Abstract. It has been shown that sunlit snow and ice plays an important role in processing atmospheric species. Photochemical production of a variety of chemicals has recently been reported to occur in snow/ice and the release of these photochemically generated species may significantly impact the chemistry of the overlying atmosphere. Nitrogen oxide and oxidant precursor fluxes have been measured in a number of snow covered environments, where in some cases the emissions significantly impact the overlying boundary layer. For example, photochemical ozone production (such as that occurring in polluted mid-latitudes) of 3-4 ppbv/day has been observed at South Pole, due to high OH and NO levels present in a relatively shallow boundary layer. Field and laboratory experiments have determined that the origin of the observed NO x flux is the photochemistry of nitrate within the snowpack, however some details of the mechanism have not yet been elucidated. A variety of low molecular weight organic compounds have been shown to be emitted from sunlit snowpacks, the source of which has been proposed to be either direct or indirect photo-oxidation of natural organic materials present in the snow. Although myriad studies have observed active processing of species within irradiated snowpacks, the fundamental chemistry occurring remains poorly understood. Here we consider the nature of snow at a fundamental, physical level; photochemical processes within snow and the caveats needed for comparison to atmospheric photochemistry; our current understanding of nitrogen, oxidant, halogen and organic photochemistry within snow; the current limitations faced by the field and implications for the future.

Nitrate photolysis proceeds via two major channels at illumination wavelengths above 290 nm: NO 3 -+ hν (+H + ) f NO 2 + • OH (1) and NO 3 -+ hν f NO 2 -+ O( 3 P) (2). A recent study determined the quantum yield of reaction 1 on ice by measuring NO 2 production, but suggested their values might be lower bounds because of incomplete recoveries of NO 2 . We measured the quantum yield of pathway 1 using an alternate approach, i.e., by following the formation of • OH. Our quantum yields for • OH (Φ OH ) at 263 K were independent of nitrate concentration and illumination wavelength (λ > 300 nm), but were dependent upon pH. Values of Φ OH decreased from (3.6 ( 0.6) × 10 -3 at pH 7.0 to (2.1 ( 0.8) × 10 -3 at pH 2.0, where the listed pH values are those of the sample solution prior to freezing. Temperature dependence experiments (239-318 K; pH 5.0) showed that values of Φ OH in ice pellets and aqueous solutions were both well described by the same regression line, ln(Φ OH ) ) ln(Φ 1 ) ) -(2400 ( 480)(1/T) + (3.6 ( 0.8) (where errors represent (1σ), suggesting that the photolysis of nitrate on ice occurs in a "quasi-liquid layer" rather than in the bulk ice. Our ice quantum yields between 268 and 240 K are 3-9 times higher, respectively, than Φ 1 values determined previously in ice. Applying our quantum yields to past field experiments indicates that nitrate photolysis can account for the flux of NO x from sunlit snow in the Antarctic and at Summit, Greenland, but that nitrate was only a minor source of the snowpack NO x measured during the Alert 2000 campaign in the Canadian Arctic. Additional calculations show that the photolysis of nitrate on cirrus clouds in the upper troposphere is a minor source of NO x that cannot account for the apparent underestimation of the ratio of NO x /HNO 3 in current numerical models.

Abstract. Phenolic compounds, which are emitted in significant amounts from biomass burning, can undergo fast reactions in atmospheric aqueous phases to form secondary organic aerosol (aqSOA). In this study, we investigate the reactions of phenol (compound with formula C6H5OH)), guaiacol (2-methoxyphenol), and syringol (2,6-dimethoxyphenol) with two major aqueous-phase oxidants – the triplet excited states of an aromatic carbonyl (3C*) and hydroxyl radical (· OH). We thoroughly characterize the low-volatility species produced from these reactions and interpret their formation mechanisms using aerosol mass spectrometry (AMS), nanospray desorption electrospray ionization mass spectrometry (nano-DESI MS), and ion chromatography (IC). A large number of oxygenated molecules are identified, including oligomers containing up to six monomer units, functionalized monomer and oligomers with carbonyl, carboxyl, and hydroxyl groups, and small organic acid anions (e.g., formate, acetate, oxalate, and malate). The average atomic oxygen-to-carbon (O / C) ratios of phenolic aqSOA are in the range of 0.85–1.23, similar to those of low-volatility oxygenated organic aerosol (LV-OOA) observed in ambient air. The aqSOA compositions are overall similar for the same precursor, but the reactions mediated by 3C* are faster than · OH-mediated reactions and produce more oligomers and hydroxylated species at the point when 50% of the phenolic compound has reacted. Profiles determined using a thermodenuder indicate that the volatility of phenolic aqSOA is influenced by both oligomer content and O / C ratio. In addition, the aqSOA shows enhanced light absorption in the UV–visible region, suggesting that aqueous-phase reactions of phenols may contribute to formation of secondary brown carbon in the atmosphere, especially in regions influenced by biomass burning.

No abstract

Condensed-phase chemistry plays a significant role in the formation and evolution of atmospheric organic aerosols. Past studies of the aqueous photoformation of secondary organic aerosol (SOA) have largely focused on hydroxyl radical oxidation, but here we show that triplet excited states of organic compounds ((3)C*) can also be important aqueous oxidants. We studied the aqueous photoreactions of three phenols (phenol, guaiacol, and syringol) with the aromatic carbonyl 3,4-dimethoxybenzaldehyde (DMB); all of these species are emitted by biomass burning. Under simulated sunlight, DMB forms a triplet excited state that rapidly oxidizes phenols to form low-volatility SOA. Rate constants for these reactions are fast and increase with decreasing pH and increasing methoxy substitution of the phenols. Mass yields of aqueous SOA are near 100% for all three phenols. For typical ambient conditions in areas with biomass combustion, the aqueous oxidation of phenols by (3)C* is faster than by hydroxyl radical, although rates depend strongly on pH, oxidant concentrations, and the identity of the phenol. Our results suggest that (3)C* can be the dominant aqueous oxidant of phenols in areas impacted by biomass combustion and that this is a significant pathway for forming SOA.