Diel variations of H 2 O 2 in Greenland: A discussion of the cause and effect relationship

Atmospheric hydrogen peroxide (H 2 O 2 ) measurements at Summit, Greenland, in May–June, 1993 exhibited a diel variation, with afternoon highs typically 1–2 parts per billion by volume (ppbv) and nighttime lows about 0.5 ppbv lower. This variation closely followed that for temperature; specific humidity exhibited the same general trend. During a 17‐day snowfall‐free period, surface snow was accumulating H 2 O 2 , apparently from nighttime cocondensation of H 2 O and H 2 O 2 . Previous photochemical modeling (Neftel et al., 1995) suggests that daytime H 2 O 2 should be about 1 ppbv, significantly lower than our measured values. Previous equilibrium partitioning measurements between ice and gas phase (Conklin et al., 1993) suggest that air in equilibrium with H 2 O 2 concentrations measured in surface snow (15–18 μM) should have an H 2 O 2 concentration 2–3 times what we measured 0.2–3.5 m above the snow surface. A simple eddy diffusion model, with vertical eddy diffusion coefficients calculated from balloon soundings, suggested that atmospheric H 2 O 2 concentrations should be affected by any H 2 O 2 degassed from surface snow. However, field measurements showed the absence of either high concentrations of H 2 O 2 or a measurable concentration gradient between inlets 0.2 and 3 m above the snow. A surface resistance to degassing, that is, slow release of H 2 O 2 from the ice matrix, is a plausible explanation for the differences between observations and modeled atmospheric profiles. Degassing of H 2 O 2 at a rate below our detection limit would still influence measured atmospheric concentrations and help explain the difference between measurements and photochemical modeling. The cumulative evidence suggests that surface snow adjusts slowly to drops in atmospheric H 2 O 2 concentration, over timescales of at least weeks. The H 2 O 2 losses previously observed in pits sampled over more than 1 year are thought to have occurred later in the summer or fall, after the May–July field season.