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.
