H₂O₂ and O₃ in the atmosphere of Los Angeles and its vicinity: Factors controlling their formation and their role as oxidants of SO₂

Factors regulating the formation of gaseous H 2 O 2 and O 3 , which are considered to be dominant oxidants for SO 2 in the atmosphere, were studied in Los Angeles and its vicinity. The concentration of gaseous H 2 O 2 , as well as O 3 , was highest in the early afternoon at all the sites studied when the sky was clear or partially cloudy, whereas when it was hazy and foggy, H 2 O 2 concentrations were substantially lower than those under clear or partially cloudy conditions. To study the seasonal variation of gaseous H 2 O 2 , its concentration was measured in west Los Angeles in the early afternoon (1200–1600 LT) during August 1985 to September 1988 as well as at other southern California sites. The observed total range of gaseous H 2 O 2 concentration was 0.03–1.35 ( n = 150), 0.20–2.04 ( n = 16), and 0.43–1.72 ( n = 13) ppb at Los Angeles, the Mojave Desert, and the San Bernardino Mountains, respectively. The mean seasonal H 2 O 2 concentrations at Westwood, Los Angeles, were 1 ppb in summer and approximately 0.2 ppb in winter. The O 3 concentration during each H 2 O 2 measurement period ranged from 10 to 230, 48 to 95, and 120 to 160 ppb for the same sampling locations, and seasonal variation was not as pronounced as for H 2 O 2 . Statistical analysis indicated that (1) H 2 O 2 was most correlated with solar radiation, whereas O 3 was most correlated with temperature, the height of inversion layer, and the concentration of NO x ; and (2) H 2 O 2 concentration was inversely correlated with SO 2 during warm months of 1987–1988, when haze events were frequently observed. From our observations we deduce that in Los Angeles air (1) the production rate of sulfuric acid from SO 2 is significantly lower in winter than summer due to lack of sufficient amounts of oxidants, and (2) thus the reduction of SO 2 emission from anthropogenic sources, which has been accomplished by electric utilities, has not resulted in the equivalent reduction in sulfuric acid in rainwater, where rain events principally occur in winter.