No Evidence for S Isotope Fractionation During SO 2 Oxidation at a Continental Location

A Norman, H Krouse, J Macleod
2014 Journal of Undergraduate Research in Alberta   unpublished
S ulfur isotope fractionation during SO 2 oxidation has been shown to occur in laboratory experiments 1 but this has not been observed in whole air samples 2. Here we replicate the laboratory experiments in a systematic manner using ambient air. Particulate matter and SO 2 in ambient air were collected at Calgary in the Fall of 2012 using high volume samplers and impingers. Atmospheric SO 2 and SO 4 concentrations and isotope characteristics were determined. Variations in the concentration of
more » ... 2 and SO 4 in the atmosphere did not reflect changes in δ 34 S values, suggesting the independence of δ 34 S from concentration. Average δ 34 S SO2 (± std. dev.) from the high volume samplers was + 13.2 ± 0.2. SO 2 from the impinger method over the same sampling period yielded a δ 34 S SO2 value of +14.0 ± 0.2. Standard deviation for the impinger samples could not be calculated and hence this ± 0.2 is the precision due to the spectrometer. δ 34 S SO4 values (± std. dev.) ranged from + 9.9 ± 0.5 to +15.3 ± 0.2. δ 34 S SO2 values from the high volume and impinger samples were similar (+13.2 versus +14.0, respectively) which shows these collection methods are equivalent. Differences between the impinger and high volume sampler δ 34 S values for SO 2 and submicron aerosol SO 4 were used to gauge sulfur isotope fractionation. Standard deviations for differences were greater than averages (∆δ 34 S SO2 avg.=-0.80, s=1.76; fine ∆δ 34 S SO4 avg.=+0.28 , s=5.15), indicating little to no fractionation. Additionally, δ 34 S SO2 and δ 34 S SO4 values were compared to the maximum percent SO 2 that may have reacted to form SO 4. No pattern was evident so the conclusion is that sulfur isotope fractionation in ambient air is negligible under the conditions sampled. Literature: 1.Harris, E., Sinha, B., Hoppe, P., Foley, S., Borrmann, S. (2012). Fractionation of sulfur isotopes during heterogeneous oxidation of SO 2 on sea salt aerosol: a new tool to investigate non-sea salt sulfate production in the marine boundary layer.