Comparison of airborne measurements of NO, NO2, HONO, NOy, and CO during FIREX-AQ
Authors: Bourgeois Ilann , Peischl Jeff , Neuman J. Andrew , Brown Steven S. , Allen Hannah M. , Campuzano-Jost Pedro , Coggon Matthew M. , DiGangi Joshua P. , Diskin Glenn S. , Gilman Jessica B. , Gkatzelis Georgios I. , Guo Hongyu , Halliday Hannah A. , Hanisco Thomas F. , Holmes Christopher D. , Huey L. Gregory , Jimenez Jose L. , Lamplugh Aaron D. , Lee Young Ro , Lindaas Jakob , Moore Richard H. , Nault Benjamin A. , Nowak John B. , Pagonis Demetrios , Rickly Pamela S. , Robinson Michael A. , Rollins Andrew W. , Selimovic Vanessa , St. Clair Jason M. , Tanner David , Vasquez Krystal T. , Veres Patrick R. , Warneke Carsten , Wennberg Paul O. , Washenfelder Rebecca A. , Wiggins Elizabeth B. , Womack Caroline C. , Xu Lu , Zarzana Kyle J. , Ryerson Thomas B. 
Published: August 2022

Abstract. 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 (NOy) and from the sum of individually measured species (ΣNOy)), 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 NOy measurements using the CL instrument were compared with
ΣNOy (= NO + NO2 + HONO + nitric acid (HNO3) + acyl peroxy nitrates (APNs) + submicrometer particulate nitrate
(pNO3)). Other NOy species were not included in ΣNOy
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) NOy 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 NOy 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 NOy 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.