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Formic acid (HCOOH) is an important component of atmospheric acidity but its budget is poorly understood, with prior observations implying substantial missing sources. Here we combine pole-to-pole airborne observations from the Atmospheric Tomography Mission (ATom) with chemical transport model (GEOS-Chem CTM) and back trajectory analyses to provide the first global in-situ characterization of HCOOH in the remote atmosphere. ATom reveals sub-100 ppt HCOOH concentrations over most of the remote oceans, punctuated by large enhancements associated with continental outflow. Enhancements correlate with known combustion tracers and trajectory-based fire influences. The GEOS-Chem model underpredicts these in-plume HCOOH enhancements, but elsewhere we find no broad indication of a missing HCOOH source in the background free troposphere. We conclude that missing non-fire HCOOH precursors inferred previously are predominantly short-lived. We find indications of a wet scavenging underestimate in the model consistent with a positive HCOOH bias in the tropical upper troposphere. Observations reveal episodic evidence of ocean HCOOH uptake, which is well-captured by GEOS-Chem; however, despite its strong seawater undersaturation HCOOH is not consistently depleted in the remote marine boundary layer. Over fifty fire and mixed plumes were intercepted during ATom with widely varying transit times and source regions. HCOOH:CO normalized excess mixing ratios in these plumes range from 3.4 to >50 ppt/ppb CO and are often over an order of magnitude higher than expected primary emission ratios. HCOOH is thus a major reactive organic carbon reservoir in the aged plumes sampled during ATom, implying important missing pathways for in-plume HCOOH production.

The global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half of the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations from the first two deployments of the NASA ATom aircraft campaign during July-August 2016 and January-February 2017 to evaluate the oxidation capacity over the remote oceans and its representation by the GEOS-Chem chemical transport model. The model successfully simulates the magnitude and vertical profile of remote OH within the measurement uncertainties. Comparisons against the drivers of OH production (water vapor, ozone, and NO y concentrations, ozone photolysis frequencies) also show minimal bias, with the exception of wintertime NO y . The severe model overestimate of NO y during this period may indicate insufficient wet scavenging and/or missing loss on sea-salt aerosols. Large uncertainties in these processes require further study to improve simulated NO y partitioning and removal in the troposphere, but preliminary tests suggest that their overall impact could marginally reduce the model bias in tropospheric OH. During the ATom-1 deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and this is not captured by the sum of its measured components (cOHRobs) or by the model (cOHRmod). This enhancement could suggest missing reactive VOCs but cannot be explained by a comprehensive simulation of both biotic and abiotic ocean sources of VOCs. Additional sources of VOC reactivity in this region are difficult to reconcile with the full suite of ATom measurement constraints. The model generally reproduces the magnitude and seasonality of cOHRobs but underestimates the contribution of oxygenated VOCs, mainly acetaldehyde, which is severely underestimated throughout the troposphere despite its calculated lifetime of less than a day. Missing model acetaldehyde in previous studies was attributed to measurement uncertainties that have been largely resolved. Observations of peroxyacetic acid (PAA) provide new support for remote levels of acetaldehyde. The underestimate in both model acetaldehyde and PAA is present throughout the year in both hemispheres and peaks during Northern Hemisphere summer. The addition of ocean sources of VOCs in the model increases cOHRmod by 3% to 9% and improves model-measurement agreement for acetaldehyde, particularly in winter, but cannot resolve the model summertime bias. Doing so would require 100 Tg yr-1 of a long-lived unknown precursor throughout the year with significant additional emissions in the Northern Hemisphere summer. Improving the model bias for remote acetaldehyde and PAA is unlikely to fully resolve previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.

Active bromine (e.g., Br2, BrCl, BrO, HOBr) promotes atmospheric ozone destruction and mercury removal. Here we report a previously unidentified participant in active-Br chemistry, cyanogen bromide (BrCN), measured during the NASA Atmospheric Tomography (ATom) mission. BrCN was confined to polar boundary layers, often appearing at concentrations higher than other Br compounds. The chemistry of BrCN determines whether it promotes or inhibits ozone and mercury removal. This dataset provides evidence that much of the BrCN was from atmospheric Br chemistry involving surface reactions with reduced nitrogen compounds. Since gas phase loss processes are known to be relatively slow, surface reactions must also be the major loss processes, with vertical profiles implying a BrCN atmospheric lifetime in the range 1-10 days. Liquid phase reactions of BrCN tend to convert Br to bromide (Br?) or C-Br bonded organics, constituting a loss of active Br. Thus, accounting for BrCN chemistry is crucial to understanding polar Br cycling. [ABSTRACT FROM AUTHOR]

Atmospheric hydroperoxides are a significant component of the atmosphere's oxidizing capacity. Two of the most abundant hydroperoxides, hydrogen peroxide (H2O2) and methyl hydroperoxide (MHP, CH3OOH), were measured in the remote atmosphere using chemical ionization mass spectrometry aboard the NASA DC‐8 aircraft during the Atmospheric Tomography Mission. These measurements present a seasonal investigation into the global distribution of these two hydroperoxides, with near pole‐to‐pole coverage across the Pacific and Atlantic Ocean basins and from the marine boundary layer to the upper troposphere and lower stratosphere. H2O2 mixing ratios are highest between 2 and 4 km altitude in the equatorial region of the Atlantic Ocean basin, where they reach global maximums of 3.6–6.5 ppbv depending on season. MHP mixing ratios reach global maximums of 4.3–8.6 ppbv and are highest between 1 and 3 km altitude, but peak in different regions depending on season. A major factor contributing to the global H2O2 distribution is the influence of biomass burning emissions in the Atlantic Ocean basin, encountered in all four seasons, where the highest H2O2 mixing ratios were found to correlate strongly with increased mixing ratios of the biomass burning tracers hydrogen cyanide (HCN) and carbon monoxide (CO). This biomass burning enhanced H2O2 by a factor of 1.3–2.2, on average, in the Atlantic compared with the Pacific Ocean basin. Plain Language Summary: Hydroperoxides, a large class of compounds that contain the R–OOH chemical structure, exist in the gas phase in the atmosphere. These compounds are key to the chemistry of the atmosphere because of the role they play in the atmosphere's ability to process and ultimately remove chemical species. Two of the most abundant atmospheric hydroperoxides were measured as part of the Atmospheric Tomography Mission, which collected samples of the atmosphere over the Pacific and Atlantic Ocean basins far from human influences. This paper presents a summary of the global distribution of these hydroperoxides across the four different seasons (winter, spring, summer, fall) and investigates the role that smoke from large‐scale fires on the continents plays in altering the amount of atmospheric hydroperoxides above the Atlantic Ocean. Key Points: The Atmospheric Tomography Mission provides an unprecedented investigation into the global seasonal distribution of hydroperoxidesChemical Ionization Mass Spectrometry is a sensitive technique for studying hydroperoxides in the remote atmosphereBiomass burning emissions increase H2O2 mixing ratios in the Atlantic Ocean compared to corresponding latitudes in the Pacific Ocean basin [ABSTRACT FROM AUTHOR]

Hydrogen peroxide (H2O2) and methyl hydroperoxide (MHP, CH3OOH) serve as HOx (OH and HO2 radicals) reservoirs and therefore as useful tracers of HOx chemistry. Both hydroperoxides were measured during the 2016–2018 Atmospheric Tomography Mission as part of a global survey of the remote troposphere over the Pacific and Atlantic Ocean basins conducted using the NASA DC‐8 aircraft. To assess the relative contributions of chemical and physical processes to the global hydroperoxide budget and their impact on atmospheric oxidation potential, we compare the observations with two models, a diurnal steady‐state photochemical box model and the global chemical transport model Goddard Earth Observing System (GEOS)‐Chem. We find that the models systematically under‐predict H2O2 by 5%–20% and over‐predict MHP by 40%–50% relative to measurements. In the marine boundary layer, over‐predictions of H2O2 in a photochemical box model are used to estimate H2O2 boundary‐layer mean deposition velocities of 1.0–1.32 cm s−1, depending on season; this process contributes to up to 5%–10% of HOx loss in this region. In the upper troposphere and lower stratosphere, MHP is under‐predicted and H2O2 is over‐predicted by a factor of 2–3 on average. The differences between the observations and predictions are associated with recent convection: MHP is under‐estimated and H2O2 is over‐estimated in air parcels that have experienced recent convective influence. Plain Language Summary: Hydrogen peroxide (H2O2) and methyl hydroperoxide (MHP, CH3OOH) in the atmosphere can act as reservoirs for one of the main drivers of atmospheric chemistry, HOx (HOx = OH and HO2). Both H2O2 and MHP were measured during the 2016–2018 Atmospheric Tomography Mission (ATom), which investigated the atmosphere over the oceans far from direct human influence. The measurements are compared to two types of models to assess our understanding of the chemical and physical processes that control their abundance. We find that these models consistently predict H2O2 to be lower and MHP to be higher than was measured during ATom. We use the discrepancy between the model and the measurements to investigate the role of deposition (removal of compounds from the Earth's atmosphere due to interactions with surfaces and with liquid water) on H2O2 in the lowest portion of atmosphere and the role of convection (vertical transport during storms and other meteorological events) on MHP between 6 and 12 km altitudes. Key Points: The global distribution of H2O2 and methyl hydroperoxide (MHP) reflects the influence of photochemistry, convective transport, and wet and dry depositionDeposition of H2O2 plays a key role in removing HOx within the marine boundary layerMHP in the upper troposphere is highly sensitive to convective transport and the Goddard Earth Observing System (GEOS)‐Chem model substantially underestimates MHP advective mass fluxes [ABSTRACT FROM AUTHOR]

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 ( ?NOy)) 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  ?NOy (= NO + NO2 + HONO + nitric acid (HNO3) + acyl peroxy nitrates (APNs) + submicron particulate nitrate (pNO3)). The aircraft instrument intercomparisons demonstrate the following: 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. [ABSTRACT FROM AUTHOR]

The hydroxyl radical (OH) reacts with thousands of chemical species in the atmosphere, initiating their removal and the chemical reaction sequences that produce ozone, secondary aerosols, and gas-phase acids. OH reactivity, which is the inverse of OH lifetime, influences the OH abundance and the ability of OH to cleanse the atmosphere. The NASA Atmospheric Tomography (ATom) campaign used instruments on the NASA DC-8 aircraft to measure OH reactivity and more than 100 trace chemical species. ATom presented a unique opportunity to test the completeness of the OH reactivity calculated from the chemical species measurements by comparing it to the measured OH reactivity over two oceans across four seasons. Although the calculated OH reactivity was below the OH reactivity instrument's limit-of-detection for much of the free troposphere, the OHR instrument was able to measure the OH reactivity in and just above the marine boundary layer. The average measured value of OH reactivity in the marine boundary layer across all latitudes and all ATom phases was 1.9 s-1, which 0.5 s-1 larger than the average calculated OH reactivity. Concurrently, missing OH reactivity, the difference between the measured and calculated OH reactivity, was measured to be ~ 0.5-2.0 s-1 at some locations in the tropics and midlatitudes. Correlations of missing OH reactivity with formaldehyde, dimethyl sulfide, butanal, and sea surface temperature suggest the presence of unmeasured or unknown volatile organic compounds or oxygenated volatile organic compounds associated with ocean emissions. [ABSTRACT FROM AUTHOR]

Nighttime reaction of nitrate radicals (NO3) with biogenic volatile organic compounds (BVOC) has been proposed as a potentially important but also highly uncertain source of secondary organic aerosol (SOA). The southeastern United States has both high BVOC and nitrogen oxide (NOx) emissions, resulting in a large model-predicted NO3-BVOC source of SOA. Coal-fired power plants in this region constitute substantial NOx emissions point sources into a nighttime atmosphere characterized by high regionally widespread concentrations of isoprene. In this paper, we exploit nighttime aircraft observations of these power plant plumes, in which NO3 radicals rapidly remove isoprene, to obtain field-based estimates of the secondary organic aerosol yield from NO3 Cisoprene. Observed in-plume increases in nitrate aerosol are consistent with organic nitrate aerosol production from NO3 Cisoprene, and these are used to determine molar SOA yields, for which the average over nine plumes is 9% (±5%). Corresponding mass yields depend on the assumed molecular formula for isoprene-NO3-SOA, but the average over nine plumes is 27% (±14%), on average larger than those previously measured in chamber studies (12%-14% mass yield as 1OA=1VOC after oxidation of both double bonds). Yields are larger for longer plume ages. This suggests that ambient aging processes lead more effectively to condensable material than typical chamber conditions allow. We discuss potential mechanistic explanations for this difference, including longer ambient peroxy radical lifetimes and heterogeneous reactions of NO3-isoprene gas phase products. More in-depth studies are needed to better understand the aerosol yield and oxidation mechanism of NO3 radicalCisoprene, a coupled anthropogenic-biogenic source of SOA that may be regionally significant. [ABSTRACT FROM AUTHOR]

Nighttime reaction of nitrate radicals (NO3) with biogenic volatile organic compounds (BVOC) has been proposed as a potentially important but also highly uncertain source of secondary organic aerosol (SOA). The southeast United States has both high BVOC and nitrogen oxide (NOx) emissions, resulting in a large model-predicted NO3-BVOC source of SOA. Coal-fired power plants in this region constitute substantial NOx emissions point sources into a nighttime atmosphere characterized by high regionally widespread concentrations of isoprene. In this paper, we exploit nighttime aircraft observations of these power plant plumes, in which NO3 radicals rapidly remove isoprene, to obtain field-based estimates of the secondary organic aerosol yield from NO3 + isoprene. Observed in-plume increases in nitrate aerosol are consistent with organic nitrate aerosol production from NO3 + isoprene, and these are used to determine molar SOA yields, for which the average over 9 plumes is 9 %. Corresponding mass yields depend on the assumed molecular formula for isoprene-NO3-SOA, but the average over 9 plumes is 27 %, larger than those previously measured in chamber studies (12–14 % after oxidation of both double bonds). Yields are larger for longer plume ages. This suggests that ambient aging processes lead more effectively to condensable material than typical chamber conditions allow. We discuss potential mechanistic explanations for this difference, including ambient peroxy radical lifetimes and heterogeneous reactions of NO3-isoprene gas phase products. Future studies of aerosol composition from NO3 radical + isoprene are needed to better understand the oxidation chemistry producing this potentially important coupled anthropogenic – biogenic source of SOA. [ABSTRACT FROM AUTHOR]

The reaction of HO 2 with NO is one of the most important steps in radical cycling throughout the stratosphere and troposphere. Previous literature experimental work revealed a small yield of nitric acid (HONO 2 ) directly from HO 2 + NO. Atmospheric models previously treated HO 2 + NO as radical recycling, but inclusion of this terminating step had large effects on atmospheric oxidative capacity and the concentrations of HONO 2 and ozone (O 3 ), among others. Here, the yield of HONO 2 , φ HONO 2 , from the reaction of HO 2 + NO was investigated in a flow tube reactor using mid-IR pulsed-cavity ringdown spectroscopy. HO 2 , produced by pulsed laser photolysis of Cl 2 in the presence of methanol, reacted with NO in a buffer gas mixture of N 2 and CO between 300 and 700 Torr at 278 and 300 K. HONO 2 and its weakly bound isomer HOONO were directly detected by their v 1 absorption bands in the mid-IR region. CO was used to suppress HONO 2 produced from OH + NO 2 and exploit a chemical amplification scheme, converting OH back to HO 2 . Under the experimental conditions described here, no evidence for the formation of either HONO 2 or HOONO was observed from HO 2 + NO. Using a comprehensive chemical model, constrained by observed secondary reaction products, all HONO 2 detected in the system could be accounted for by OH + NO 2 . At 700 ± 14 Torr and 300 ± 3 K, φ HONO 2 = 0.00 ± 0.11% (2σ) with an upper limit of 0.11%. If all of the observed HONO 2 was attributed to the HO 2 + NO reaction, φ HONO 2 = 0.13 ± 0.07% with an upper limit of 0.20%. At 278 ± 2 K and 718 ± 14 Torr, we determine an upper limit, φ HONO 2 ≤ 0.37%. Our measurements are significantly lower than those previously reported, lying outside of the uncertainty of the current experimental and recommended literature values.