Evidence for aqueous phase hydrogen peroxide synthesis in the troposphere

Abstract

Recent studies of the oxidation of sulfur dioxide by hydrogen peroxide in solution indicate that this process may be the dominant mechanism for the conversion of SO2 to H2SO4 in cloud water where the typical pH is less than 5.5. In the interpretation of theoretical calculations and previous hydrogen peroxide measurements it has generally been assumed that the limiting factor in the acidification of precipitation is the amount of gas‐phase H2O2 available to the cloud‐precipitation system. Field observations of H2O2 during Acid Precipitation Experiment (APEX) missions revealed that in addition to vapor phase H2O2, there were present one or more atmospheric constituents which produced H2O2 in the aqueous phase, and there were other species which consumed the collected H2O2 vapor. We report here an investigation of these phenomena undertaken to ensure that the instrumentation was responding to H2O2 alone and to identify the species which interfered in the aqueous collection and measurement of H2O2 vapor. Our results showed that (1) the H2O2 found within the collectors frequently exceeded the amount initially present as vapor, (2) the species which produced H2O2 were relatively insoluble, (3) NO2 did not enhance H2O2 formation, (4) O3 enhanced H2O2 formation, and (5) SO2 depleted aqueous H2O2. The amount of aqueous H2O2 lost when SO2 vapor was added to ambient air was larger than expected. The results of these experiments have important implications for cloud water chemistry. If the processes occurring within the impinger can be extrapolated to droplets, the amount of H2O2 available for SO2 oxidation would not be limited by the amount of H2O2 vapor in precloud air but would also depend upon the concentration of the precursors capable of generating H2O2 in droplets.

The role of clouds in tropospheric photochemistry

“…The production of HzO 2 in the cloud-interstitial gas phase (G21) is strongly reduced due to scavenging of HO 2 by the droplets. However, in-cloud production of H202 can be quite efficient (Heikes et al, 1982;Chameides, 1984;Jacob, 1986). Actually, the reactions (A9)-(A10) are more efficient than their gas phase counterpart (G21) in the absence of clouds.…”

Aqueous oxidation of SO2 by hydrogen peroxide

“…Both as a result of the importance of the aqueous oxidation of S(IV) to S(VI) by H20: and the use of H20:-HSO3-reaction kinetics to determine H:O: [Heikes et al, 1982] a study was undertaken to verify the reaction rate constants determined as a function of pH by other investigators. Four major differences in experimental conditions existed between this study and the work of other investigators [Hoffman and Edwards, 1975;Penkett et al, 1979;Martin and Darnschen, 1981 3.…”

Measurement of gas phase hydrogen peroxide in air by tunable diode laser absorption spectroscopy

“…Bufalini Heikes, 1984]. In addition, the aqueous trap methods may suffer from SO2 interferences [Heikes et al, 1982] and from interference by organic peroxides, which are also expected to be present in tropospheric air [Logan et al, 1980]. Although progress has been made in overcoming these artifacts (A. L. Lazrus, private communi-Direct measurement of H202 in the gas phase avoids these artifacts effects.…”