Aqueous oxidation of aerosol and cloud water samples at Whistler, BC: Secondary organic aerosol formation via oxidative cloud processing

Secondary organic aerosols (SOA) modeled to form from traditional gas-phase oxidation mechanism cannot match observed SOA loadings and their degree of oxidation. Aqueous chemistry has been proposed to explain these discrepancies. In particular, cloud processing is important in modifying chemical composition and properties of atmospheric particulate matter. Extremely high water content in cloud droplets favors additional partitioning of water-soluble gaseous organics to cloud water, which is much less significant in aerosol particles. Subsequent aqueous oxidation of those cloud organics followed by droplet evaporation can be a source of SOA. In this study, the water-extractable fraction of filter aerosol samples (PM1) and cloud water collected at Whistler, British Columbia during summer 2010 (Whistler Aerosol and Cloud Study (WACS) 2010) was aerosolized and then analyzed using an Aerodyne Aerosol Mass Spectrometer (AMS) on site. This is the first study to report AMS spectra of cloud water and to make direct comparison between the AMS spectra of cloud water and filter aerosol particles collected at the same location. While sulfate was the most abundant inorganic component in aerosol samples, a comparable amount of inorganic nitrate and sulfate were observed in cloud water, indicating a unique nitrate formation pathway in cloud water. It was found that fresh biogenic organic aerosols (OA) produced at Whistler forest area were generally less oxygenated than aged OA and cloud organics. In addition, a simple aqueous photochemical reaction vessel was used to oxidize both aerosol filter extracts and cloud water samples. Here we have experimental evidence that aqueous oxidation of water-soluble organic aerosol components generate high vapor pressure species that led to significant organic mass loss through volatilization. On the contrary, formation of AMS-measurable organic mass can be observed during aqueous oxidation of cloud organics. This observation suggests that oxidative processing of cloud water can significantly reduce the vapor pressure of volatile/semi-volatile cloud organics and hence is potentially important to produce atmospheric SOA.