Observations and Modeling of NOx Photochemistry and Fate in Fresh Wildfire Plumes

With large primary emissions of nitrogen-containing compounds, wildfires impact the tropospheric oxidizing capacity, ozone (O-3), and formation of secondary organic and inorganic aerosol. The fate of reactive nitrogen in daytime fresh wildfire plumes was examined using airborne measurements over the western U.S. during the Wildfire Experiment for Cloud chemistry, Aerosol absorption, and Nitrogen (WE-CAN) campaign in the summer of 2018 together with a photochemical box model. For four wildfire plumes sampled in a pseudo-Lagrangian manner, the model predicts that the majority of emitted NOx (96 +/- 2%) is converted into peroxyacetyl nitrate (PAN) (27 +/- 8%) and the sum of gas and particulate HNO3 (29 +/- 5%) within a few hours of plume evolution. In two of the plumes with the highest initial NOx and HONO, the default model significantly underestimates the observed dilutionnormalized decay rate of NOx with plume age. We investigated several potential causes of this discrepancy and found that the model likely does not accurately represent the formation of a suite of oxidized organic nitrogen species such as alkyl and acyl peroxynitrates in these fire plumes, consistent with a suite of organic nitrogen compounds measured by chemical ionization mass spectrometry. This organic nitrogen reservoir can be similar in magnitude to that of PAN and thus represents an important fate of NOx with uncertain impacts on downwind O-3 and aerosol nitrate formation depending on whether these are acyl peroxynitrates (APNs), alkyl nitrates (RONO2), or nitro-aromatics.