Formation of organic aerosol through cloud chemistry: insights from the OH radical oxidation of filtered rainwater

Organic particulate matter in the atmosphere plays an important role in climate forcing, visibility, and adverse health effects. Atmospheric organic aerosol is predominantly of secondary origin, formed in the atmosphere. Laboratory photooxidation experiments, atmospheric aerosol measurements below vs. above clouds and at increasing humidity, and modeling studies all suggest that secondary organic aerosol (SOA) forms from water-soluble gases through aqueous chemistry in clouds and wet aerosols (aqSOA). Previous laboratory experiments are simple compared to the atmospheric water media (single compound deionize water solutions), thus a more realistic approach is needed for the understanding of SOA formation through aqueous chemistry. We conducted batch photooxidation experiments with three different rainwater samples from Camden and Pinelands, NJ and hydroxyl radicals (formed from 150 μM H2O2 + UV radiation). We used rainwater (RW) as a surrogate for cloud water in these experiments. SOA precursors and products were identified by real-time Electrospray Ionization – Mass Spectrometry (ESI-MS, continuous online sampling) and by Ion Chromatography (discrete samples). Precursors were found predominantly in the positive mode, suggesting the presence of aldehydes, alcohols and organic peroxides, and products were found predominantly in the negative mode, suggesting the presence of organic acids. A decrease in the abundance of ions with the same unit mass-to-charge ratio as standards of glyoxal, methylglyoxal and glycolaldehyde and an increase in the abundance of ions associated with organic acids (e.g., oxalic and pyruvic acid) suggest that these aldehydes were present and reacting. The evidence is strongest for methylglyoxal (three RW samples). Glyoxal oxidation appears to occur in two RW samples; evidence for glycolaldehyde is not as strong. Other potential contributors to SOA formation (precursor and products) were identified based on their percentage of change and absolute change in ion abundance across the reaction.M.S.Includes bibliographical referencesby Anjuli Ramos-Buso