Use of Oxygen Isotopy in the Study of SO sub 2 to Sulfates in the Atmosphere

摘要

Previous experimental results have indicated that the exp 18 O/ exp 16 O ratios in water vapor, precipitation water, and sulfate contained in precipitation water vary seasonally (higher in summer, lower in winter), while the exp 18 0/ exp 16 0 ratio in atmospheric particulate sulfate appears to vary randomly with season. The samples were arbitrarily intermittent and therefore did not necessarily represent continuous, temporal variations of the oxygen isotopes, except during the three-day sampling period within each month. It was postulated on the basis of the data obtained, that since the two-year averages of exp 18 0 content in particulate sulfate and precipitation sulfate were about equal, both were probably formed by the same aqueous oxidation mechanism in clouds. In recent studies, sequential seven-day samples of water vapor, particulate sulfate and sulfur dioxide were continuously collected throughout the four seasons. Samples of rain and snow were also collected for most of the precipitation events during this period. The results obtained from this more intensive sampling program give an alternative and possibly more accurate interpretation of the apparent lack of agreement between the isotopic qualities of precipitation and particulate sulfates. Results indicate that SO sub 2 in the atmosphere is converted to sulfate in the late fall, winter, and early spring predominantly by an aqueous oxidation mechanism that yields sulfates isotopically similar, if not identical, to sulfates found in precipitation water: while in the late spring, summer, and early fall, SO sub 2 is converted predominantly by a sun-induced mechanism that yields sulfates more depleted in exp 18 0. The predominating summer mechanism(s) may involve reactions of SO sub 2 with 0H radicals or other free radicals, or reactions with photochemically produced aqueous-phase oxidants, such as H sub 2 0 sub 2 . (ERA citation 05:012418)