Daytime Oxidized Reactive Nitrogen partitioning in western U.S. wildfire plumes

The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen (WE-CAN) project deployed the NCAR/NSF C-130 research aircraft in summer 2018 to sample wildfire smoke during its first day of atmospheric evolution. We present a summary of the partitioning (amount, species, and phase) and evolution of oxidized nitrogen in the wildfire plumes sampled during WE-CAN. Our analysis includes samples from 20 wildfires in the Western US.

Our analysis shows that, on average, within 6 hours ~50% of the oxidized N is in the form of PAN and PPN; 80% of the initial NO _x (NO + NO ₂ ) is typically converted to more oxidized forms in this time frame. The average initial contribution of HONO to total oxidized nitrogen is ~35% across the plumes, and this decreases rapidly in the first two hours, dropping to 5% after 6 hours. The percent of oxidized nitrogen present as particulate nitrate (NO ₃ ^- ) started at 16% and increased to 38% after 6 hours.

We focus our attention on three wildfires plumes with strong PAN formation: Bear Trap Fire (BT), South Sugarloaf Fire (SS) and Taylor Creek Fire (TC). In BT, the PAN and PPN contribution to total oxidized nitrogen increased from 22% to 56% in 5 hours, in SS the relative contributions increased from 8% to 35% and in TC the relative contributions increased from 10% to 51%, both in 3 hours. We compare our PAN enhancement ratios to measurements from past field campaigns and observations from satellite platforms. Finally, we examine the total enhancement of NO _y relative to CO as the plume ages to determine if we are likely to be missing measurements of key species.