Modeling atmospheric sulfate oxidation chemistry via the oxygen isotope anomaly using the Community Multiscale Air Quality Model (CMAQ)

Atmospheric sulfate formation influences climate and air quality, yet its chemical pathways remain difficult to constrain. This study utilizes the oxygen isotope anomaly (Δ17O) of sulfate aerosol (ASO4) as a tracer to distinguish formation processes. This work presents a simulation of Δ17O(ASO4) within the contiguous United States, conducted over full annual cycles, which enables the quantification of seasonal and spatial patterns of sulfate oxidation pathways and their response to major emission reductions, for the first time at this scale and temporal coverage. In 2019, Δ17O(ASO4) values were predicted to be below 1 ‰ in the Gulf Coast, indicating acidic, ASO4-rich conditions dominated by S(IV) + H2O2 oxidation, while values above 2 ‰ in the West suggested less acidic conditions, leading to enhanced ASO4 production via S(IV) + O3 oxidation. Peak Δ17O(ASO4) values of ∼4.5 ‰ in April across the Western US reflected O3-driven ASO4 formation during high ammonia (NH3) emissions from fertilization. Between 2006 and 2019, mean Δ17O(ASO4) was predicted to increase by up to 2 ‰, driven by declining sulfur dioxide (SO2) emissions from regulatory measures. Model comparisons with historical measurements show reasonable agreement in the acidic southeastern US (RMSE = 0.20 ‰, Baton Rouge, LA). However, the model overpredicts Δ17O(ASO4) in the Western US with RMSE values of 0.36 ‰ (La Jolla, CA) and 1.9 ‰ (White Mountain Research Center, CA). This overestimation suggests an excessive model response to aqueous S(IV) + O3 reactions. These findings underscore the diagnostic potential of Δ17O(ASO4) for assessing sulfate formation mechanisms and pinpointing shortcomings in chemical transport models. However, Δ17O(ASO4) observations across the United States remain exceedingly limited, with most available data dating back to the late 1990s and early 2000s, highlighting the need for renewed measurement efforts.