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Reactive nitrogen (Nr) within smoke plumes plays important roles in the production of ozone, the formation of secondary aerosols, and deposition of fixed N to ecosystems. The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) field campaign sampled smoke from 23 wildfires throughout the western U.S. during summer 2018 using the NSF/NCAR C‐130 research aircraft. We empirically estimate Nr normalized excess mixing ratios and emission factors from fires sampled within 80 min of estimated emission and explore variability in the dominant forms of Nr between these fires. We find that reduced N compounds comprise a majority (39%–80%; median = 66%) of total measured reactive nitrogen (ΣNr) emissions. The smoke plumes sampled during WE‐CAN feature rapid chemical transformations after emission. As a result, within minutes after emission total measured oxidized nitrogen (ΣNOy) and measured total ΣNHx (NH3 + pNH4) are more robustly correlated with modified combustion efficiency (MCE) than NOx and NH3 by themselves. The ratio of ΣNHx/ΣNOy displays a negative relationship with MCE, consistent with previous studies. A positive relationship with total measured ΣNr suggests that both burn conditions and fuel N content/volatilization differences contribute to the observed variability in the distribution of reduced and oxidized Nr. Additionally, we compare our in situ field estimates of Nr EFs to previous lab and field studies. For similar fuel types, we find ΣNHx EFs are of the same magnitude or larger than lab‐based NH3 EF estimates, and ΣNOy EFs are smaller than lab NOx EFs. Plain Language Summary: Smoke from large wildfires in the western U.S. degrades air quality across the whole U.S. Smoke contains a mixture of many different gases and particles, including carbon compounds like carbon dioxide and carbon monoxide, as well as nitrogen compounds such as ammonia and nitrogen oxides. Gases containing nitrogen are important for the production of ozone and the formation of more or larger particles as the smoke moves downwind. During the summer of 2018, we used the National Science Foundation/National Center for Atmospheric Research C130 research aircraft to fly through smoke across the western U.S. and measure many of the most abundant nitrogen compounds. We find that the smoke plumes we sampled emitted more nitrogen in a reduced form than in an oxidized form, and chemical reactions change the form and phase of nitrogen very quickly in the smoke. We compare our field measurements with laboratory measurements with the goal of using them together to improve our forecasts of how and where wildfire smoke will impact air quality. Key Points: Normalized excess mixing ratios of nitrogen species are reported for western U.S. wildfires; median of 66% of emissions are in reduced formWithin minutes after emission, rapid chemistry changes the distribution (species/amount/phase) of reactive nitrogen in fresh smoke plumesFor similar fuel types, NHx emissions factors are similar to or higher than lab‐based estimates; NOy emission factors are smaller [ABSTRACT FROM AUTHOR]

Sulfur dioxide (SO2) is emitted in large quantities from coal‐burning power plants and leads to various harmful health and environmental effects. In this study, we use plume intercepts from the Wintertime INvestigation of Transport, Emission and Reactivity (WINTER) campaign to estimate the oxidation rates of SO2 under wintertime conditions and the factors that determine SO2 removal. Observations suggest that OH governs the rate SO2 oxidation in the eastern United States during winter. The range of mean oxidation rates during the day from power plants were 0.22–0.71%/hr, producing SO2 lifetimes of 13–43 days, if SO2 consumption is assumed to occur during 10.5 hr of daylight in cloudless conditions. Though most nighttime rate measurements were zero within uncertainty, there is some evidence of nighttime removal, which suggests alternate oxidation mechanisms. The fastest nighttime observed SO2 oxidation rate was 0.25±0.07%/hr, producing a combined day/night SO2 lifetime of 8.5–21 days. The upper limit of the oxidation rate (the mean+1σ of the fastest day and night observations) is 16.5%/day, corresponding to a lifetime of 6.1 days. The analysis also quantifies the primary emission of sulfate from power plants. The median mole percentage of SO4‐2 from observed plumes was 1.7% and the mean percentage sulfate was 2.8% for intercepts within 1 hr of transit to power plants. The largest value observed from close intercepts was over 7% sulfate, and the largest extrapolated value was 18%, based on intercepts further from their source and fastest observed oxidation rate. Key Points: The daytime conversion rate of SO2 to SO4‐2 was 0.22‐0.71%/hr in winter under clear‐sky conditions, with lifetimes of 140‐450 hrFor 10.5 hr of daylight, the upper limit of the oxidation rate is 16.5%/day, corresponding to a lifetime of 6.1 daysDirect emissions of SO4‐2 relative to total sulfur (SO2 + SO4‐2) had mean and median values of 1.7% and 2.8%, respectively [ABSTRACT FROM AUTHOR]