Mass fluxes of radical precursors in wildfire plumes: a remote sensing perspective

Emissions from biomass burning form a complicated and poorly characterized mix of both gases and particles that are transported in the atmosphere and thus can affect wide areas. Secondary chemistry leads to downwind formation of ozone and particles that affect climate and adversely impact public health. Gas phase radicals play important roles in many of these processes, such as secondary organic aerosol formation and ozone production or destruction. Quantifying the emissions of radical precursors such as NO₂, HONO, and carbonyls such as HCHO is an important first step to constraining their role in smoke chemistry.

Here we use a combination of satellite and aircraft measurements to constrain the emissions and effects of various radical sources emitted by biomass burning. The Tropospheric Monitoring Instrument (TROPOMI) on the Sentinel-5 Precursor satellite provides daily measurements of NO₂, HCHO, and HONO over a large area covering many different land types at a resolution high enough to resolve individual fire plumes. Aircraft measurements were made from the University of Wyoming King Air research aircraft during the 2018 "Biomass Burning fluxes of trace gases and aerosols" (BB-FLUX) campaign based in Boise, ID. The aircraft performed a total of 38 research flights with over 125 plume underpasses to characterize column abundances of various species, and over 60 vertical profiles inside the plumes for wind and other in situ measurements. In situ ozone measurements were made using a chemiluminescence instrument from the Karlsruhe Institute of Technology. The University of Colorado Airborne Differential Optical Absorption Spectroscopy (DOAS) instrument used ultraviolet-visible spectroscopy in a zenith sky geometry to measure HONO, HCHO, and NO₂ columns, while the University of Colorado Solar Occultation Flux (SOF) instrument measured columns of HCHO and several other relevant trace gases at mid-infrared wavelengths in a direct sun geometry. The combination of repeated aircraft measurements on a single fire and coincident satellite measurements provides a unique dataset for evaluating plume radical chemistry.