Measurement and interpretation of concentrations of H₂O₂ and related species in the upper midwest during summer

Abstract

Measurements of H 2 O 2 and related species were made by aircraft on 18 flights over 11 days during June 1987 over Ohio. Measurements typically encompassed an altitude range from the surface to 5.5 km above mean sea level and were conducted under a variety of meteorological conditions, but focussed on conditions conducive to the formation of convective storms. Concentrations of H 2 O 2 ranged from <0.2 to 7 ppb and exhibited as much as a factor of 10 variability even on a given day at fixed altitude, as well as between days. Hydrogen peroxide concentration typically was low near the surface, rose to a maximum near the top of the boundary layer, then slowly decreased with altitude. In contrast, SO 2 and NO y were largely confined to the surface layer. A simple photochemical calculation shows that where NO x concentrations are low, [H 2 O 2 ] is strongly influenced by the water vapor concentration. The [H 2 O 2 ] tracked changes in dew point, and measured and calculated H 2 O 2 concentrations were in general agreement. The low [H 2 O 2 ] in the boundary layer is ascribed to consumption of radicals by reactions other than combination reactions, and to loss of H 2 O 2 by dry deposition. Comparison of the profile shapes for H 2 O 2 and SO 2 suggests that mixing processes will have a large influence on the extent to which these two species will react in convective systems.