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

We investigate snowpack fluxes of formaldehyde (HCHO) into the South Pole boundary layer using steady-state 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 (CH₃OOH): 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 (H₂O₂) and nitrate (NO₃^-) 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, H₂O₂ and NO₃ ^- 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.