Multiple oxygen and sulfur isotope compositions of secondary atmospheric sulfate in the city of Wuhan, central China

Secondary atmospheric sulfate (SAS) is the oxidation product and sink for sulfur gases of biological, volcanic, and anthropogenic origins on Earth. SAS can be produced from gas-phase OH-radical oxidation and five aqueous-phase chemical reactions including aqueous-phase S (IV) oxidation reactions by H2O2, O3, oxygen catalyzed by Fe3+ and Mn2+, and methyle hydrogen peroxide and peroxyacetic acid. The tropospheric sulfur oxidation pathway is therefore determined by cloud-water pH, dissolved [Fe2+] or [Mn2+] content, S emission rate, meteorological condition, and other factors. The S isotope composition is a good tracer for the source while the O isotopes, especially the triple O isotope compositions are a good tracer for S oxidation pathway. Jerkins and Bao (2006) provided the first set of multiple stable isotope compositions (δ34S, δ18O and Δ17O) for SAS collected from bulk atmosphere in Baton Rouge in the relatively rural southern USA. Their study revealed a long-tern average Δ17O value of ~+0.7‰ for SAS, and speculated that much of the Earth mid-latitudes may have a similar average SAS Δ17O value. Additional sampling campaign at different sites is necessarily for constructing and testing models on sulfur oxidation and transport in the troposphere. A total of 33 sulfate samples were collected from bulk atmospheric deposition over a 950-day period from May 2009 to December 2011 in the city of Wuhan, Hubei Province, China. Differing from Baton Rouge, Wuhan is an industrial metropolis with a population of 9.8 million and a high particulate matter content (115 μg/m3). It also has a subtropical monsoon climate, with rainwater pH at ~5.3 year-around. The rainwater ion concentrations have seasonal variations, typically low in summer and high in winter. The anions are dominated by SO42-, at an average concentration of 8.5 mg/L. There is little sulfate contribution from sea-salt (SS) sulfate or dusts in Wuhan. The isotopic compositions for bulk atmospheric sulfate range from 0.00‰ to 1.02‰ for the Δ17O, 8.0‰ to 17.8‰ for the δ18O, and 2.1‰ to 24.1‰ for the δ34S. No apparent correlation is found among Δ17O, δ18O, or δ34S values. No significant temporal pattern exists for the Δ17O over the collection period. The positive Δ17O values for SAS have a time-weighted average of 0.52 ± 0.23‰, lower than the average in Baton Rouge, raising the possibility that the high particulate matter content in Wuhan may have played a role in promoting S oxidation via surface and/or Fe(III)-catalyzed pathways that do not generate positive 17O anomaly in product sulfate. The average Δ17O value also supports the assertion that the long-term average Δ17O value for SAS in the mid-latitude sites fall within a range (0.6~0.8‰) that is much lower than that in polar areas. The SAS δ18O values in Wuhan lie within the range reported for other sites (+5.0‰ to +19.6‰), with a time-weighted average value of 12.0 ± 2.3‰. Not counting three outlier (>13‰), the δ34S values are at a narrow range with a time-weighted average of +4.5 ±1.3‰ (n=30), which is higher than those from Baton Rouge but is typical for the heavily populated regions in China.