A Comprehensive 0-D Chemical Model Simulation of Wildfire Plume Composition Evolution: Insights into Radical Oxidant Sources, NOx Lifetime, and the Formation of Secondary Products

Abstract:

Photolysis of radical precursors is a critical driver in daytime wildfire plume chemistry, along with a range of factors such as the emissions and their mixing with background air. Photolysis rates in fire plumes typically increase downwind and towards the plume edges due to decreased light extinction from increased dilution. This behavior implies a complex variation in the chemical processing of wildfire plumes with distinct longitudinal and lateral profiles. Here, we describe an integrated model approach to study wildfire photochemistry encompassing the overall plume volume. We used a zero-dimensional time-dependent photochemical box model with near-explicit chemical mechanisms to simulate plume evolution along multiple plume streamlines identified using airborne measurements over the western U.S. during the summer 2018 Western wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN). The importance and effects of reactive nitrogen chemistry are examined, and a comparison of model output with observations shows that the model can reasonably reproduce the plume evolution of major primary and secondary species such as nitrous acid (HONO), ozone (O₃), peroxyacetyl nitrate (PAN), and total (gas+particle) inorganic nitrate. We use the suite of simulations to derive insights into factors controlling NO_x (NO_x = NO + NO₂) lifetime, hydroxyl radical (OH) concentrations, and secondary product formation across the plume transects, from dilute edges to concentrated plume cores. We discuss how such evaluations can be used to incorporate the effects of sub grid-scale wildfire plume chemistry into regional and global scale chemical transport models.