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We present a comparison of instruments measuring nitrogen oxide species from an aircraft during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the northeast United States. Instrument techniques compared here include chemiluminescence (CL), thermal dissociation laser‐induced fluorescence (TD‐LIF), cavity ring‐down spectroscopy (CRDS), high‐resolution time of flight, iodide‐adduct chemical ionization mass spectrometry (I‐CIMS), and aerosol mass spectrometry. Species investigated include NO2, NO, total nitrogen oxides (NOy), N2O5, ClNO2, and HNO3. Particulate‐phase nitrate is also included for comparisons of HNO3and NOy. Instruments generally agreed within reported uncertainties, with individual flights sometimes showing much better agreement than the data set taken as a whole, due to flight‐to‐flight slope changes. NO measured by CRDS and CL showed an average relative slope of 1.16 ± 0.01 across all flights, which is outside of combined uncertainties. The source of the error was not identified. For NO2measured by CRDS and TD‐LIF the average was 1.02 ± 0.00; for NOymeasured by CRDS and CL the average was 1.01 ± 0.00; and for N2O5measured by CRDS and I‐CIMS the average was 0.89 ± 0.01. NOybudget closure to within 20% is demonstrated. We observe nonlinearity in NO2and NOycorrelations at concentrations above ~30 ppbv that may be related to the NO discrepancy noted above. For ClNO2there were significant differences between I‐CIMS and TD‐LIF, potentially due in part to the temperature used for thermal dissociation. Although the fraction of particulate nitrate measured by the TD‐LIF is not well characterized, it improves comparisons to include particulate measurements. Nitrogen oxides are accurately and precisely measured by a number of independent methodsNew chemical ionization approaches compare well with more established methodsClosure of the NOy budget was observed

Sulfur dioxide (SO2) is emitted in large quantities from coal‐burning power plants and leads to various harmful health and environmental effects. In this study, we use plume intercepts from the Wintertime INvestigation of Transport, Emission and Reactivity (WINTER) campaign to estimate the oxidation rates of SO2under wintertime conditions and the factors that determine SO2removal. Observations suggest that OH governs the rate SO2oxidation in the eastern United States during winter. The range of mean oxidation rates during the day from power plants were 0.22–0.71%/hr, producing SO2lifetimes of 13–43 days, if SO2consumption is assumed to occur during 10.5 hr of daylight in cloudless conditions. Though most nighttime rate measurements were zero within uncertainty, there is some evidence of nighttime removal, which suggests alternate oxidation mechanisms. The fastest nighttime observed SO2oxidation rate was 0.25±0.07%/hr, producing a combined day/night SO2lifetime of 8.5–21 days. The upper limit of the oxidation rate (the mean+1σof the fastest day and night observations) is 16.5%/day, corresponding to a lifetime of 6.1 days. The analysis also quantifies the primary emission of sulfate from power plants. The median mole percentage of SO4‐2from observed plumes was 1.7% and the mean percentage sulfate was 2.8% for intercepts within 1 hr of transit to power plants. The largest value observed from close intercepts was over 7% sulfate, and the largest extrapolated value was 18%, based on intercepts further from their source and fastest observed oxidation rate. The daytime conversion rate of SO2to SO4‐2was 0.22‐0.71%/hr in winter under clear‐sky conditions, with lifetimes of 140‐450 hrFor 10.5 hr of daylight, the upper limit of the oxidation rate is 16.5%/day, corresponding to a lifetime of 6.1 daysDirect emissions of SO4‐2relative to total sulfur (SO2+ SO4‐2) had mean and median values of 1.7% and 2.8%, respectively

Although urban NOxlifetimes have been examined extensively during summertime conditions, wintertime NOxchemistry has been comparatively less studied. We use measurements of NOxand its oxidation products from the aircraft‐based WINTER (Wintertime INvestigation of Transport, Emissions, and Reactivity) experiment over the northeastern United States during February–March 2015 to describe the NOxlifetime during conditions when days are shorter, actinic flux is reduced, and temperatures are colder. By analyzing regional outflow from the East Coast, we show that NOxis long lived during the winter, with a longer daytime lifetime (29 hr) than nighttime lifetime (6.3 hr). We demonstrate that wintertime NOxemissions have an overall lifetime controlled by the nighttimeconversion of NOxto nitric acid (HNO3) via N2O5heterogeneous chemistry, and we discuss constraints on the rates of NOxconversion to HNO3. Additionally, analysis of the nighttime Oxbudget suggests that approximately 15% of O3is lost overnight through N2O5production and subsequent reaction with aerosol to form HNO3. The atmospheric lifetime (how long something persists in the atmosphere) and fate of nitrogen oxides in urban areas during the summer has been studied extensively, but relatively few studies have looked at the lifetime of nitrogen oxides in the atmosphere during winter. We use aircraft data from the East Coast of the United States during February–March 2015 to characterize the wintertime lifetime of nitrogen oxides when days are shorter, sunlight is reduced, and temperatures are colder. We are able to measure the wintertime lifetime of nitrogen oxides and assess the relative roles of mixing, deposition, and chemistry on their fate. We determine that nitrogen oxide loss during winter is dominated by nighttime rather than daytime chemistry and that this nighttime chemistry effectively removes ozone from the atmosphere. NOxis long lived during winter, with a longer daytime lifetime (29 hr) than nighttime lifetime (6.3 hr)Wintertime NOxlifetime is controlled by the nighttime conversion of NOxto HNO3via N2O5heterogeneous chemistryFifteen percent of O3is removed at night via the dark reactions of N2O5

We describe the University of Washington airborne high‐resolution time‐of‐flight chemical ionization mass spectrometer (HRToF‐CIMS) and evaluate its performance aboard the NCAR‐NSF C‐130 aircraft during the recent Wintertime INvestigation of Transport, Emissions and Reactivity (WINTER) experiment in February–March of 2015. New features include (i) a computer‐controlled dynamic pinhole that maintains constant mass flow‐rate into the instrument independent of altitude changes to minimize variations in instrument response times; (ii) continuous addition of low flow‐rate humidified ultrahigh purity nitrogen to minimize the difference in water vapor pressure, hence instrument sensitivity, between ambient and background determinations; (iii) deployment of a calibration source continuously generating isotopically labeled dinitrogen pentoxide (15N2O5) for in‐flight delivery; and (iv) frequent instrument background determinations to account for memory effects resulting from the interaction between sticky compounds and instrument surface following encounters with concentrated air parcels. The resulting improvements to precision and accuracy, along with the simultaneous acquisition of these species and the full set of their isotopologues, allow for more reliable identification, source attribution, and budget accounting, for example, by speciating the individual constituents of nocturnal reactive nitrogen oxides (NOz= ClNO2+ 2 × N2O5+ HNO3+ etc.). We report on an expanded set of species quantified using iodide‐adduct ionization such as sulfur dioxide (SO2), hydrogen chloride (HCl), and other inorganic reactive halogen species including hypochlorous acid, nitryl chloride, chlorine, nitryl bromide, bromine, and bromine chloride (HOCl, ClNO2, Cl2, BrNO2, Br2, and BrCl, respectively). Series of upgrades implemented on the University of Washington aircraft‐based HRToF‐CIMS improves accuracy and robustness of measurementsWe report on the detection of HCl, SO2, ClNO2, and a suite of reactive halogen and nitrogen oxide species utilizing iodide adduct ionizationObservations from WINTER (Cl,Br species in coal‐fired power plant exhaust and full speciation of NOzspecies) are presented as examples

Strong correlations of O3‐CH2O, O3‐CO and CO‐CH2O were observed during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER‐AQ) aircraft experiment in July 2011 over the Washington‐Baltimore area. The linear regression slopes of observed O3‐CH2O, O3‐CO, and CO‐CH2O do not vary significantly with time (11 a.m. to 4 p.m.) or altitude in the boundary layer. These observed relationships are simulated well by a regional chemical transport model. Using tagged‐tracer simulations, we find that biogenic isoprene oxidation makes the largest contribution to the regression slope of O3‐CH2O across much of the eastern United States, providing a good indicator for O3enhanced by biogenic isoprene oxidation. In contrast, the regression slope of O3‐CO is controlled by both anthropogenic and biogenic emissions. Therefore, we use the CO‐CH2O relationship to separate biogenic from anthropogenic contributions to CO. By combining these regressions, we can track the contributions to surface O3by anthropogenic and biogenic factors and build a fast‐response ozone estimator using near‐surface CH2O and CO concentrations as inputs. We examine the quality of O3estimator by increasing or decreasing anthropogenic emissions by up to 50%. The estimated O3distribution is in reasonably good agreement with the full‐model simulations (R2> 0.77 in the range of −30% to +50% of anthropogenic emissions). The analysis provides the basis for using high‐quality geostationary satellites with UV, thermal infrared, or near‐infrared instruments for observing CH2O and CO to improve surface O3distribution monitoring. The estimation model can also be applied to derive observation‐derived regional metrics to evaluate and improve full‐fledged 3‐D air quality models. Observations show robust correlations and consistent linear regression slopes of O3‐CH2O, O3‐CO and CO‐CH2OBiogenic isoprene oxidation makes the largest contribution to the regression slope of O3‐CH2O across much of the eastern United StatesWe build a fast‐response O3estimator using input near‐surface CH2O and CO concentrations to improve surface O3distribution monitoring

We quantify the stratospheric injection of brominated very short‐lived substances (VSLS) based on aircraft observations acquired in winter 2014 above the Tropical Western Pacific during the CONvective TRansport of Active Species in the Tropics (CONTRAST) and the Airborne Tropical TRopopause EXperiment (ATTREX) campaigns. The overall contribution of VSLS to stratospheric bromine was determined to be 5.0 ± 2.1 ppt, in agreement with the 5 ± 3 ppt estimate provided in the 2014 World Meteorological Organization (WMO) Ozone Assessment report (WMO 2014), but with lower uncertainty. Measurements of organic bromine compounds, including VSLS, were analyzed using CFC‐11 as a reference stratospheric tracer. From this analysis, 2.9 ± 0.6 ppt of bromine enters the stratosphere via organic source gas injection of VSLS. This value is two times the mean bromine content of VSLS measured at the tropical tropopause, for regions outside of the Tropical Western Pacific, summarized in WMO 2014. A photochemical box model, constrained to CONTRAST observations, was used to estimate inorganic bromine from measurements of BrO collected by two instruments. The analysis indicates that 2.1 ± 2.1 ppt of bromine enters the stratosphere via inorganic product gas injection. We also examine the representation of brominated VSLS within 14 global models that participated in the Chemistry‐Climate Model Initiative. The representation of stratospheric bromine in these models generally lies within the range of our empirical estimate. Models that include explicit representations of VSLS compare better with bromine observations in the lower stratosphere than models that utilize longer‐lived chemicals as a surrogate for VSLS. Based on winter 2014 observations, very short‐lived bromocarbons produced by oceanic biology contribute 5 ± 2 ppt to stratospheric bromineOf the bromine from very short‐lived substances that reaches the stratosphere, 60% enters as organic species and 40% as inorganic speciesRepresentation of stratospheric bromine within global models is greatly improved upon consideration of very short‐lived bromocarbons

The San Joaquin Valley (SJV) of California experiences high concentrations of particulate matter NH4NO3during episodes of meteorological stagnation in winter. A rich data set of observations related to NH4NO3formation was acquired during multiple periods of elevated NH4NO3during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER‐AQ) field campaign in SJV in January and February 2013. Here NH4NO3is simulated during the SJV DISCOVER‐AQ study period with the Community Multiscale Air Quality (CMAQ) model, diagnostic model evaluation is performed using the DISCOVER‐AQ data set, and integrated reaction rate analysis is used to quantify HNO3production rates. Simulated NO3−generally agrees well with routine monitoring of 24‐hr average NO3−, but comparisons with hourly average NO3−measurements in Fresno revealed differences at higher time resolution. Predictions of gas‐particle partitioning of total nitrate (HNO3+ NO3−) and NHx (NH3+ NH4+) generally agree well with measurements in Fresno, although partitioning of total nitrate to HNO3is sometimes overestimated at low relative humidity in afternoon. Gas‐particle partitioning results indicate that NH4NO3formation is limited by HNO3availability in both the model and ambient. NH3mixing ratios are underestimated, particularly in areas with large agricultural activity, and additional work on the spatial allocation of NH3emissions is warranted. During a period of elevated NH4NO3, the model predicted that the OH + NO2pathway contributed 46% to total HNO3production in SJV and the N2O5heterogeneous hydrolysis pathway contributed 54%. The relative importance of the OH + NO2pathway for HNO3production is predicted to increase as NOx emissions decrease. NH4NO3formation is limited by HNO3availability in both the model and ambient based on gas‐particle partitioning resultsThe OH + NO2pathway contributed 46% to modeled HNO3production in SJV, and the N2O5heterogeneous hydrolysis pathway contributed 54%The relative importance of the OH + NO2pathway for HNO3production is predicted to increase as NOx emissions decrease

Nocturnal dinitrogen pentoxide (N2O5) heterogeneous chemistry impacts regional air quality and the distribution and lifetime of tropospheric oxidants. Formed from the oxidation of nitrogen oxides, N2O5is heterogeneously lost to aerosol with a highly variable reaction probability, γ(N2O5), dependent on aerosol composition and ambient conditions. Reaction products include soluble nitrate (HNO3or NO3−) and nitryl chloride (ClNO2). We report the first‐ever derivations of γ(N2O5) from ambient wintertime aircraft measurements in the critically important nocturnal residual boundary layer. Box modeling of the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the eastern United States derived 2,876 individual γ(N2O5) values with a median value of 0.0143 and range of 2 × 10−5to 0.1751. WINTER γ(N2O5) values exhibited the strongest correlation with aerosol water content, but weak correlations with other variables, such as aerosol nitrate and organics, suggesting a complex, nonlinear dependence on multiple factors, or an additional dependence on a nonobserved factor. This factor may be related to aerosol phase, morphology (i.e., core shell), or mixing state, none of which are commonly measured during aircraft field studies. Despite general agreement with previous laboratory observations, comparison of WINTER data with 14 literature parameterizations (used to predict γ(N2O5) in chemical transport models) confirms that none of the current methods reproduce the full range of γ(N2O5) values. Nine reproduce the WINTER median within a factor of 2. Presented here is the first field‐based, empirical parameterization of γ(N2O5), fit to WINTER data, based on the functional form of previous parameterizations. Aircraft measurements over the eastern United States provide the largest number of N2O5uptake coefficient γ(N2O5) determinations during winterDespite a large range and variability, several γ(N2O5) dependencies are statistically significant, particularly with aerosol liquid waterStandard γ(N2O5) parameterizations do not capture the variability but several, including an empirical form derived here, capture the median

A number of satellite‐based instruments have become an essential part of monitoring emissions. Despite sound theoretical inversion techniques, the insufficient samples and the footprint size of current observations have introduced an obstacle to narrow the inversion window for regional models. These key limitations can be partially resolved by a set of modest high‐quality measurements from airborne remote sensing. This study illustrates the feasibility of nitrogen dioxide (NO2) columns from the Geostationary Coastal and Air Pollution Events Airborne Simulator (GCAS) to constrain anthropogenic NOxemissions in the Houston‐Galveston‐Brazoria area. We convert slant column densities to vertical columns using a radiative transfer model with (i) NO2profiles from a high‐resolution regional model (1 × 1 km2) constrained by P‐3B aircraft measurements, (ii) the consideration of aerosol optical thickness impacts on radiance at NO2absorption line, and (iii) high‐resolution surface albedo constrained by ground‐based spectrometers. We characterize errors in the GCAS NO2columns by comparing them to Pandora measurements and find a striking correlation (r> 0.74) with an uncertainty of 3.5 × 1015molecules cm−2. On 9 of 10 total days, the constrained anthropogenic emissions by a Kalman filter yield an overall 2–50% reduction in polluted areas, partly counterbalancing the well‐documented positive bias of the model. The inversion, however, boosts emissions by 94% in the same areas on a day when an unprecedented local emissions event potentially occurred, significantly mitigating the bias of the model. The capability of GCAS at detecting such an event ensures the significance of forthcoming geostationary satellites for timely estimates of top‐down emissions. We constrain anthropogenic NOxemissions by Kalman filter using the NO2observations from GCAS sensorConstrained emissions counterbalance the well‐documented positive bias of 1 × 1 km2regional model against surface observationsGCAS provides a unique opportunity to detect and constrain an emission event anomaly in the Houston area

Modeled source attribution information from the Community Multiscale Air Quality model was coupled with ambient data from the 2011 Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality Baltimore field study. We assess source contributions and evaluate the utility of using aircraft measured CO and NOyrelationships to constrain emission inventories. We derive ambient and modeled ΔCO:ΔNOyratios that have previously been interpreted to represent CO:NOyratios in emissions from local sources. Modeled and measured ΔCO:ΔNOyare similar; however, measured ΔCO:ΔNOyhas much more daily variability than modeled values. Sector‐based tagging shows that regional transport, on‐road gasoline vehicles, and nonroad equipment are the major contributors to modeled CO mixing ratios in the Baltimore area. In addition to those sources, on‐road diesel vehicles, soil emissions, and power plants also contribute substantially to modeled NOyin the area. The sector mix is important because emitted CO:NOxratios vary by several orders of magnitude among the emission sources. The model‐predicted gasoline/diesel split remains constant across all measurement locations in this study. Comparison of ΔCO:ΔNOyto emitted CO:NOyis challenged by ambient and modeled evidence that free tropospheric entrainment, and atmospheric processing elevates ambient ΔCO:ΔNOyabove emitted ratios. Specifically, modeled ΔCO:ΔNOyfrom tagged mobile source emissions is enhanced 5–50% above the emitted ratios at times and locations of aircraft measurements. We also find a correlation between ambient formaldehyde concentrations and measured ΔCO:ΔNOysuggesting that secondary CO formation plays a role in these elevated ratios. This analysis suggests that ambient urban daytime ΔCO:ΔNOyvalues are not reflective of emitted ratios from individual sources. Modeling and ambient analysis suggest that atmospheric processing can impact the ΔCO:ΔNOymeasured by aircraft over the Baltimore regionFor about one third of measurements, NOyvalues derived from different instruments lead to observed ΔCO:ΔNOythat are statistically inconsistentWe urge caution in using ΔCO:ΔNOyderived from aircraft measurement to quantify emissions errors from specific sources

Nitric oxide (NO) is emitted in large quantities from coal‐burning power plants. During the day, the plumes from these sources are efficiently mixed into the boundary layer, while at night, they may remain concentrated due to limited vertical mixing during which they undergo horizontal fanning. At night, the degree to which NO is converted to HNO3and therefore unable to participate in next‐day ozone (O3) formation depends on the mixing rate of the plume, the composition of power plant emissions, and the composition of the background atmosphere. In this study, we use observed plume intercepts from the Wintertime INvestigation of Transport, Emissions and Reactivity campaign to test sensitivity of overnight NOxremoval to the N2O5loss rate constant, plume mixing rate, background O3, and background levels of volatile organic compounds using a 2‐D box model of power plant plume transport and chemistry. The factor that exerted the greatest control over NOxremoval was the loss rate constant of N2O5. At the lowest observed N2O5loss rate constant, no other combination of conditions converts more than 10% of the initial NOxto HNO3. The other factors did not influence NOxremoval to the same degree. N2O5loss rate is main factor controlling overnight NOxloss via conversion to HNO3in power plant plumesFate of organic nitrates is important factor in degree of overnight NOxremoval from power plant plumesExtremely low N2O5loss rate constant and uptake coefficients observed in plume are below most prior observations

Formaldehyde (HCHO) directly affects the atmospheric oxidative capacity through its effects on HOx. In remote marine environments, such as the tropical western Pacific (TWP), it is particularly important to understand the processes controlling the abundance of HCHO because model output from these regions is used to correct satellite retrievals of HCHO. Here we have used observations from the Convective Transport of Active Species in the Tropics (CONTRAST) field campaign, conducted during January and February 2014, to evaluate our understanding of the processes controlling the distribution of HCHO in the TWP as well as its representation in chemical transport/climate models. Observed HCHO mixing ratios varied from ~500 parts per trillion by volume (pptv) near the surface to ~75 pptv in the upper troposphere. Recent convective transport of near surface HCHO and its precursors, acetaldehyde and possibly methyl hydroperoxide, increased upper tropospheric HCHO mixing ratios by ~33% (22 pptv); this air contained roughly 60% less NO than more aged air. Output from the CAM‐Chem chemistry transport model (2014 meteorology) as well as nine chemistry climate models from the Chemistry‐Climate Model Initiative (free‐running meteorology) are found to uniformly underestimate HCHO columns derived from in situ observations by between 4 and 50%. This underestimate of HCHO likely results from a near factor of two underestimate of NO in most models, which strongly suggests errors in NOxemissions inventories and/or in the model chemical mechanisms. Likewise, the lack of oceanic acetaldehyde emissions and potential errors in the model acetaldehyde chemistry lead to additional underestimates in modeled HCHO of up to 75 pptv (~15%) in the lower troposphere. Global chemistry climate models (CCMs) underestimate observed HCHO in the tropical western Pacific troposphere during CONTRAST by between 4 and 50%Errors in NOxchemistry and emissions are significant drivers of the measurement versus model discrepancy for HCHO in the CCMsLack of oceanic emissions and missing in situ production of acetaldehyde leads to additional global chemistry model underestimates of HCHO

Deep convective transport of surface moisture and pollution from the planetary boundary layer to the upper troposphere and lower stratosphere affects the radiation budget and climate. This study analyzes the deep convective transport in three different convective regimes from the 2012 Deep Convective Clouds and Chemistry field campaign: 21 May Alabama air mass thunderstorms, 29 May Oklahoma supercell severe storm, and 11 June mesoscale convective system (MCS). Lightning data assimilation within the Weather Research and Forecasting (WRF) model coupled with chemistry (WRF‐Chem) is utilized to improve the simulations of storm location, vertical structure, and chemical fields. Analysis of vertical flux divergence shows that deep convective transport in the 29 May supercell case is the strongest per unit area, while transport of boundary layer insoluble trace gases is relatively weak in the MCS and air mass cases. The weak deep convective transport in the strong MCS is unexpected and is caused by the injection into low levels of midlevel clean air by a strong rear inflow jet. In each system, the magnitude of tracer vertical transport is more closely related to the vertical distribution of mass flux density than the vertical distribution of trace gas mixing ratio. Finally, the net vertical transport is strongest in high composite reflectivity regions and dominated by upward transport. Lightning data assimilation improves model simulations of storm location and vertical structureRear inflow jets within mesoscale convective systems may weaken boundary layer trace gas vertical transportThe tracer vertical transport is more controlled by the vertical gradient of mass flux than the vertical gradient of trace gas

Gas‐phase volatile organic compounds (VOCs) were measured at three vertical levels between 0.6 m and 5.4 m in the Arctic boundary layer in Barrow, Alaska, for the Ocean‐Atmosphere‐Sea Ice‐Snowpack (OASIS)‐2009 field campaign during March–April 2009. C4‐C8nonmethane hydrocarbons (NMHCs) and oxygenated VOCs (OVOCs), including alcohols, aldehydes, and ketones, were quantified multiple times per hour, day and night, during the campaign using in situ fast gas chromatography‐mass spectrometry. Three canister samples were also collected daily and subsequently analyzed for C2‐C5NMHCs. The NMHCs and aldehydes demonstrated an overall decrease in mixing ratios during the experiment, whereas acetone and 2‐butanone showed increases. Calculations of time‐integrated concentrations of Br atoms, ∫[Br]dt, yielded values as high as (1.34 ± 0.27) × 1014cm−3s during the longest observed ozone depletion event (ODE) of the campaign and were correlated with the steady state Br calculated at the site during this time. Both chlorine and bromine chemistry contributed to the large perturbations on the production and losses of VOCs. Notably, acetaldehyde, propanal, and butanal mixing ratios dropped below the detection limit of the instrument (3 parts per trillion by volume (pptv) for acetaldehyde and propanal, 2 pptv for butanal) during several ODEs due to Br chemistry. Chemical flux calculations of OVOC production and loss are consistent with localized high Cl‐atom concentrations either regionally or within a very shallow surface layer, while the deeper Arctic boundary layer provides a continuous source of precursor alkanes to maintain the OVOC mixing ratios. Integrated Cl and Br atom concentrations calculated from NMHC are decoupledStrong positive correlation observed between ozone and aldehyde depletion eventsAlkane‐Cl reactions play a key role in OVOC chemical fluxes in the Arctic

We analyze the effects of the diurnal cycle of fire emissions (DCFE) and plume rise on U.S. air quality using the MUSICAv0 (Multi‐Scale Infrastructure for Chemistry and Aerosols Version 0) model during the FIREX‐AQ (Fire Influence on Regional to Global Environments and Air Quality) and WE‐CAN (Western wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen) field campaigns. To include DCFE in the model, we employ two approaches: a DCFE climatology and DCFE derived from a satellite fire radiative power product. We also implemented two sets of plume‐rise climatologies, and two plume‐rise parameterizations. We evaluate the model performance with airborne measurements, U.S. EPA Air Quality System surface measurements, and satellite products. Overall, including plume rise improves model agreement with observations such as aircraft observations of CO and NOxfor FIREX‐AQ and WE‐CAN. Applying DCFE also improves model performance, such as for surface PM2.5in fire‐impacted regions. The impact of plume rise is larger than the impact of DCFE. Plume rise can greatly enhance modeled long‐range transport of fire‐emitted pollutants. The simulations with plume‐rise parameterizations generally perform better than the simulations with plume‐rise climatologies during FIREX‐AQ, but not for WE‐CAN. The 2019 Williams Flats Fire case study demonstrates that DCFE and plume rise change fire impacts because fire emissions are subject to different meteorology and chemistry when emitted at different times of a day and altitudes. Moreover, DCFE and plume rise also impact local‐to‐regional meteorology and chemical reaction rates. DCFE and plume rise will be included in future MUSICA versions. Fires have significant impacts on U.S. air quality. The characteristics of fires such as diurnal variation and plume rise can largely alter these impacts. We study the impact of fire diurnal variation and plume rise on U.S. air quality using a newly developed modeling system—Multi‐Scale Infrastructure for Chemistry and Aerosols Version 0 (MUSICAv0). Specifically, we employ two approaches to include the diurnal cycle of fire emissions (DCFE) and four approaches to include fire plume rise in the model. The model experiments were conducted for two campaign periods, FIREX‐AQ (Fire Influence on Regional to Global Environments and Air Quality) and WE‐CAN (Western wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen). We compare the model results with airborne measurements, surface measurements, and satellite products over the continental U.S. during the two campaign periods. Overall, including plume rise and/or DCFE improves model agreement with observations of the studied air‐quality‐related chemical species, and the impact of fire plume rise is larger than the impact of the DCFE. A case study also demonstrates that fire plume rise and the DCFE also impact local‐to‐regional meteorology and chemical reaction rates. Including plume rise and diurnal cycle of fire emissions (DCFE) overall improves MUSICAv0 model agreement with observationsSimulations with plume rise parameterizations generally perform better than the simulations with plume rise climatologies during FIREX‐AQPlume rise and DCFE impact local‐to‐regional meteorology and chemical reaction rates Including plume rise and diurnal cycle of fire emissions (DCFE) overall improves MUSICAv0 model agreement with observations Simulations with plume rise parameterizations generally perform better than the simulations with plume rise climatologies during FIREX‐AQ Plume rise and DCFE impact local‐to‐regional meteorology and chemical reaction rates

We aim to reduce uncertainties in CH2O and other volatile organic carbon (VOC) emissions through assimilation of remote sensing data. We first update a three‐dimensional (3D) chemical transport model, GEOS‐Chem with the KORUSv5 anthropogenic emission inventory and inclusion of chemistry for aromatics and C2H4, leading to modest improvements in simulation of CH2O (normalized mean bias (NMB): −0.57 to −0.51) and O3(NMB: −0.25 to −0.19) compared against DC‐8 aircraft measurements during KORUS‐AQ; the mixing ratio of most VOC species are still underestimated. We next constrain VOC emissions using CH2O observations from two satellites (OMI and OMPS) and the DC‐8 aircraft during KORUS‐AQ. To utilize data from multiple platforms in a consistent manner, we develop a two‐step Hybrid Iterative Finite Difference Mass Balance and four‐dimensional variational inversion system (Hybrid IFDMB‐4DVar). The total VOC emissions throughout the domain increase by 47%. The a posteriori simulation reduces the low biases of simulated CH2O (NMB: −0.51 to −0.15), O3(NMB: −0.19 to −0.06), and VOCs. Alterations to the VOC speciation from the 4D‐Var inversion include increases of biogenic isoprene emissions in Korea and anthropogenic emissions in Eastern China. We find that the IFDMB method alone is adequate for reducing the low biases of VOCs in general; however, 4D‐Var provides additional refinement of high‐resolution emissions and their speciation. Defining reasonable emission errors and choosing optimal regularization parameters are crucial parts of the inversion system. Our new hybrid inversion framework can be applied for future air quality campaigns, maximizing the value of integrating measurements from current and upcoming geostationary satellite instruments. A two step Hybrid IFDMB‐4DVar inversion systemVOC DC‐8 emissions in Northeast Asia were constrained using CH2O concentrations observed by Ozone Monitoring Instrument, Ozone Mapping and Profiler Suite, and DC‐8 aircraft during KORUS‐AQVOC emissions were increased by 47% leading to improved performance in simulation of O3and speciated VOCs A two step Hybrid IFDMB‐4DVar inversion system VOC DC‐8 emissions in Northeast Asia were constrained using CH2O concentrations observed by Ozone Monitoring Instrument, Ozone Mapping and Profiler Suite, and DC‐8 aircraft during KORUS‐AQ VOC emissions were increased by 47% leading to improved performance in simulation of O3and speciated VOCs

Wildfires are a major source of gas‐phase ammonia (NH3) to the atmosphere. Quantifying the evolution and fate of this NH3is important to understanding the formation of secondary aerosol in smoke and its accompanying effects on radiative balance and nitrogen deposition. Here, we use data from the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) to add new empirical constraints on the e‐folding loss timescale of NH3and its relationship with particulate ammonium (pNH4) within wildfire smoke plumes in the western U.S. during summer 2018. We show that the e‐folding loss timescale of NH3with respect to particle‐phase partitioning ranges from ∼24 to ∼4000 min (median of 55 min). Within these same plumes, oxidation of nitrogen oxides is observed concurrent with increases in the fraction of pNH4in each plume sampled, suggesting that formation of ammonium nitrate (NH4NO3) is likely. We find wide variability in how close our in situmeasurements of NH4NO3are to those expected in a dry thermodynamic equilibrium, and find that NH4NO3is most likely to form in fresh, dense smoke plumes injected at higher altitudes and colder temperatures. In chemically older smoke we observe correlations between both the fraction of pNH4and the fraction of particulate nitrate (pNO3) in the aerosol with temperature, providing additional evidence of the presence of NH4NO3and the influence of injection height on gas‐particle partitioning of NH3. We estimate e‐folding loss timescales for ammonia in eight western U.S. smoke plumes from 24 to 4000 min old (median = 55 min)Ammonium nitrate formation is favorable in fresh, dense plumes injected higher into the troposphere, nitric acid is normally limitingObserved ammonium and nitrate fractions increase with lower temperatures in medium (1–3 days) and old (>3 days) chemical aged smoke We estimate e‐folding loss timescales for ammonia in eight western U.S. smoke plumes from 24 to 4000 min old (median = 55 min) Ammonium nitrate formation is favorable in fresh, dense plumes injected higher into the troposphere, nitric acid is normally limiting Observed ammonium and nitrate fractions increase with lower temperatures in medium (1–3 days) and old (>3 days) chemical aged smoke

Reactive nitrogen (Nr) within smoke plumes plays important roles in the production of ozone, the formation of secondary aerosols, and deposition of fixed N to ecosystems. The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) field campaign sampled smoke from 23 wildfires throughout the western U.S. during summer 2018 using the NSF/NCAR C‐130 research aircraft. We empirically estimate Nrnormalized excess mixing ratios and emission factors from fires sampled within 80 min of estimated emission and explore variability in the dominant forms of Nrbetween these fires. We find that reduced N compounds comprise a majority (39%–80%; median = 66%) of total measured reactive nitrogen (ΣNr) emissions. The smoke plumes sampled during WE‐CAN feature rapid chemical transformations after emission. As a result, within minutes after emission total measured oxidized nitrogen (ΣNOy) and measured total ΣNHx(NH3+ pNH4) are more robustly correlated with modified combustion efficiency (MCE) than NOxand NH3by themselves. The ratio of ΣNHx/ΣNOydisplays a negative relationship with MCE, consistent with previous studies. A positive relationship with total measured ΣNrsuggests that both burn conditions and fuel N content/volatilization differences contribute to the observed variability in the distribution of reduced and oxidized Nr. Additionally, we compare our in situ field estimates of NrEFs to previous lab and field studies. For similar fuel types, we find ΣNHxEFs are of the same magnitude or larger than lab‐based NH3EF estimates, and ΣNOyEFs are smaller than lab NOxEFs. Smoke from large wildfires in the western U.S. degrades air quality across the whole U.S. Smoke contains a mixture of many different gases and particles, including carbon compounds like carbon dioxide and carbon monoxide, as well as nitrogen compounds such as ammonia and nitrogen oxides. Gases containing nitrogen are important for the production of ozone and the formation of more or larger particles as the smoke moves downwind. During the summer of 2018, we used the National Science Foundation/National Center for Atmospheric Research C130 research aircraft to fly through smoke across the western U.S. and measure many of the most abundant nitrogen compounds. We find that the smoke plumes we sampled emitted more nitrogen in a reduced form than in an oxidized form, and chemical reactions change the form and phase of nitrogen very quickly in the smoke. We compare our field measurements with laboratory measurements with the goal of using them together to improve our forecasts of how and where wildfire smoke will impact air quality. Normalized excess mixing ratios of nitrogen species are reported for western U.S. wildfires; median of 66% of emissions are in reduced formWithin minutes after emission, rapid chemistry changes the distribution (species/amount/phase) of reactive nitrogen in fresh smoke plumesFor similar fuel types, NHxemissions factors are similar to or higher than lab‐based estimates; NOyemission factors are smaller Normalized excess mixing ratios of nitrogen species are reported for western U.S. wildfires; median of 66% of emissions are in reduced form Within minutes after emission, rapid chemistry changes the distribution (species/amount/phase) of reactive nitrogen in fresh smoke plumes For similar fuel types, NHxemissions factors are similar to or higher than lab‐based estimates; NOyemission factors are smaller

This study quantifies the immediate influences of aerosols from urban anthropogenic and shipping emission sources on carbon and water fluxes in East Asia on a cloudy day in spring 2016 when strong regional pollution transport occurred and intensive field campaign measurements are available. Within National Aeronautics and Space Administration (NASA)'s Land Information System (LIS), a long‐term offline Noah‐multiparameterization (MP) simulation with dynamic vegetation is performed. Modeled soil moisture and leaf area index are evaluated with satellite observations to ensure that land surface conditions are moderately well reproduced. The LIS output is then used to initialize several coupled NASA‐Unified Weather Research and Forecasting model simulations with online chemistry in which urban anthropogenic and shipping emissions are (1) largely based on the Hemispheric Transport of Air Pollution Phase 2 inventory for 2010, (2) reduced by 20% and 50% for all chemical species, and (3) adjusted only for nitrogen oxides (NOx) using satellite observations. Overall, modeled gross primary productivity and evapotranspiration almost linearly increase with the all‐species emission reductions, but their responses to emission‐induced aerosol optical depth (AOD) changes show strong spatial variability resulting from combined radiation and temperature impacts. Using satellite‐observation‐constrained NOxemissions, modeled nitrogen species and AOD better match various measurements at some locations/times. All‐species and NOx‐only emission adjustments lead to different gross primary productivity and evapotranspiration changes with AOD, especially over South Korea. This study demonstrates the importance of accurately quantifying emission impacts on atmosphere‐biosphere interactions. Improving more species' emission inputs for Earth system models, including applying effective chemical data assimilation methods, is strongly encouraged. Carbon and water fluxes are calculated by Noah‐MP (with dynamic vegetation) coupled with atmospheric weather and chemistryOverall, GPP and ET almost linearly increase with the all‐species urban/shipping emission reductions on a cloudy day in East AsiaGPP and ET responses to emission‐induced AOD changes differ by region and for all‐species and NOx‐only emission adjustments

Nitryl chloride (ClNO2) plays an important role in the budget and distribution of tropospheric oxidants, halogens, and reactive nitrogen species. ClNO2is formed from the heterogeneous uptake and reaction of dinitrogen pentoxide (N2O5) on chloride‐containing aerosol, with a production yield, ϕ(ClNO2), defined as the moles of ClNO2produced relative to N2O5lost. The ϕ(ClNO2) has been increasingly incorporated into 3‐D chemical models where it is parameterized based on laboratory‐derived kinetics and currently accepted aqueous‐phase formation mechanism. This parameterization models ϕ(ClNO2) as a function of the aerosol chloride to water molar ratio. Box model simulations of night flights during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign derived 3,425 individual ϕ(ClNO2) values with a median of 0.138 and range of 0.003 to 1. Comparison of the box model median to those predicted by two other field‐based ϕ(ClNO2) derivation methods agreed within a factor of 1.3, within the uncertainties of each method. In contrast, the box model median was 75–84% lower than predictions from the laboratory‐based parameterization (i.e., [parameterization − box model]/parameterization). An evaluation of factors influencing this difference reveals a positive dependence of ϕ(ClNO2) on aerosol water, opposite to the currently parameterized trend. Additional factors may include aqueous‐phase competition reactions for the nitronium ion intermediate and/or direct ClNO2loss mechanisms. Further laboratory studies of ClNO2formation and the impacts of aerosol water, sulfate, organics, and ClNO2aqueous‐phase reactions are required to elucidate and quantify these processes on ambient aerosol, critical for the development of a robust ϕ(ClNO2) parameterization. Box modeling of wintertime measurements over the eastern United States provides the first determinations of ClNO2yields (ϕ(ClNO2)) from aircraftThe median ϕ(ClNO2) value derived from the box model is overpredicted by 75‐84% by the current laboratory‐based ϕ(ClNO2) parameterizationModeled and parameterized ϕ(ClNO2) values show opposite trends with aerosol water, suggesting an incorrectly parameterized dependence

We present airborne observations of gaseous reactive halogen species (HCl, Cl2, ClNO2, Br2, BrNO2, and BrCl), sulfur dioxide (SO2), and nonrefractory fine particulate chloride (pCl) and sulfate (pSO4) in power plant exhaust. Measurements were conducted during the Wintertime INvestigation of Transport, Emissions, and Reactivity campaign in February–March of 2015 aboard the NCAR‐NSF C‐130 aircraft. Fifty air mass encounters were identified in which SO2levels were elevated ~5 ppb above ambient background levels and in proximity to operational power plants. Each encounter was attributed to one or more potential emission sources using a simple wind trajectory analysis. In case studies, we compare measured emission ratios to those reported in the 2011 National Emissions Inventory and present evidence of the conversion of HCl emitted from power plants to ClNO2. Taking into account possible chemical conversion downwind, there was general agreement between the observed and reported HCl:SO2emission ratios. Reactive bromine species (Br2, BrNO2, and/or BrCl) were detected in the exhaust of some coal‐fired power plants, likely related to the absence of wet flue gas desulfurization emission control technology. Levels of bromine species enhanced in some encounters exceeded those expected assuming all of the native bromide in coal was released to the atmosphere, though there was no reported use of bromide salts (as a way to reduce mercury emissions) during Wintertime INvestigation of Transport, Emissions, and Reactivity observations. These measurements represent the first ever in‐flight observations of reactive gaseous chlorine and bromine containing compounds present in coal‐fired power plant exhaust. Gaseous inorganic chlorine and bromine species were observed in the exhaust of coal‐fired power plants during the WINTER flight campaignWe present evidence of the conversion of hydrogen chloride (HCl) emitted from power plants to nitryl chloride (ClNO2) downwind of a sourceObserved levels of Br2, BrNO2, and BrCl in some plumes strongly suggest artificial enhancement of bromide in the coal