Emissions of Reactive Nitrogen From Western U.S. Wildfires During Summer 2018

Abstract

Abstract 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 ( ΣN 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 ( Σ NO y ) and measured total Σ NH x (NH 3 + p NH 4 ) are more robustly correlated with modified combustion efficiency (MCE) than NO x and NH 3 by themselves. The ratio of ΣNH x /ΣNO y displays a negative relationship with MCE, consistent with previous studies. A positive relationship with total measured ΣN 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 Σ NH x EFs are of the same magnitude or larger than lab‐based NH 3 EF estimates, and Σ NO y EFs are smaller than lab NO x EFs.