Factors affecting the levels of hydrogen peroxide in rainwater

Measurements of hydrogen peroxide (H2O2) and several meteorological and chemical parameters were made for 34 rain events which occurred in Miami, Florida between April, 1995 and October, 1996. The measured H2O2 concentrations ranged from 0.3 to 38.6 μM with an average concentration of 6.9 μM. A strong seasonal dependence for H2O2 concentrations was observed during this period, with highest concentrations in the summer and lower levels in the winter, which corresponds to the stronger solar radiation and higher vaporization of volatile organic compounds (VOCs) in the summer and fall, and the weaker sunlight and lower vaporization in the winter and spring. Measurements also showed a significant increase trend of H2O2 with increasing ambient rainwater temperature. Rains that were out from lower latitude were exposed to higher solar irradiation and contained relatively higher levels of H2O2 than those from the north. All these observations indicate that photochemical reactions that involved volatile organic compounds are the predominant source of H2O2 observed in rainwater. During several individual rainstorms, H2O2 concentration was found to increase as a function of time due to electrical storm activities. This finding suggests that lightning could be an important factor that determines the level of H2O2 during thunderstorms. Statistical data showed that the highest concentrations of H2O2 were observed only in rains containing low levels of nonsea-salt sulfate (NSS), nitrate and hydrogen ion. H2O2 concentrations in continental originated rains were much lower than marine originated ones, indicating that air pollutants in continental rains could significantly deplete the H2O2 concentration in atmospheric gas-phase, clouds and rainwater.