Investigating the photo-oxidative and heterogeneous chemical production of HCHO in the snowpack at the South Pole, Antarctica

Abstract

Environmental context. Snowpacks present a surprisingly active environment for photochemistry, leading to sunlight-induced oxidation of deposited organic matter and the subsequent emission of a variety of photochemically active trace gases. We seek to address questions regarding the ultimate fate of organic matter deposited onto snow in the remote regions of the world. The work is relevant to atmospheric composition and climate change.Abstract. We investigate snowpack fluxes of formaldehyde (HCHO) into the South Pole boundary layer using steadystate photochemical models. We study two chemical sources of HCHO within the snowpack. First, we study chemical production of HCHO from the processing of methyl hydroperoxide (CH 3 OOH): photolysis, reaction with the hydroxyl radical (OH ), and by an acid catalysed rearrangement. Assuming surface layer concentration effects for acidic solutes, we show that the acid catalysed production of HCHO within ice could contribute a non-negligible source to the snowpack HCHO budget. This novel source of HCHO complements existing explanations of HCHO fluxes based on physical emission of HCHO from snow. Secondly, we investigate HCHO production from the oxidation of organic matter (OM) by OH within snow to explain observed fluxes of photochemical origin from the South Pole snowpack. This work shows that laboratory-derived photochemical production rates of HCHO and our standard model are inconsistent with field observations, which has implications for the distribution of OM relative to oxidants within ice particles. We resolve this inconsistency using new laboratory measurements of the molecular dynamics of the OH photofragment from hydrogen peroxide (H 2 O 2 ) and nitrate (NO 3 À ) photolysis, which show that only OH produced in the outermost monolayers can contribute to gas phase and surface layer chemistry. Using these new measurements in conjunction with realistic treatments of ice grain size, H 2 O 2 and NO 3 À distribution within ice grains, diffusion of gas species within solid ice, and observed OM particle size distributions yields snowpack HCHO photochemical production rates more consistent with observations.

Heterogeneous Photochemistry in the Atmosphere

“…For example, Hamer et al modeled photochemical production of formaldehyde in a snowpack from a variety of different sources, and concluded that CH 3 OOH photolysis is a relatively minor source of formaldehyde. 254 …”

“…Despite the theoretical interest in CH 3 OOH, its ice photochemistry may be too slow to affect the polar environments. For example, Hamer et al modeled photochemical production of formaldehyde in a snowpack from a variety of different sources, and concluded that CH 3 OOH photolysis is a relatively minor source of formaldehyde …”

“…For example, Hamer et al modeled photochemical production of formaldehyde in a snowpack from a variety of different sources, and concluded that CH 3 OOH photolysis is a relatively minor source of formaldehyde. 254 Many of the larger organic molecules that are of interest in the environmental chemistry of ice and snow absorb actinic radiation in the wavelength region reaching the ice at ground level and may therefore participate in direct photochemical degradation or may initiate indirect photochemistry by acting as photosensitizers. For some smaller oxygenated VOCs, the absorption properties depend strongly on their precise phase behavior once in contact with ice.…”

Surface Snow, Firn and Ice Core Composition in Polar Areas in Relation to Atmospheric Aerosol and Gas Concentrations: Critical Aspects

“…Many studies have observed significant changes in the concentration of gases contained in the deposited snow due to metamorphism, photolysis, diffusion out of the snow crystals, etc. [104]- [106] In addition, the distribution of impurities between the different domains of the condensed phase inside the snowpack (at the grain surface, in the bulk of the solid, in liquid trapped in confined pockets between grains), the presence of a multiphase system where liquid solutions and ice coexist, and changes in snow structure and properties affect both physical processes and Snow-grain metamorphism ATMOSPHERE chemical reactivity [107].…”