Measurements of hydrogen peroxide and formaldehyde exchange between the atmosphere and surface snow at Summit, Greenland

Tower-based measurements of hydrogen peroxide (H2O2) and formaldehyde (HCHO)exchange were performed above the snowpack of the Greenland ice sheet. H2O2 andHCHO fluxes were measured continuously between June 16 and July 7, 2000, at theSummit Environmental Observatory. The fluxes were determined using coilscrubber-aqueous phase fluorometry systems together with micrometeorologicaltechniques. Both compounds exhibit strong diel cycles in the observed concentrationsas well as in the fluxes with emission from the snow during the day and the eveningand deposition during the night. The averaged diel variations of the observed fluxeswere in the range of +1.3 · 10^13 molecules m^-2 s^-1 (deposition) and-1.6 · 10^13 molecules m^-2 s^-1 (emission) for H2O2 and +1.1 · 10^12molecules m^-2 s^-1 and -4.2 · 10^12 molecules m^-2 s^-1 for HCHO, while the netexchange per day for both compounds were much smaller. During the study period of22 days on average 0.8 (+4.6/-4.3) · 10^17 molecules m^-2 of H2O2 were depositedand 7.0 (+12.6/-12.2) · 10^16 molecules m^-2 of HCHO were emitted from the snowper day. A comparison with the inventory in the gas phase demonstrates that theexchange influences the diel variations in the boundary layer above snow covered areas.Flux measurements during and after the precipitation of new snow shows that less than16 % of the H2O2 and more than 25 % of the HCHO originally present in the new snowwere available for fast release to the ABL within hours after precipitation. This releasecan effectively disturb the normally observed diel variations of the exchange betweenthe surface snow and the atmosphere, thus perturbing also the diel variations ofcorresponding gas phase concentrations

Discrepancies between formaldehyde measurements and methane oxidation model predictions in the Antarctic troposphere: An assessment of other possible formaldehyde sources

Abstract. Formaldehyde (HCHO) is a key intermediate in the photooxidation of methane by hydroxyl radicals. Through its photolysis it is also a source for free radicals in the troposphere. Owing to these reactions, HCHO influences the oxidation capacity of the atmosphere and is a suitable species to test our current understanding of atmospheric oxidation pathways. Especially in polar regions, discrepancies between measurements and model calculations exist. Though recent investigations in the Arctic suggest that HCHO emissions from the snow surface might act as the missing source, the question remains unresolved for the Antarctic. We compare year-round HCHO measurements in Antarctica with model results from a simple photochemical box model. The observed ambient HCHO mixing ratios cannot be explained by methane photooxidation alone. Inclusion of HCHO emissions from the snow surface makes the model results and measurements consistent, but significantly higher emissions than those derived in the Arctic are needed to explain the observed HCHO mixing ratios. We discuss other possible sources such as oxidation of dimethylsulfide (DMS), isoprene, ethene, propene, and the effect of halogens, that may be responsible for the enhanced HCHO mixing ratios in the marine Antarctic troposphere. We find that, for the largest HCHO mixing ratio measured, methane is likely to produce only about 9% of the required HCHO; isoprene (including generated propene) about 22%; and ethene, DMS and halogens together only 7%. If the remaining HCHO is produced by a flux from the snow, the flux required is about 1.9 x 1013 molecules m-2 s-1