Photochemical and dynamical processes affecting gaseous H2O2 concentrations in the lower troposphere

Abstract

Measurements of gas phase H2O2, O3, and SO2 in ambient air, along with other meteorological parameters, were made at the Summit (1.5 km, above mean sea level, amsl) and at the Lodge (0.6 km, amsl) on Whiteface Mountain situated in the Adirondack region of northern New York state. These measurements were made during the summer of 1997 (July‐August) to study the dynamical and chemical processes that control the distribution of H2O2 at a pristine mountainous site. H2O2 and O3 exhibited considerable variability from day to day. Gas phase H2O2 ranged from 0.3 to 4.3 ppb with a mean of 1.2±0.6 at the Summit, whereas at the Lodge the range was 0.10 to 4.6 ppb with a mean of 1.1±0.8. H2O2 exhibited a weak reverse diurnal variation at the Summit, whereas a diurnal variation was observed in the H2O2 levels at the Lodge. The daytime H2O2 concentrations at the Lodge were significantly higher than those at the Summit, while the nighttime values were higher at the Summit. The observed diurnal variations at the Lodge have been explained in terms of photochemical production of H2O2 during the day and scavenging by aqueous aerosols during the nighttime under the influence of a stable nocturnal boundary layer (NBL). Ozone was observed to have a reverse diurnal variation at both sites though the concentrations at the Summit were higher than at the Lodge. Nocturnal maxima in H2O2 and O3 concentrations under certain meteorological conditions were observed at both sites and are believed to be due to transport of these pollutants above the NBL after they are mixed down in the presence of a weak NBL. Two case studies are presented to elucidate the influence of atmospheric chemical and dynamical processes on the ambient concentration of H2O2 and O3. Results of multivariate statistical analysis show that photochemical production is most important in regulating the formation of H2O2 at the Lodge, whereas at the Summit meteorological processes are most important.