Abstract
The estimated contribution of “primary” combustion sulfate to the total atmospheric sulfate flux is inconsistent, hindering our interpretation of the atmospheric “missing sulfate”. We expect combustion sulfate to inherit the characteristic triple oxygen isotope composition of air O2 to variable degrees. To test the hypothesis, we extracted sulfate from soot samples collected from 48 residential chimneys throughout China. The mean sulfate's Δ′17O0.5305 and δ18O values are −0.27 ± 0.06‰ and 12.8 ± 3.6‰, respectively, and the Δ′17O0.5305 and δ18O values correlate negatively and extend to air O2's triple oxygen isotope composition as an endmember, suggesting that combustion sulfate is generated dominantly by S(IV) multiphase oxidation via O2 in the plume, with possible additional pathways through •OH in aqueous films or droplets and NO2 at soot surface. Oxygen sources (i.e., air O2, fuel, ambient moisture) and oxidation kinetics determine the triple oxygen isotope composition of combustion sulfate. The characterization of combustion sulfate's triple isotope composition helps to improve global and regional atmospheric sulfur chemistry models.
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