We present a comparison of fast-response instruments installed onboard the NASA DC-8 aircraft that measured nitrogen oxides (NO and NO
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 km
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 (NO
List of measured species and instruments, including the
corresponding uncertainties, during FIREX-AQ. For uncertainties given as
Nitric oxide (NO) and nitrogen dioxide (NO
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 NO
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, NO
The FIREX-AQ campaign (
Example DC-8 flight tracks from western wildfires and eastern
agricultural fires. Panel
Most wildfire flights were designed to sample background mixing ratios, fresh emissions, and aged smoke, whereas the eastern fire flights typically transected numerous fresh smoke plumes several times each. For wildfires, the NASA DC-8 first flew upwind of the fire to characterize ambient conditions unaffected by targeted fire emissions. Subsequent cross-wind plume transects were conducted as close as possible to the fire to sample the emissions with the least possible atmospheric aging. Plume transects were designed to be perpendicular to the wind direction and through the center of the vertical extent of the plume, terrain permitting. The vertical structure of the plume was systematically assessed using a differential absorption lidar during a lengthwise overpass above the plume from end to start. The aircraft transected the smoke plume successively further downwind, at approximately 15–40 km intervals, to characterize smoke evolution in a “lawnmower” pattern (Fig. 1a). For several wildfires, the DC-8 also executed flight transects along the plume axis, both toward and away from the fire source. Most eastern fires sampled during FIREX-AQ did not produce plumes large enough to enable regularly spaced plume transects. Most smoke plumes were therefore sampled repetitively at the same location, sometimes with varying altitude and/or approach angle (Fig. 1b).
The NOAA chemiluminescence (CL) instrument has been frequently used for both ground-based and
airborne measurements of NO, NO
The NOAA NO-LIF measurements were performed using a custom-built
laser-induced fluorescence instrument as detailed in Rollins et al. (2020). Air was continuously
sampled from outside the aircraft through an optical cell in the DC-8 cabin
held to near 90 hPa. The instrument utilizes a fiber laser system with a
narrow-band laser tuned to a rotationally resolved NO spectral feature near
215 nm. Rapid dithering on and off of this resonance achieves 0.1 s
measurements with a continuously monitored background to reduce uncertainty
in the instrument zero. The laser-induced excitation of NO is followed by
red-shifted fluorescence, which is detected by a photomultiplier tube
operated in single-photon counting mode. The laser is directed through both
a sampling and reference cell in a single pass for continuous monitoring of
any changes in the instrument sensitivity due to changes in the laser
spectrum or pressure of the optical cells. A total of 500 ppbv of NO in air was flown
at 50 sccm through the reference cell to ensure that measurements are
occurring with the laser tuned to the peak online wavelength. A constant
flow of approximately 2500 sccm is maintained within the sampling cell
through the use of a custom inlet valve (Gao et
al., 1999), and the exhausts of both cells are tied together, allowing for any
changes in sensitivity due to pressure fluctuations to be accounted for
during data reduction. Hourly calibrations were performed during each flight
in which 2–10 sccm of 5 ppmv NO in N
The NASA Compact Airborne NO
During FIREX-AQ, ambient air was sampled using a shared inlet that provided a large (10–25 standard liter per minute, slpm) bypass flow to the instrument rack. The inlet tube is a 45 cm length of 0.94 cm inner diameter Silcosteel (Restek) coated with FluoroPel (Cazorla et al., 2015). The CANOE instrument pulled its 750 sccm sample flow from a shared manifold (with another four instruments) at the instrument rack. An inline particle filter on the sample line prevented laser scatter by fine aerosol that were not removed by the particle-rejecting inlet. A manual three-way valve outside the instrument was used to sample from a scrubber (Drierite/molecular sieve) and provides a zero before and periodically during the flight. Pressure in the CANOE detection cell was maintained at 53 hPa by a pressure controller that precedes the cell in the flow path.
NO
Ambient air is pulled through the inlet into the two optical cavities at a
flow rate of 5.4 volumetric liters per minute per cavity by a scroll pump.
The air passes through two 1
The measured absorption spectrum is fit to a linear combination of
literature or reference spectra of absorbing gas-phase species and a
polynomial to account for drifts in the cavity stability or light source
intensity, as detailed by Min et al. (2016), using a Levenberg–Marquardt least-squares fitting algorithm. For the
365 nm channel, those species are NO
HONO was measured using a modified commercial time-of-flight chemical
ionization mass spectrometer (TOF CIMS, Aerodyne Research, Inc.; Lee
et al., 2014; Veres et al., 2020). Trace gases are ionized by mixing ambient
air with reagent ions made in flight, and the resulting product ions are
detected. Ions are separated by mass-to-charge ratio (
Ambient air was sampled through a mass-flow-controlled (6 slpm) heated
perfluoroalkoxy (PFA) inlet (70 cm length, 0.64 cm inner diameter). A
pressure control region upstream of a critical orifice at the entrance to
the IMR was maintained at 140 hPa, and thus a constant flow of 1.2 slpm
ambient air entered the IMR to mix with the 1 slpm ion source flow. A small
nitrogen flow of about 20 sccm containing water vapor was added directly
into the IMR region and controlled to maintain a measured I
HONO was detected as a cluster with I
To determine the extent of budget closure for reactive odd nitrogen species
during FIREX-AQ, we compare measured NO Observations of HNO Particulate nitrate ( APNs were measured using a thermal dissociation–chemical ionization mass
spectrometer (TD-CIMS) method. The CIMS instrument used during the FIREX-AQ
campaign was similar to that described in Slusher et al. (2004) and Lee et al. (2020). Briefly,
ambient air is sampled into the TD-CIMS through heated Teflon tubing at a
temperature of approximately 150 Nitromethane (CH N C
CO was measured using a modified commercial off-axis ICOS instrument (Los
Gatos Research (LGR) N
Air was sampled from a ram-air intake inlet through 0.64 cm (outside
diameter) stainless steel tubing. Cavity pressure was maintained at 113.3
After the campaign, the H
Carbon monoxide (CO) was measured by tunable diode laser absorption
spectroscopy (TDLAS) using the DACOM (Differential Absorption Carbon
monOxide Measurement) instrument (Sachse et al., 1987). The TDLAS
instrument configuration used during FIREX-AQ also included channels for
measurements of methane (CH
The lasers were operated in a wavelength-modulated mode, each at an independent frequency, and line-locked to the centers of the species' selected absorption lines. Lines were selected to provide both good sensitivity and good isolation from any potential spectral interferences. Detector signals were demodulated at twice the lasers' modulation frequencies (2F detection) and normalized by average detected laser intensity.
Ambient air was sampled through an inlet probe, compressed, and passed through a permeable membrane dryer to remove water vapor prior to being introduced into the Herriott cell. Due to the need for very fast time response during FIREX-AQ, the instrument was operated with a flow of approximately 14 slpm with the Herriott cell at a pressure of approximately 67 hPa. The resulting time response, verified with a fast-acting valve, was faster than 0.2 s. Data were reported at both 0.2 and 1 s time steps.
The TDLAS instrument was calibrated using the same gas standards as for the ICOS instrument, nominally with a 4 min period but often advanced or delayed in time to avoid calibrating during fire plume encounters. Calibrations provided both slope and intercept values tying signals to species concentrations. The very large CO concentrations encountered necessitated post-campaign correction calibrations to account for response nonlinearity.
Post-campaign analysis of the TDLAS CO data indicated that measurement
precision (1
H
The age of smoke from emission to sampling by the aircraft was determined
from an ensemble of upwind trajectories from the aircraft
(Holmes et al., 2020). Trajectories were computed with HYSPLIT (Stein et
al., 2015) using three meteorological datasets (HRRR, NAM CONUS Nest, and
GFS 0.25
This study focuses on comparing the different techniques used for the
measurements of one or several reactive nitrogen species and CO
during FIREX-AQ. Here we compare both archived 1 s data (
We first calculated the slope of the linear least-squares (LLS) orthogonal
distance regression (ODR; Boggs et al.,
1987) to characterize the percent difference between measurements of a pair
of instruments weighted by the inverse of the instrument precision. Here, we
used a mixing ratio-independent instrument precision that corresponded to
the 1
The 1 Hz data comparison between the CL and LIF instruments is shown in
Fig. 2. The overall comparison slope (
NO measurements by LIF vs. CL with
The 1 s measurements of
The 1 s measurements of
A histogram of the absolute difference between LIF and CL (
Histograms of the absolute difference of 1 s measurements of
Measurement difference (1 s data) for
Overall, the comparison between the two NO instruments shows an agreement
within stated uncertainties. While the single-photon LIF detection of NO is
a new technique that was evaluated for the first time during FIREX-AQ
(Rollins et al., 2020), there are several
studies that compared CL detection of NO to other measurement techniques
during airborne field campaigns. The Global Tropospheric Experiment Chemical
Instrumentation Test and Evaluation (GTE-CITE) was designed in the 1990s to
intercompare airborne measurement techniques for trace species including NO,
NO
Three instruments measured NO
NO
Histograms of the absolute difference between CES, LIF, and CL (
Previous comparisons of NO
The 1 Hz data comparison between the CES and the CIMS instruments is shown in Fig. 8, and time series of HONO measurements in wildfires and eastern fires
are shown in Figs. 3c and 4c, respectively. The correlation between the
CES and CIMS was very high in each plume transect (Figs. 3c and 4c), but
the overall comparison yielded a slope (
The same as Fig. 2 but comparing HONO measurements by CES and CIMS.
No slope is given for the Los Angeles (LA) flights in panel
However, further laboratory studies, field measurements, and examination of
this comparison has revealed that the CIMS sensitivity to HONO is reduced
when the instrument reaches temperatures greater than 30
The interpretation of the literature suggests that HONO measurements are notoriously difficult due to the potential for artifacts associated with inlet surfaces and interferences associated with some methods (e.g., Kleffmann et al., 2006; Xu et al., 2019). Past ground-based intercomparisons often revealed significant discrepancies in HONO measurements. For example, six ground-based HONO measurement techniques including a CIMS instrument were compared during the Study of Houston Atmospheric Radical Precursors (SHARP) campaign in 2009 (Pinto et al., 2014). While three out of six of these techniques agreed within 20 %, larger deviations were found when the other three instruments were considered and attributed to the physical separation of these instruments. Three different techniques, including a CIMS instrument, were used to measure HONO in the urban area of Shanghai, China (Bernard et al., 2016). The percent difference between these measurements ranged from 27 % to 46 %. In 2019, six HONO measurement techniques were again compared in a Chinese urban area, this time in Beijing, and included a CIMS instrument and two broadband cavity-enhanced absorption spectrometers (BBCEAS) (Crilley et al., 2019). Percent differences up to 39 % were observed during this intercomparison and again attributed to the physical distance separating inlets coupled to high spatial heterogeneity of HONO mixing ratios. Airborne measurements of HONO by CIMS and CES were made during the Southeast Nexus Experiment (SENEX), and the CES instrument was approximatively 25 % higher than the CIMS instrument (Neuman et al., 2016).
The 1 Hz data comparison between the total NO
The same as Fig. 2 but comparing the sum of individually measured
NO
Contribution of individually measured reactive odd nitrogen
species to the total NO
Despite this correlation, two modes are apparent in the overall distribution
of the absolute difference (
Histograms of
Higher
We calculated the fraction of measured NO
On the other hand, the positive
Scatterplots of
Overall, the agreement between the total NO
The 1 Hz data comparison between the ICOS and the TDLAS instruments is shown
in Fig. 14. The overall comparison yielded a slope (
The same as Fig. 2 but comparing CO measurements by TDLAS and ICOS.
Overall, the comparison between the two CO instruments shows an agreement
well within stated uncertainties. We find that the agreement between the two
CO instruments used during FIREX-AQ is well in line with past
intercomparisons. During the GTE-CITE experiment, the comparison of a TDLAS
technique with two grab sample and gas chromatograph methods for detection of CO
showed agreement across the instruments – within the combined instrument
uncertainties and strong correlations (
In this study, we compare airborne measurements of NO, NO Comparison of NO measurements by LIF and CL showed an overall agreement
well within instrument uncertainties. Flight-to-flight agreement was
generally more variable during the eastern fires sampling period than during the wildfires sampling period, which was attributed to the heterogeneous nature of smoke plumes combined with the physical separation of inlets. Both measurements are considered reliable for FIREX-AQ, although the LIF instrument has better 1 Hz precision (1 pptv) than the CL instrument (6 pptv), and the CL instrument exhibited slower time response. Comparison of NO The 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 from the entire campaign was 1.80. The HONO measured by CIMS was on average 74 % of that measured by CES during the wildfires sampling period, and on average 40 % of CES during the eastern fires sampling period. The higher
precision data from the CIMS are most useful for analysis of HONO when
mixing ratios are lower. The redundancy of HONO measurements during FIREX-AQ led to the discovery that the CIMS sensitivity to HONO was reduced in a high-temperature environment. This intercomparison has initiated further studies of the CIMS sensitivity to HONO and other compounds. Closure of the NO Comparison of CO measurements by TDLAS and ICOS showed an agreement well
within the combined instrument uncertainties. An offset of Integrating data across smoke plume transects generally improved the
correlation between independent measurements and may be necessary for
fire-science-related analyses, especially for smaller plumes with greater
spatial heterogeneity compared to the distance between the sampling
locations on a large aircraft.
All data used in this paper are archived online and available at
The supplement related to this article is available online at:
IB and TBR designed the research. All authors performed FIREX-AQ measurements. PCJ, HG, and JLJ performed the flow modeling analysis. All authors analyzed the data. IB, JP, JAN, and SSB wrote the original draft and all authors edited and revised the paper.
The contact author has declared that none of the authors has any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We would like to thank the NOAA/NASA FIREX-AQ science and aircraft operation
teams. We acknowledge Armin Whistaler, Felix Piel, and Laura Tomsche for providing the
NH
This research has been supported by the National Oceanic and Atmospheric Administration (grant nos. NA17OAR4320101, NA16OAR4310100, and NA17OAR4310004) and the National Aeronautics and Space Administration (grant nos. 80NSSC18K0660 and 80NSSC18K0630).
This paper was edited by Hang Su and reviewed by three anonymous referees.