Heterogeneous Chemistry on Biomass Burning Aerosol: Aircraft Measurements of N2O5 and ClNO2 during FIREX-AQ

Zachary Decker, STEVEN S. BROWN, Kenneth Aikin, Ilann Bourgeios, Pedro Campuzano-Jost, Matthew Coggon, Josh DiGangi, Glenn Diskin, Frank Flocke, Alessandro Franchin, Carley D. Fredrickson, Georgios Gkatzelis, Hongyu Guo, Sam Hall, Hannah Halliday, Katherine L. Hayden, Christopher D. Holmes, Jose-Luis Jimenez, Melinda Beaver, Ann M. Middlebrook, D. D. Montzka, Richard Moore, J. Andrew Neuman, John Nowak, et al., National Oceanic and Atmospheric Administration

     Abstract Number: 513
     Working Group: Aerosol Chemistry

Abstract
Biomass burning (BB) emissions contain large amounts of both reactive nitrogen (NO, NO₂, HONO) and particulate chloride (pCl^-). Oxidation of reactive nitrogen species by ozone produces dinitrogen pentoxide, N₂O₅, which reacts heterogeneously to form soluble nitrate and gas-phase nitryl chloride, ClNO₂. Subsequent photolysis releases atomic chlorine and recycles NO₂. Here, we analyze in situ aircraft measurements of aerosol composition, surface area, N₂O₅, ClNO₂, nitrogen oxides and ozone in BB plumes sampled during the 2019 FIREX-AQ campaign and report the first aircraft observations of N₂O₅ and ClNO₂ in fire plumes.

The NASA DC-8 and NOAA Twin Otter sampled wildfire plumes across the western U.S., and the DC-8 further sampled agricultural emissions in the southeast U.S. Normalized excess mixing ratios (NEMRs) of pCl^- were consistent with laboratory derived emission factors for different fuel types and were larger in agricultural fire plumes. Plume enhancements of N₂O₅ and ClNO₂ were routinely observed from the DC-8 during daytime flights and from the Twin Otter on targeted night flights, with up to 340 pptv of ClNO₂ at night. We present comparisons of an observationally constrained Lagrangian box model determination of the N₂O₅ uptake coefficient, γ(N₂O₅), and ClNO₂ yield, φ(ClNO₂), to parameterizations based on aerosol composition and calculated liquid water content.

Box model determinations of γ(N₂O₅) were consistently below the range of values reported from laboratory studies (γ < 10^-3). Parameterized φ(ClNO₂) from pCl^- and liquid water had median values of 0.82 and 0.87 for wildfire and agricultural fire smoke, respectively, while the box model for six wildfire plumes showed lower values, consistent with previous work and laboratory determinations. These results have implications for the total budget of tropospheric halogen activation through biomass burning emissions and chemistry, as well as for ozone loss induced by heterogeneous reactions following injections from large wildfire plumes to the stratosphere.