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

Abstract

Abstract 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 + NO 2 ) 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 ( p NO 3 ) (∼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 (HNO 3 ) were observed in plumes sampled in a pseudo‐Lagrangian fashion, implying HNO 3 ‐limited ammonium nitrate (NH 4 NO 3 ) 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 p NO 3 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.