DIEL VARIATIONS OF H2O2 IN GREENLAND - A DISCUSSION OF THE CAUSE AND EFFECT RELATIONSHIP

Atmospheric hydrogen peroxide (H2O2) measurements at Summit, Greenland , in May-June, 1993 exhibited a diel variation, with afternoon highs t ypically 1-2 parts per billion by volume (ppbv) and nighttime lows abo ut 0.5 ppbv lower, This variation closely followed that for temperatur e; specific humidity exhibited the same general trend, During a 17-day snowfall-free period, surface snow was accumulating H2O2, apparently from nighttime cocondensation of H2O and H2O2. Previous photochemical modeling (Neftel et al., 1995) suggests that daytime H2O2 should be ab out I ppbv, significantly lower than our measured values, Previous equ ilibrium partitioning measurements between ice and gas phase (Conklin et al., 1993) suggest that air in equilibrium with H2O2 concentrations measured in surface snow (15-18 mu M) should have an H2O2 concentrati on 2-3 times what we measured 0.2-3.5 m above the snow surface, A simp le eddy diffusion model, with vertical eddy diffusion coefficients cal culated from balloon soundings, suggested that atmospheric H2O2 concen trations should be affected by any H2O2 degassed from surface snow, Ho wever, field measurements showed the absence of either high concentrat ions of H2O2 or a measurable concentration gradient between inlets 0.2 and 3 m above the snow. A surface resistance to degassing, that is, s low release of H2O2 from the ice matrix, is a plausible explanation fo r the differences between observations and modeled atmospheric profile s, Degassing of H2O2 at a rate below our detection limit would still i nfluence measured atmospheric concentrations and help explain the diff erence between measurements and photochemical modeling, The cumulative evidence suggests that surface snow adjusts slowly to drops in atmosp heric H2O2 concentration, over timescales of at least weeks. The H2O2 losses previously observed in pits sampled over more than 1 year are t hought to have occurred later in the summer or fall, after the May-Jul y field season.