Daytime Oxidized Reactive Nitrogen Partitioning in Western U.S. Wildfire Smoke Plumes

The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) deployed the NSF/NCAR C-130 aircraft in summer 2018 across the western U.S. to sample wildfire smoke during its first days of atmospheric evolution. We present a summary of a subset of reactive oxidized nitrogen species (NO_y) in plumes sampled in a pseudo-Lagrangian fashion. Emissions of nitrogen oxides (NO_x = NO + NO₂) and nitrous acid (HONO) are rapidly converted to more oxidized forms. Within 4 h, ∼86% of the ΣNO_y is in the form of peroxy acyl nitrates (PANs) (∼37%), particulate nitrate (pNO₃) (∼27%), and gas-phase organic nitrates (Org N_(g)) (∼23%). The average e-folding time and distance for NO_x are ∼90 min and ∼40 km, respectively. Nearly no enhancements in nitric acid (HNO₃) were observed in plumes sampled in a pseudo-Lagrangian fashion, implying HNO₃-limited ammonium nitrate (NH₄NO₃) formation, with one notable exception that we highlight as a case study. We also summarize the observed partitioning of NO_y in all the smoke samples intercepted during WE-CAN. In smoke samples intercepted above 3 km above sea level (ASL), the contributions of PANs and pNO₃ to ΣNO_y increase with altitude. WE-CAN also sampled smoke from multiple fires mixed with anthropogenic emissions over the California Central Valley. We distinguish samples where anthropogenic NO_x emissions appear to lead to an increase in NO_x abundances by a factor of four and contribute to additional PAN formation.