Characterization of aerosol and cloud water at a mountain site during WACS 2010: secondary organic aerosol formation through oxidative cloud processing

The water-soluble fractions of aerosol filter samples and cloud water collected during the Whistler Aerosol and Cloud Study (WACS 2010) were analyzed using an Aerodyne aerosol mass spectrometer (AMS). This is the first study to report AMS organic spectra of re-aerosolized cloud water, and to make direct comparison between the AMS spectra of cloud water and aerosol samples collected at the same location. In general, the mass spectra of aerosol were very similar to those of less volatile cloud organics. By using a photochemical reactor to oxidize both aerosol filter extracts and cloud water, we find evidence that fragmentation of water-soluble organics in aerosol increases their volatility during photochemical oxidation. By contrast, enhancement of AMS-measurable organic mass by up to 30 % was observed during the initial stage of oxidation of cloud water organics, which was followed by a decline at the later stages of oxidation. These observations are in support of the general hypothesis that cloud water oxidation is a viable route for SOA formation. In particular, we propose that additional SOA material was produced by functionalizing dissolved organics via OH oxidation, where these dissolved organics are sufficiently volatile that they are not usually part of the aerosol. This work demonstrates that water-soluble organic compounds of intermediate volatility (IVOC), such as cispinonic acid, produced via gas-phase oxidation of monoterpenes, can be important aqueous-phase SOA precursors in a biogenic-rich environment.