Abstract. Smoke from wildfires is a significant source of air pollution, which can adversely impact air quality and ecosystems downwind. With the recently increasing intensity and severity of wildfires, the threat to air quality is expected to increase. Satellite-derived biomass burning emissions can fill in gaps in the absence of aircraft or ground-based measurement campaigns and can help improve the online calculation of biomass burning emissions as well as the biomass burning emissions inventories that feed air quality models. This study focuses on satellite-derived NOx emissions using the high-spatial-resolution TROPOspheric Monitoring Instrument (TROPOMI) NO2 dataset. Advancements and improvements to the satellite-based determination of forest fire NOx emissions are discussed, including information on plume height and effects of aerosol scattering and absorption on the satellite-retrieved vertical column densities. Two common top-down emission estimation methods, (1) an exponentially modified Gaussian (EMG) and (2) a flux method, are applied to synthetic data to determine the accuracy and the sensitivity to different parameters, including wind fields, satellite sampling, noise, lifetime, and plume spread. These tests show that emissions can be accurately estimated from single TROPOMI overpasses. The effect of smoke aerosols on TROPOMI NO2 columns (via air mass factors, AMFs) is estimated, and these satellite columns and emission estimates are compared to aircraft observations from four different aircraft campaigns measuring biomass burning plumes in 2018 and 2019 in North America. Our results indicate that applying an explicit aerosol correction to the TROPOMI NO2 columns improves the agreement with the aircraft observations (by about 10 %–25 %). The aircraft- and satellite-derived emissions are in good agreement within the uncertainties. Both top-down emissions methods work well; however, the EMG method seems to output more consistent results and has better agreement with the aircraft-derived emissions. Assuming a Gaussian plume shape for various biomass burning plumes, we estimate an average NOx e-folding time of 2 ±1 h from TROPOMI observations. Based on chemistry transport model simulations and aircraft observations, the net emissions of NOx are 1.3 to 1.5 times greater than the satellite-derived NO2 emissions. A correction factor of 1.3 to 1.5 should thus be used to infer net NOx emissions from the satellite retrievals of NO2.

Biomass burning is an important and increasing source of trace gases and aerosols relevant to air quality and climate. The Biomass Burning Flux Measurements of Trace Gases and Aerosols (BB-FLUX) field campaign deployed the University of Colorado Airborne Solar Occultation Flux (CU AirSOF) instrument aboard the University of Wyoming King Air research aircraft during the 2018 Pacific Northwest wildfire season (July–September). CU AirSOF tracks the sun even through thick smoke plumes using short-wave infrared wavelengths to minimize scattering from smoke particles, and uses Fourier transform infrared spectroscopy (FTS) to measure the column absorption of multiple trace gases at mid-infrared wavelengths. The instrument is described, characterized, and evaluated using colocated ground-based remote sensing and airborne in situ data sets. Vertical column density (VCD) measurements agree well with a colocated stationary high-resolution FTS for carbon monoxide (CO, slope within 2%), formaldehyde (HCHO, 3%), formic acid (HCOOH, 18%), ethane (C2H6, 4%), ammonia (NH3, 4%), hydrogen cyanide (HCN, 10%), and peroxyacyl nitrate (PANFTS, 1%; we distinguish the molecule PAN from PANFTS, which includes similar molecules and is measured as a sum by FTS). Airborne VCD measurements are compared with in situ measurements aboard the NSF/NCAR C-130 aircraft during a coordinated mission to the Rabbit Foot Fire near Boise, Idaho by digesting VCDs into normalized excess column ratios (NEMRs). Column NEMRs from CU AirSOF, expressed as VCD enhancements over background and normalized to CO enhancements, are found to agree with the in situ NEMRs within 20% for HCHO, methanol (CH3OH), ethylene (C2H4), C2H6, NH3, and HCN and within 30–66% for HCOOH and PAN. CU AirSOF integrates over plume heterogeneity, is inherently calibrated, and provides an innovative, flexible, and quantitative tool to measure emission mass fluxes from wildfires.

Abstract. Oxidation flow reactors (OFRs) complement environmental smog chambers as a portable, low-cost technique for exposing atmospheric compounds to oxidants such as ozone (O3), nitrate (NO3) radicals, and hydroxyl (OH) radicals. OH is most commonly generated in OFRs via photolysis of externally added O3 at λ=254 nm (OFR254) or combined photolysis of O2 and H2O at λ=185 nm plus photolysis of O3 at λ=254 nm (OFR185) using low-pressure mercury (Hg) lamps. Whereas OFR254 radical generation is influenced by [O3], [H2O], and photon flux at λ=254 nm (I254), OFR185 radical generation is influenced by [O2], [H2O], I185, and I254. Because the ratio of photon fluxes, I185:I254, is OFR-specific, OFR185 performance varies between different systems even when constant [H2O] and I254 are maintained. Thus, calibrations and models developed for one OFR185 system may not be applicable to another. To investigate these issues, we conducted a series of experiments in which I185:I254 emitted by Hg lamps installed in an OFR was systematically varied by fusing multiple segments of lamp quartz together that either transmitted or blocked λ=185 nm radiation. Integrated OH exposure (OHexp) values achieved for each lamp type were obtained using the tracer decay method as a function of UV intensity, humidity, residence time, and external OH reactivity (OHRext). Following previous related studies, a photochemical box model was used to develop a generalized OHexp estimation equation as a function of [H2O], [O3], and OHRext that is applicable for I185:I254≈0.001 to 0.1.