High-Precision Measurements of [superscript 33]S and [superscript 34]S Fractionation during SO[subscript 2] Oxidation Reveal Causes of Seasonality in SO[subscript 2] and Sulfate Isotopic Composition

Abstract

This study presents high-precision isotope ratio-mass spectrometric measurements of isotopic fractionation during oxidation of SO[subscript 2] by OH radicals in the gas phase and H[subscript 2]O[subscript 2] and transition metal ion catalysis (TMI-catalysis) in the aqueous phase. Although temperature dependence of fractionation factors was found to be significant for H[subscript 2]O[subscript 2] and TMI-catalyzed pathways, results from a simple 1D model revealed that changing partitioning between oxidation pathways was the dominant cause of seasonality in the isotopic composition of sulfate relative to SO[subscript 2]. Comparison of modeled seasonality with observations shows the TMI-catalyzed oxidation pathway is underestimated by more than an order of magnitude in all current atmospheric chemistry models. The three reactions showed an approximately mass-dependent relationship between [superscript 33]S and [superscript 34]S. However, the slope of the mass-dependent line was significantly different to 0.515 for the OH and TMI-catalyzed pathways, reflecting kinetic versus equilibrium control of isotopic fractionation. For the TMI-catalyzed pathway, both temperature dependence and [superscript 33]S/[superscript 34]S relationship revealed a shift in the rate-limiting reaction step from dissolution at lower temperatures to TMI-sulfite complex formation at higher temperatures. 1D model results showed that although individual reactions could produce Δ[superscript 33]S values between −0.15 and +0.2‰, seasonal changes in partitioning between oxidation pathways caused average sulfate Δ[superscript 33]S values of 0‰ throughout the year.