The CU Airborne Solar Occultation Flux Instrument: Performance Evaluation during BB-FLUX

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 (PAN_FTS, 1%; we distinguish the molecule PAN from PAN_FTS, 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.