High-Precision Measurements of ~(33)S and ~(34)S Fractionation during SO_2 Oxidation Reveal Causes of Seasonality in SO_2 and Sulfate Isotopic Composition

摘要

This study presents high-precision isotope ratio-mass spectrometric measurements of isotopic fractionation during oxidation of SO_2 by OH radicals in the gas phase and H_2O_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_2O_2 and TMI-catalyzed pathways, results from a simple ID model revealed that changing partitioning between oxidation pathways was the dominant cause of seasonality in the isotopic composition of sulfate relative to SO_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 ~(33)S and ~(34)S. However, the slope ot 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 ~(33)S/~(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. ID model results showed that although individual reactions could produce △~(33)S values between -0.15 and +0.2‰, seasonal changes in partitioning between oxidation pathways caused average sulfate △~(33)S values of 0‰ throughout the year.