Biomass burning (BB) can take multiple forms (e.g., wildfires, prescribed fires, agricultural burns, grass fires, peat fires) and accounts for a large fraction of global carbon emissions with consequences for climate (Bowman et al., 2009; van der Werf et al., 2010, 2017) and biogeochemical cycles (Crutzen and Andreae, 2016). BB also contributes substantially to the atmospheric burden of trace gases and aerosols (Andreae, 2019), causing poor air quality on regional to continental scales (Jaffe et al., 2020; O’Dell et al., 2019; Wotawa, 2000) and posing a major threat to public health (Johnston et al., 2012, 2021). In the United States (US), wildfires mainly occur in the western states and in Alaska and burned over 18 000 km2 in 2019 (US National Interagency Fire Center, https://www.nifc.gov/fire-information, last access: 15 November 2021). Wildfire frequency and severity are predicted to increase in response to a warmer, drier climate (Burke et al., 2021; Westerling, 2016) and also to increasing human-caused ignition (Balch et al., 2017). In comparison, prescribed fires, which are common practice in the southeastern US, burned an estimated 40 000 km2 in 2019, to which agricultural burns added another 8000-12 000 km2 (Melvin, 2020). While agricultural burns are usually smaller and less intense than wildfires or prescribed fires, they occur more frequently and throughout the whole year and can significantly impact local air quality (Dennis et al., 2002; McCarty, 2011).
Rising interest in the impact of fires on climate and air quality over the past decades has resulted in a series of laboratory studies of BB emissions in the US, such as the FLAME-4 experiment in 2012 (e.g., Stockwell et al., 2014) and the FIRELAB study in 2016 (e.g., Selimovic et al., 2018). Recent large-scale field studies such as AMMA (e.g., Liousse et al., 2010), BBOP (e.g., Collier et al., 2016), and WE-CAN (e.g., Juncosa Calahorrano et al., 2021) have been dedicated to sampling and characterizing emissions and atmospheric chemistry from fires. The focus of the joint National Oceanic and Atmospheric Administration (NOAA)/National Aeronautics and Space Administration (NASA) Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) airborne campaign was to provide comprehensive observations to investigate the impact of summer time wildfires, prescribed fires, and agricultural burns on air quality and climate across the conterminous US (Warneke et al., 2022).
Accurate measurements facilitate understanding of fire emissions, processing, and impacts. In situ, fast-response measurements of trace gases in the atmosphere conducted from airborne platforms provide unique datasets that enhance our understanding of atmospheric composition and chemistry. One method for evaluating measurement accuracy is by comparison of independent measurements using different techniques. A relatively small body of literature reported comparisons of methods for in-flight detection of tropospheric carbon monoxide (CO) and reactive odd nitrogen species measured both as the total (NOy) and from the sum of individually measured species (ΣNOy), and these studies have shown that such comparisons are valuable for identifying instrument artifacts and quantifying measurement uncertainties (Eisele et al., 2003; Gregory et al., 1990a, b; Hoell et al., 1987a, b; Sparks et al., 2019). During FIREX-AQ, a large suite of airborne instruments, detailed in the following sections, performed independent in situ tropospheric measurements of one or more fire-science-relevant reactive nitrogen species and CO aboard the NASA DC-8 aircraft (Table 1). Additionally, FIREX-AQ provides a unique opportunity to investigate measurement accuracy in concentrated smoke plumes where hundreds of species coexist.
Nitric oxide (NO) and nitrogen dioxide (NO2) are among the largest components of the reactive nitrogen budget emitted by biomass burning and are produced by the oxidation of reduced nitrogen species present in the fuel in the flaming stage of combustion (Roberts et al., 2020). NOx, defined as the sum of NO and NO2, directly affects atmospheric oxidation rates and ozone (O3) production within fire plumes (Bourgeois et al., 2021; Robinson et al., 2021; Xu et al., 2022). It also contributes to the formation of secondary aerosols and N transport and deposition to ecosystems downwind (Galloway et al., 2003; Kroll and Seinfeld, 2008; Ziemann and Atkinson, 2012). Therefore, two independent NO measurements and three independent NO2 measurements were part of FIREX-AQ to provide continuous in situ observations, as described in Sect. 2 below.
Nitrous acid (HONO) is emitted directly to the atmosphere through various combustion processes including BB. The rapid production of OH from HONO at the early stage of smoke plume formation (Peng et al., 2020) results in rapid initiation of photochemistry, with a strong influence on downwind chemical evolution of smoke plumes Robinson et al., 2021; Theys et al., 2020).
Total NOy can be measured through conversion of individual species to NO (Fahey et al., 1985). It is a more conserved tracer for NOx emissions than NOx itself since it accounts for NOx oxidation products, and it provides a mean to assess the accuracy of ΣNOy budget closure from a mass balance approach (Bollinger et al., 1983; Fahey et al., 1986; Williams et al., 1997). Fahey et al. (1986) define ΣNOy as the sum of important nitrogen species, as illustrated by Eq. (1).
Other nitrogen compounds that can contribute to ΣNOy include alkyl nitrates (Day et al., 2003), acyl peroxynitrates (APNs; Juncosa Calahorrano et al., 2021), non-acyl peroxynitrates (RO2NO2; Murphy et al., 2004), nitryl chloride (ClNO2; Kenagy et al., 2018), and nitro compounds and nitroaromatics (Decker et al., 2021).
Carbon monoxide (CO) is emitted from incomplete combustion in fires and other sources, and is especially important for characterizing the combustion stage of fires (i.e., flaming vs. smoldering) through the use of the modified combustion efficiency (Yokelson et al., 1996). Due to its relatively long chemical lifetime, CO is commonly used as a conserved tracer to account for dilution with ambient air as smoke plumes are transported downwind, and accurate CO measurements are necessary to better constrain emission factors (EFs) used in emission inventories.
This study builds on past airborne instrument comparisons and extends these analyses to a new species (HONO), new measurement techniques (the first airborne deployment of the NOAA NO-LIF (laser-induced fluorescence) and the NOAA CO-ICOS (integrated cavity output spectroscopy) instruments) and new environments (concentrated fire smoke). In this paper we present a comparison of NO, NO2, HONO, NOy, and CO measurements, which are compounds of major interest for fire-related science, air quality, and climate. First, we describe the FIREX-AQ campaign, the deployed instruments, and the methodology used to perform the comparisons. Following this, we provide a detailed instrument comparison for each species.