Emissions of reactive nitrogen from Western U.S. wildfires during summer 2018

Reactive nitrogen (N-r) 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 N-r normalized excess mixing ratios and emission factors from fires sampled within 80 min of estimated emission and explore variability in the dominant forms of N-r between these fires. We find that reduced N compounds comprise a majority (39%-80%; median = 66%) of total measured reactive nitrogen (Sigma N-r) 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 (Sigma NOy) and measured total Sigma NHx (NH3 + pNH(4)) are more robustly correlated with modified combustion efficiency (MCE) than NOx and NH3 by themselves. The ratio of Sigma NHx/Sigma NOy displays a negative relationship with MCE, consistent with previous studies. A positive relationship with total measured Sigma N-r suggests that both burn conditions and fuel N content/volatilization differences contribute to the observed variability in the distribution of reduced and oxidized N-r. Additionally, we compare our in situ field estimates of N-r EFs to previous lab and field studies. For similar fuel types, we find Sigma NHx EFs are of the same magnitude or larger than lab-based NH3 EF estimates, and Sigma NOy EFs are smaller than lab NOx EFs.