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 SO 2 to H 2 SO 4 in cloud water where the typical p H 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 H 2 O 2 available to the cloud‐precipitation system. Field observations of H 2 O 2 during Acid Precipitation Experiment (APEX) missions revealed that in addition to vapor phase H 2 O 2 , there were present one or more atmospheric constituents which produced H 2 O 2 in the aqueous phase, and there were other species which consumed the collected H 2 O 2 vapor. We report here an investigation of these phenomena undertaken to ensure that the instrumentation was responding to H 2 O 2 alone and to identify the species which interfered in the aqueous collection and measurement of H 2 O 2 vapor. Our results showed that (1) the H 2 O 2 found within the collectors frequently exceeded the amount initially present as vapor, (2) the species which produced H 2 O 2 were relatively insoluble, (3) NO 2 did not enhance H 2 O 2 formation, (4) O 3 enhanced H 2 O 2 formation, and (5) SO 2 depleted aqueous H 2 O 2 . The amount of aqueous H 2 O 2 lost when SO 2 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 H 2 O 2 available for SO 2 oxidation would not be limited by the amount of H 2 O 2 vapor in precloud air but would also depend upon the concentration of the precursors capable of generating H 2 O 2 in droplets.