| 09 Sep 2026
Direct Measurements of N2O5 Reactivity in Salt Lake City Suggest the Aerosol Nitrate Effect Suppresses Additional PM2.5 Accumulation
Abstract. Salt Lake City (SLC) frequently experiences severe wintertime pollution due to the accumulation of fine particulate matter, which is dominated by ammonium nitrate (AN) during atmospheric inversion events. Recent measurements in SLC have shown that heterogeneous reactions between gaseous dinitrogen pentoxide (N2O5) and ambient aerosol drives AN accumulation in SLC. However, information is lacking on the ambient aerosol sources present, and how efficiently these particles react with N2O5 to form AN. As such, we deployed an ambient aerosol flow tube coupled to a chemical ionization mass spectrometer and particle sizing instrumentation at the University of Utah in winter to probe the reactive uptake of N2O5 (γN2O5) that leads to AN formation. Aerosol particle composition was measured from offline filter collections and ion chromatography to determine how different particle sources react with N2O5. Under inversion conditions, we measured suppressed values of γN2O5 (median γN2O5 = 0.03 ± 0.04) that decreased to as low as 0.001 when high AN was measured, suggesting that the nitrate effect suppresses γN2O5 by ~ 0.02 per night. Under clean conditions, we observed elevated values of γN2O5 (median γN2O5 = 0.06 ± 0.05) due to efficient heterogeneous reactions with mineral dust, ammonium sulfate, and road salt aerosol with no notable decrease in γN2O5 throughout clean periods. Using a 1-dimensional chemical box model, we find that aerosol nitrate accumulation is limited when γN2O5 is less than 0.01 throughout the evening. These results suggest that the most important influence on γN2O5 in wintertime SLC is the nitrate effect.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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