Comparison of airborne measurements of NO, NO2, HONO, NOy, and CO during FIREX-AQ

We present a comparison of fast-response instruments installed onboard the NASA DC-8 aircraft that measured nitrogen oxides (NO and NO₂), nitrous acid (HONO), total reactive odd nitrogen (measured both as the total (NO_y) and from the sum of individually measured species (ΣNO_y)), and carbon monoxide (CO) in the troposphere during the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. By targeting smoke from summertime wildfires, prescribed fires, and agricultural burns across the continental United States, FIREX-AQ provided a unique opportunity to investigate measurement accuracy in concentrated plumes where hundreds of species coexist. Here, we compare NO measurements by chemiluminescence (CL) and laser-induced fluorescence (LIF); NO₂ measurements by CL, LIF, and cavity-enhanced spectroscopy (CES); HONO measurements by CES and iodide-adduct chemical ionization mass spectrometry (CIMS); and CO measurements by tunable diode laser absorption spectrometry (TDLAS) and integrated cavity output spectroscopy (ICOS). Additionally, total NO_y measurements using the CL instrument were compared with ΣNO_y (= NO + NO₂ + HONO + nitric acid (HNO₃) + acyl peroxy nitrates (APNs) + submicrometer particulate nitrate (pNO₃)). Other NO_y species were not included in ΣNO_y as they either contributed minimally to it (e.g., C₁-C₅ alkyl nitrates, nitryl chloride (ClNO₂), dinitrogen pentoxide (N₂O₅)) or were not measured during FIREX-AQ (e.g., higher oxidized alkyl nitrates, nitrate (NO₃), non-acyl peroxynitrates, coarse-mode aerosol nitrate). The aircraft instrument intercomparisons demonstrate the following points: (1) NO measurements by CL and LIF agreed well within instrument uncertainties but with potentially reduced time response for the CL instrument; (2) NO₂ measurements by LIF and CES agreed well within instrument uncertainties, but CL NO₂ was on average 10 % higher; (3) CES and CIMS HONO measurements were highly correlated in each fire plume transect, but the correlation slope of CES vs. CIMS for all 1 Hz data during FIREX-AQ was 1.8, which we attribute to a reduction in the CIMS sensitivity to HONO in high-temperature environments; (4) NO_y budget closure was demonstrated for all flights within the combined instrument uncertainties of 25 %. However, we used a fluid dynamic flow model to estimate that average pNO₃ sampling fraction through the NO_y inlet in smoke was variable from one flight to another and ranged between 0.36 and 0.99, meaning that approximately 0 %-24 % on average of the total measured NO_y in smoke may have been unaccounted for and may be due to unmeasured species such as organic nitrates; (5) CO measurements by ICOS and TDLAS agreed well within combined instrument uncertainties, but with a systematic offset that averaged 2.87 ppbv; and (6) integrating smoke plumes followed by fitting the integrated values of each plume improved the correlation between independent measurements.