Multiple oxygen and sulfur isotope compositions of secondary atmospheric sulfate in a mega-city in central China

Sulfate aerosol is an important atmosphere constituent that can be formed secondarily through the oxidation of sulfur gases. Atmospheric sulfur oxidation can take different pathways depending on meteorological conditions, which affects sulfate aerosol size and composition and therefore local or global climate. The magnitude of O-17 enrichment (Delta O-17) in secondary atmospheric sulfate (SAS) is a tracer for the apportionment of different sulfur oxidation pathways. Atmospheric chemistry-transport models predict a low O-17 enrichment (Delta O-17 < 1 parts per thousand) for SAS in mid-latitude continental sites. However, there are few long-term site observations to test the prediction, and data from interior metropolitan sites are entirely absent. We report here multiple oxygen and sulfur isotope compositions (Delta O-17, delta O-18, and delta S-34) of SAS collected over a 950-day period in the city of Wuhan, central China, and to compare to data from a similar sampling campaign in the city of Baton Rouge, LA, U.S.A. The isotope compositions of bulk atmospheric sulfate closely reflect those of SAS in Wuhan, with the Delta O-17 ranging from 0.14 parts per thousand to 1.02 parts per thousand, the delta O-18 from 8.0 parts per thousand to 16.1 parts per thousand, and the delta S-34 from 2.1 parts per thousand to 7.3 parts per thousand. The average Delta O-17 value at 0.53 parts per thousand-0.59 parts per thousand is consistent with model prediction for continental interior, mid-latitude sites. The Asian monsoon-influenced meteorological condition in Wuhan appears to produce a weak but discernible seasonal pattern for Delta O-17 and delta O-18 of the SAS. The average rainwater pH value is higher in Wuhan than in Baton Rouge (5.47 versus 4.78) while the two cities have a statistically identical average SAS Delta O-17 value. We suggest that the higher pH does result in a higher fraction of SAS generated by aqueous 03 oxidation, but the resulted higher Delta O-17 value for SAS is diluted by the O-17-normal SAS generated from an enhanced transition-metal-catalyzed O-2 oxidation pathway. The enhancement is corroborated with the much higher content of atmospheric particulate matter especially mineral dusts in Wuhan, a point to be considered by future effort to quantify the climate impact of SO2. (C) 2013 Elsevier Ltd. All rights reserved.