Photoformation of hydroxyl radical and hydrogen peroxide in aerosol particles from Alert, Nunavut: implications for aerosol and snowpack chemistry in the Arctic

We have studied oxidant photoformation in aqueous extracts of aerosol particles collected from Alert, Nunavut, Canada during springtime. Absorption spectra of the extracts reveal that Alert particles have tremendous light absorption coefficients (e.g., α300 100 cm−1) as a result of unidentified, pH-dependent, water-soluble chromophores. Illumination of the extracts leads to the rapid formation of both hydroxyl radical (√OH) and hydrogen peroxide (HOOH). Based on our laboratory results, the calculated rate of √OH photoformation in Alert particles is very rapid, 1 mM h−1 (midday, 1 April, 248 K), with nitrate photolysis contributing only 10% of the total rate. Deposition of these aerosol particles, in conjunction with smaller contributions from gaseous chromophores, leads to estimated rates of √OH photoformation in the quasi-liquid layer of surface snow of 20–40 μM h−1; approximately a third of this reactivity is from nitrate photolysis. The estimated 24-h-average rate of HOOH photoformation in Alert particles ( 9 mM h−1 on 1 April) is large enough to be a major source of HOOH to both the particles and the gas phase. In contrast, particle-derived reactions in the snow appear to be a minor source of HOOH to the surface snowpack. The effects of √OH and HOOH photoformation in particles and snowpack likely include the oxidation of organic carbon, halides, and S(IV) species to yield products such as volatile aldehydes and carboxylic acids, photoactive halogens, and sulfuric acid. In addition, √OH and HOOH photoformation within the snowpack might significantly alter snow and ice core records of HOOH and other trace gases.