The delta-O-17 and delta-O-18 measurements of atmospheric sulfate from a coastal and high alpine region - A mass-independent isotopic anomaly

they possess mass-independent oxygen isotopic compositions, unlike laboratory nonatmospheric standards. As suggested by the recent laboratory findings, the source of mass independent oxygen isotopic composition in sulfate is implied to be aqueous phase S(IV) oxidation by atmospheric H2O2 and O3. The relative S(IV) oxidation contribution by H2O2 and O3, which together generally represent the overall aqueous phase S(IV) oxidation, is estimated using the average O-17 enrichment value of La Jolla rainwater sulfate samples as an index. The estimate indicates a relative dominance of the aqueous over gas phase S(IV) oxidation. Although inconclusive owing to small data sets, both La Jolla and WMRS aerosol sulfate samples display seasonal 170 enrichment variations with a maximum in winter and a minimum in summer. Such seasonal isotopic variations are believed to derive from climatic effects that favor aqueous phase S(IV) oxidation in winter compared to summer. (Author)