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 Just Accepted

This article has been peer reviewed and accepted for publication. It is in production and has not been edited, so may differ from the final published form.


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

Paul Hamer, Dudley Shallcross, Akihiro Yabushita, Masahiro Kawasaki, Virginie Marécal, Chris Boxe

Abstract

We investigate snowpack fluxes of formaldehyde (HCHO) into the boundary layer at the South Pole using steady state photochemical models. We study two chemical sources of HCHO within the snowpack. In the first instance, we study the chemical production of HCHO from the processing of methyl hydroperoxide (CH3OOH) via photolysis, reaction with the hydroxyl radical (OH), and via 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 budget of HCHO. This newly proposed source of HCHO directly complements existing explanations of HCHO fluxes that are based on physical emission of HCHO from snow grains. We investigate a second source of HCHO production from the oxidation of organic matter (OM) by OH within snow in an attempt to explain previously identified fluxes of photochemical origin from the South Pole snowpack. Initially, this second analysis 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. To deal with this inconsistency we use new laboratory measurements of the molecular dynamics of the OH photofragment from hydrogen peroxide (H2O2) and nitrate (NO3-) photolysis and show that OH generated in the outer ice monolayers can either penetrate the surface layer to participate in gas phase chemistry or reside within the surface to engage in aqueous phase chemistry in thin liquid films. Using these newly available measurements in conjunction with realistic model 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.

EN13227  Accepted 01 May 2014
 
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