Photochemical HCHO and H2O2 Processing in Snow at Summit, Greenland, and at South Pole

Heterogeneous photochemistry and temperature-driven recycling of formaldehyde (HCHO) and hydrogen peroxide (H₂O₂) in snow can significantly alter the composition of both the snow and the overlying atmospheric boundary layer. This has important consequences for the interpretation of ice-core records to understand the past oxidizing capacity of the atmosphere, as well as for the basic understanding of gas-phase photochemistry above snowpacks. Previous field and laboratory experiments combined with physically based air-snow transfer modeling showed that temperature-driven uptake and release explains at least 75% if not all of the observed net snow-air fluxes of both species at Summit, Greenland and South Pole, Antarctica. Other studies indicated that, at least for HCHO, some of the observed net flux is due to photochemical production in the snow. However, its actual contribution to the total observed flux from snowpacks into the boundary layer has not yet been quantified. Here we present new HCHO and H₂O₂ data from 2 field campaigns at Summit, Greenland, in summer 2003 and spring 2004 and from the ANTCI field campaign at South Pole in December 2003. Both species were measured in ambient air, and in firn air drawn from various depths in the snowpack while a large area of the snowpack (4m²) was intermittently shaded. Filters of various transmittances of UV radiation were used during the shading experiments in order to separate the impact of changing temperature and radiation on the firn-air mixing ratios. In addition, profiles of temperature, radiation, and concentrations of HCHO and H₂O₂ in the snow phase were measured. Combined with new modeling studies, these data are used to quantify the relative importance of both, UV and temperature on firn air mixing ratios and snow-air fluxes for the wide range of conditions encountered at both locations and during different times of the year.