Abstract
Formaldehyde (HCHO) measurements in snow, firn, atmosphere, and air in the open pore space of the firn (firn air) at Summit, Greenland, in June 1996 show that the top snow layers are a HCHO source. HCHO concentrations in fresh snow are higher than those in equilibrium with atmospheric concentrations, resulting in HCHO degassing in the days to weeks following snowfall. Maximum HCHO concentrations in firn air were 1.5-2.2 ppbv, while the mean atmospheric HCHO concentration 1 m above the surface was 0.23 ppbv. Apparent HCHO fluxes out of the snow are a plausible explanation for the discrepancy between the 0.1 ppbv atmospheric concentration predicted by photochemical modeling and the measurements. HCHO in deeper firn is near equilibrium with the lower tropospheric HCHO concentration at the annual average temperature. Thus HCHO in ice may in fact be linearly related to multiyear average atmospheric concentrations through a temperature dependent partition coefficient.
Introduction
The main source of HCHO in the remote troposphere is oxidation of methane (CH4) by OH followed by further reactions. Past atmospheric OH levels inferred from HCHO and CH4 in ice cores [Staffelbach et al., 1991] could not be reproduced with photochemical modeling [Thompson, 1995], and it became evident that a better understanding of the transfer processes relating atmospheric and ice concentrations is imperative for a quantitative interpretation of ice cores. Previous studies show that HCHO concentrations in fresh snow at Summit, Greenland are much higher than those preserved in firn, suggesting that surface snow at Summit is a HCHO source during summer [Fuhrer et al., 1996]. This source has the potential to significantly affect atmospheric HCHO concentrations as well as the amount of HCHO preserved in the ice. Further, photochemical model calculations predict summertime atmospheric HCHO concentrations of about 0.1 ppbv [Staffelbach et al., 1991], about half of average measured values [Fuhrer et al., 1996]. The purpose of this study was to test the hypothesis that the HCHO flux out of the snow pack in summer can be attributed to a temperature dependent release of HCHO from the near-surface snow and firn as the HCHO concentrations in air and ice approach equilibrium.
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