Daytime Oxidized Reactive Nitrogen Partitioning In Western Us 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 (NOy) in plumes sampled in a pseudo-Lagrangian fashion. Emissions of nitrogen oxides (NOx = NO + NO2) and nitrous acid (HONO) are rapidly converted to more oxidized forms. Within 4 h, similar to 86% of the sigma NOy is in the form of peroxy acyl nitrates (PANs) (similar to 37%), particulate nitrate (pNO(3)) (similar to 27%), and gas-phase organic nitrates (Org N-(g)) (similar to 23%). The average e-folding time and distance for NOx are similar to 90 min and similar to 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 NOy 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 pNO(3) to sigma NOy 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 NOx emissions appear to lead to an increase in NOx abundances by a factor of four and contribute to additional PAN formation.