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.

Wildfires emit significant amounts of pollutants that degrade air quality. Plumes from three wildfires in the western U.S. were measured from aircraft during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) and the Biomass Burning Observation Project (BBOP), both in summer 2013. This study reports an extensive set of emission factors (EFs) for over 80 gases and 5 components of submicron particulate matter (PM1) from these temperate wildfires. These include rarely, or never before, measured oxygenated volatile organic compounds and multifunctional organic nitrates. The observed EFs are compared with previous measurements of temperate wildfires, boreal forest fires, and temperate prescribed fires. The wildfires emitted high amounts of PM1 (with organic aerosol (OA) dominating the mass) with an average EF that is more than 2 times the EFs for prescribed fires. The measured EFs were used to estimate the annual wildfire emissions of carbon monoxide, nitrogen oxides, total nonmethane organic compounds, and PM1 from 11 western U.S. states. The estimated gas emissions are generally comparable with the 2011 National Emissions Inventory (NEI). However, our PM1 emission estimate (1530 ± 570 Gg yr−1) is over 3 times that of the NEI PM2.5 estimate and is also higher than the PM2.5 emitted from all other sources in these states in the NEI. This study indicates that the source of OA from biomass burning in the western states is significantly underestimated. In addition, our results indicate that prescribed burning may be an effective method to reduce fine particle emissions.

[1] A simple dimensionless parameter, L, is shown to determine whether or not new particle formation can occur in the atmosphere on a given day. The criterion accounts for the probability that clusters, formed by nucleation, will coagulate with preexisting particles before they grow to a detectable size. Data acquired in an intensive atmospheric measurement campaign in Atlanta, Georgia, during August 2002 (ANARChE) were used to test the validity of this criterion. Measurements included aerosol size distributions down to 3 nm, properties and composition of freshly nucleated particles, and concentrations of gases including ammonia and sulfuric acid. Nucleation and subsequent growth of particles at this site were often dominated by sulfuric acid. New particle formation was observed when L was less than $1 but not when L was greater than $1. Furthermore, new particle formation was only observed when sulfuric acid concentrations exceeded 5 Â 10 6 cm À3 . The data suggest that there was a positive association between concentrations of particles produced by nucleation and ammonia, but this was not shown definitively. Ammonia mixing ratios during this study were mostly in the 1 to 10 ppbv range.

Asian sulfate over the ocean is in the lower free troposphere (800-600 hPa), with a decrease in pressure toward land due to orographic effects. We calculate that 56% of the measured sulfate between 500-900 hPa over British Columbia is due to East Asian sources. We find evidence of a 72-85% increase in the relative contribution of East Asian sulfate to the total burden in spring off the northwest coast of the United States since 1985. Campaign-average simulations indicate anthropogenic East Asian sulfur emissions increase mean springtime sulfate in Western Canada at the surface by 0.31 µg/m 3 (∼30%) and account for 50% of the overall regional sulfate burden between 1 and 5 km. Mean measured daily surface sulfate concentrations taken in the Vancouver area increase by 0.32 µg/m 3 per 10% increase in the simulated fraction of Asian sulfate, and suggest current East Asian emissions episodically degrade local air quality by more than 1.5 µg/m 3 .

[1] A ship emission plume experiment was conducted about 100 km off the California coast during the NOAA Intercontinental Transport and Chemical Transformation (ITCT) 2K2 airborne field campaign. Measurements of chemical species were made from the NOAA WP-3D aircraft in eight consecutive transects of a ship plume around midday during 2.5 hours of flight. The measured species include NO x , HNO 3 , peroxyacetylnitrate (PAN), SO 2 , H 2 SO 4 , O 3 , CO, CO 2 , nonmethane hydrocarbons (NMHC), and particle number and size distributions. Observations demonstrate a NO x lifetime of $1.8 hours inside the ship plume compared to $6.5 hours (at noontime) in the moderately polluted background marine boundary layer of the experiment. This confirms the earlier hypothesis of highly enhanced in-plume NO x destruction. Consequently, one would expect the impact of ship emissions is much less severe than those predicted by global models that do not include rapid NO x destruction. Photochemical model calculations suggest that more than 80% of the NO x loss was due to the NO 2 + OH reaction; the remainder was by PAN formation. The model underestimated in-plume NO x loss rate by about 30%. In addition, a comparison of measured to predicted H 2 SO 4 in the plumes suggests that the photochemical model predicts OH variability reasonably well but may underestimate actual values. Predictions of in-plume O 3 production agree well with the observations, suggesting that model-predicted peroxy radical (HO 2 + RO 2 ) levels are reasonable. The model estimated ozone production efficiency ranges from 6 to 30. The largest model bias was seen in the comparison with measured HNO 3 . The model overestimated in-plume HNO 3 by about a factor of 6. This is most likely caused by underestimated HNO 3 sinks possibly involving particle scavenging. However, limited data availability precluded a conclusive test of this possible loss process.