keywords:
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- 0305 ATMOSPHERIC COMPOSITION AND STRUCTURE / Aerosols and particles;
- 0320 ATMOSPHERIC COMPOSITION AND STRUCTURE / Cloud physics and chemistry;
- 0345 ATMOSPHERIC COMPOSITION AND STRUCTURE / Pollution: urban and regional;
- 0365 ATMOSPHERIC COMPOSITION AND STRUCTURE / Troposphere: composition and chemistry