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

Abstract

With large primary emissions of nitrogen-containing compounds, wildfires impact the tropospheric oxidizing capacity, ozone (O3), 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 NO x (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 NO x and HONO, the default model significantly underestimates the observed dilution-normalized decay rate of NO x 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 NO x with uncertain impacts on downwind O3 and aerosol nitrate formation depending on whether these are acyl peroxynitrates (APNs), alkyl nitrates (RONO2), or nitro-aromatics.

Direct Constraints on Secondary HONO Production in Aged Wildfire Smoke From Airborne Measurements Over the Western US

“…As noted above, variations in these parameters may arise from aerosol particle properties, for example, pH, water content, and organic nitrate to inorganic nitrate fraction, among others. The fraction of total particulate nitrate that is inorganic nitrate in wildfire smoke may add uncertainty to our conclusions, though previous box modeling of WE‐CAN plumes suggest total fine mode nitrate can be explained with mostly inorganic nitrate in the near field (Peng et al., 2021).…”

“…In Figure 4, we compare direct HONO emissions to the secondary HONO sources evaluated above using simulations of the evolution of two fire plumes where in situ plume sampling included a sequence of downwind cross‐plume intercepts. Adding secondary HONO sources to the F0AM model (Peng et al., 2021) improved HONO simulations compared to observations especially in the older plume portions. p NO 3 photolysis becomes more important with aging and more predominant in less polluted events (i.e., lower NO x loading), while NO 2 ‐related reactions and homogeneous HONO formation are more important in early, higher NO x stages of the plume.…”

Wildfire Smoke Influence on Cloud Water Chemical Composition at Whiteface Mountain, New York

“…SO 2 is produced from anthropogenic and biomass burning emissions (Chuang et al., 2016) and is oxidized to form sulfate, which can participate in brown carbon production in the aqueous phase (De Haan et al., 2020). NO rapidly reacts in wildfire plumes to produce nitroaromatic compounds (Becidan et al., 2007; Peng et al., 2021; Selimovic et al., 2020), and anthropogenic sources of NO can mix with smoke plumes to increase O 3 levels (Jacob et al., 2010). Together, these trace gas trends give greater confidence to the smoke classification used in this study and further show the influence of the transport of wildfire smoke on air quality.…”

Atmospheric OH reactivity in the western United States determined from comprehensive gas-phase measurements during WE-CAN

“…Alternatively, box models such as the Framework for 0-D Atmospheric Modeling (F0AM) can simulate chemistry in BB plumes for tens of thousands of reactions and species using detailed explicit chemical mechanisms such as the Master Chemical Mechanism (MCM). 13,18,19 How well these models represent the tOHR and its evolution downwind is bound by the number of reactive species included in the chemical mechanisms, how well we understand their cascade oxidation, and whether these species are included in model initialization. The extent to which the MCM incorporates most of the reactive species emitted from wildfires is not well known due to historically scarce field observations to fully characterize BB smoke.…”