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During winter in the midlatitudes, photochemical oxidation is significantly slower than in summer and the main radical oxidants driving formation of secondary pollutants, such as fine particulate matter and ozone, remain uncertain, owing to a lack of observations in this season. Using airborne observations, we quantify the contribution of various oxidants on a regional basis during winter, enabling improved chemical descriptions of wintertime air pollution transformations. We show that 25–60% of NOxis converted to N2O5via multiphase reactions between gas‐phase nitrogen oxide reservoirs and aerosol particles, with ~93% reacting in the marine boundary layer to form >2.5 ppbv ClNO2. This results in >70% of the oxidizing capacity of polluted air during winter being controlled by multiphase reactions and emissions of volatile organic compounds, such as HCHO, rather than reaction with OH. These findings highlight the control local anthropogenic emissions have on the oxidizing capacity of the polluted wintertime atmosphere. During summer, rapid transformations of primary pollutants, those emitted directly into the atmosphere, into secondary pollutants, such as particulate matter and ozone, are driven by reactions with the hydroxyl radical, formed in the atmosphere when sunlight strikes ozone in the presence of water vapor. During winter, when there is less sunlight and water vapor, production of this radical is lower. Yet the conversion of primary pollutants into secondary pollutants still occurs rapidly, pointing to a misunderstanding in the chemical processes that drive this conversion during winter. Using aircraft data collected across the northeast United States during the winter of 2015, we show that reactions with radicals arising from atypical precursors, such as nitryl chloride, account for more than 70% of the reactions that directly emitted pollutants undergo. We show that during winter, the formation of these radicals is tied to human activities. Our data provide critical constraints for improving the descriptions of chemical processes in air quality models, which will help guide improved air quality policy. Other regions of the world, such as China, Europe, and northern India, also experience this seasonal chemical shift in the atmosphere. Our findings, therefore, have global scale implications for understanding wintertime pollution transformations and transport. Observations show atypical radical precursors drive >70% of atmospheric oxidation in polluted air over the northeast United States during winterAnthropogenic emissions and multiphase chemistry largely control the abundance of these atypical radical precursorsImproved descriptions of these radical sources in air quality models are needed to predict future trajectories in wintertime air pollution

The South Central United States is a hot spot for anthropogenic methane (CH4) emissions, with contributions from the oil/gas (O&G) and animal agriculture sectors. During frontal weather events, airflow combines enhancements from these emissions into a large plume. In this study, we take CH4and ethane (C2H6) observations from the Atmospheric Carbon and Transport‐America campaign and adjust O&G and animal agriculture emissions such that modeled CH4and C2H6enhancements match the observed plume. Results from the joint CH4‐C2H6optimization indicate that emissions from the O&G sector are 1.8 ± 0.7 (2σ) times larger than EPA inventory estimates. These results match synthesis work from recent literature and reject the possibility that this increase compared to inventories is due to a potential bias in daytime‐only measurements of these facilities. Successful modeling from this study raises the possibility of using trace gas measurements along frontal crossings to solve for emissions in other regions of the United States. Methane and ethane observations from U.S. Midwest frontal flights are used to solve for emissions from oil/gas and animal agricultureMethane emissions from oil/gas sectors in the South Central United States are 1.8 ± 0.7 (2σ) times larger than U.S. EPA inventory estimatesMeasurements across frontal structures can provide opportunities to solve for regional emissions of trace gases in other regions

One‐second in situ measurements of CO and CO2mole fractions were made aboard the National Aeronautics and Space Administration DC‐8 aircraft during the 2016 KORUS‐AQ joint air quality and atmospheric chemistry field campaign in South Korea. The ratio of CO to CO2enhancement is used to characterize regional combustion source signatures. Calculations of the ∆CO/∆CO2ratio were made with a short duration rolling window (60 s), filtered by the coefficient of determination (R2), and plotted as distributions to characterize air masses measured from the aircraft during the campaign. The KORUS‐AQ sampling domain was divided into analysis regions to facilitate the analysis. Over Seoul, the boundary layer shows a low‐ratio signature in the ∆CO/∆CO2ratios, with more than 50% of the correlated slopes in the boundary layer falling below 1% ∆CO/∆CO2, and 80% of the slopes between 0% and 2% ∆CO/∆CO2. However, this behavior changes to a larger ratio distribution at higher altitudes. The West Sea receptor region was divided into three analysis sectors, by meteorological regime, and used in conjunction with measurements collected over China during the KORUS‐AQ campaign time period to characterize the Chinese ∆CO/∆CO2ratio signature. Chinese‐type emissions have a slope distribution that is shifted to higher ratios and broadened compared to measurements over Seoul, with the bulk of the measurements between 2% and 4% ∆CO/∆CO2, with few negative slopes. The measured ratio trends over South Korea are consistent with inventoried CO and CO2emissions. Measurements of carbon monoxide (CO) and carbon dioxide (CO2) were made over South Korea in May and June of 2016. The ratio of CO to CO2provides information about the combustion sources in a region, as most of the CO measured in the atmosphere is sourced from combustion, such as open air fires or engines. Using a 1‐s data set, the short‐term slopes of CO to CO2enhancement were calculated by making a measurement of the CO to CO2slope at every observation using the data in the surrounding minute. Plotting the distributions of the short‐term slopes with correlated values in the calculation window reveals whether the air masses have mostly low ratios of CO/CO2, which indicates high‐efficiency combustion, or mostly high ratios of CO/CO2, from low‐efficiency combustion. The slope distributions over the Seoul megacity show low‐ratio distributions, but at high altitudes the ratios move to higher values, showing that the combustion sources change with altitude and have a less efficient combustion signature. Meanwhile, the ratios that were measured over the West Sea show that the air coming to South Korea from China has higher CO/CO2ratios than the air that is measured over South Korea. Short‐term ∆CO/∆CO2ratios characterized air masses measured by aircraft over South Korea during the 2016 KORUS‐AQ field campaignThe air mass combustion efficiency signature can be assessed with the distribution of the correlated short‐term CO to CO2slopesDifferences were found between Chinese influenced and South Korea influenced air masses, which reflect the known sources in each country

Production of coal and natural gas is responsible for one third of anthropogenic methane (CH4) emissions in the United States. Here we examine CH4emissions from coal and natural gas production in southwestern Pennsylvania. Using a top‐down methodology combining measurements of CH4and ethane, we conclude that while Environmental Protection Agency inventories appear to report emissions from coal accurately, emissions from unconventional natural gas are underreported in the region by a factor of 5 (±3). However, production‐scaled CH4emissions from unconventional gas production in the Marcellus remain small compared to other basins due to its large production per well. After normalizing emissions by energy produced, total greenhouse gas emissions from Pennsylvania unconventional natural gas production produce half the carbon footprint compared to regionally produced coal, with carbon dioxide emissions from combustion being the dominant source of greenhouse gas emissions for both sources. Methane and ethane observations from aircraft are used to quantify methane emissions from coal and natural gas production in PennsylvaniaMethane emissions from coal production align with national estimates, whereas emissions from natural gas production are underestimatedEnergy produced through natural gas production in Pennsylvania has half the carbon footprint compared to energy from coal mining

High‐frequency (20 Hz) aircraft measurements of tropical tropopause layer (TTL) cirrus and humidity are used to examine recent homogeneous freezing ice nucleation events. Several localized regions with peak ice concentrations greater than 10 cm−3are documented. These high ice concentrations are almost certainly generated by homogeneous freezing of aqueous aerosols. The high‐frequency data reveals considerable structure in the cloud properties and humidity at along‐track scales as short as 8.5 m. Simulations of vapor depletion by growing ice crystals are used to assess the time since nucleation in these high ice concentration patches. Comparisons between observed and simulated dependence of supersaturation on ice concentration indicate the nucleation events occurred within minutes (sometimes only a few minutes) of the aircraft sampling. The infrequency and short ages of the events suggest that the high ice concentrations do not persist for long times, but they may still be important for TTL cirrus microphysical properties by providing a source of ice crystals that can lead to extensive, persistent clouds with lower ice concentrations. We have also used the extensive TTL measurements provided by the high‐altitude aircraft campaigns to evaluate the threshold supersaturation for homogeneous freezing. Peak supersaturations observed are consistent with recent laboratory experiments indicating that ice saturation ratios of about 1.75–1.95 are required for homogeneous freezing of aqueous aerosols at temperatures of 185–200 K. High‐frequency aircraft measurements of cirrus clouds and humidity in the cold, tropical upper troposphere are used to examine recent ice crystal formation events. Several localized regions with very large numbers of small ice crystals are documented. The numerous ice crystals are almost certainly generated by spontaneous freezing of abundant liquid aerosol particles. The high‐frequency data reveals considerable fine‐scale structure in the cloud properties and humidity. Comparisons with calculations of water vapor depletion by growing ice crystals indicate the formation events occurred within minutes (sometimes only a few minutes) of the aircraft sampling. The infrequency and short ages of the events suggest that the high ice number densities do not persist for long times, but they may still be important for cirrus physical properties by providing a source of ice crystals that can lead to extensive, persistent clouds with lower ice numbers. We have also used the extensive measurements provided by the high‐altitude aircraft campaigns to evaluate the threshold conditions for spontaneous freezing of liquid aerosol particles. The threshold conditions are consistent with recent laboratory experiments indicating that substantial super‐cooling must occur before the liquid aerosol particles can freeze. High‐frequency airborne cloud and humidity measurements are used to examine high ice concentration events in the tropical tropopause layerThe supersaturation in some of these events indicates that ice nucleation occurred less than 12 min prior to the aircraft samplingPeak supersaturation measurements indicate a higher threshold for homogeneous freezing than previously assumed in models High‐frequency airborne cloud and humidity measurements are used to examine high ice concentration events in the tropical tropopause layer The supersaturation in some of these events indicates that ice nucleation occurred less than 12 min prior to the aircraft sampling Peak supersaturation measurements indicate a higher threshold for homogeneous freezing than previously assumed in models

Accurate fire emissions inventories are crucial to predict the impacts of wildland fires on air quality and atmospheric composition. Two traditional approaches are widely used to calculate fire emissions: a satellite‐based top‐down approach and a fuels‐based bottom‐up approach. However, these methods often considerably disagree on the amount of particulate mass emitted from fires. Previously available observational datasets tended to be sparse, and lacked the statistics needed to resolve these methodological discrepancies. Here, we leverage the extensive and comprehensive airborne in situ and remote sensing measurements of smoke plumes from the recent Fire Influence on Regional to Global Environments and Air Quality (FIREX‐AQ) campaign to statistically assess the skill of the two traditional approaches. We use detailed campaign observations to calculate and compare emission rates at an exceptionally high‐resolution using three separate approaches: top‐down, bottom‐up, and a novel approach based entirely on integrated airborne in situ measurements. We then compute the daily average of these high‐resolution estimates and compare with estimates from lower resolution, global top‐down and bottom‐up inventories. We uncover strong, linear relationships between all of the high‐resolution emission rate estimates in aggregate, however no single approach is capable of capturing the emission characteristics of every fire. Global inventory emission rate estimates exhibited weaker correlations with the high‐resolution approaches and displayed evidence of systematic bias. The disparity between the low‐resolution global inventories and the high‐resolution approaches is likely caused by high levels of uncertainty in essential variables used in bottom‐up inventories and imperfect assumptions in top‐down inventories. Smoke emitted by wildland fires is dangerous to human health and contributes to climate change. To predict and evaluate the impacts of fires, we need to know how much smoke is emitted into the atmosphere. There are two state‐of‐the‐art methods used to estimate the mass of smoke emitted by fires, but they often disagree. In this study, we use unusually detailed measurements collected using an aircraft that flew within wildland fire smoke plumes to calculate the amount of smoke emitted from fires in the Western United States. We compare emission rates derived from the exceptionally high spatial and temporal resolution approach to the two traditional, lower resolution approaches to understand why they sometimes diverge. In situ measurements of wildland fire smoke plumes provide emission rates for evaluating emissions inventories at unprecedented resolutionFire emissions inventories struggle to capture the emissions rate characteristics of individual fires but may perform well in the aggregateBottom‐up inventories suffer from major uncertainty in key variables, while top‐down inventories may have bias from imperfect assumptions In situ measurements of wildland fire smoke plumes provide emission rates for evaluating emissions inventories at unprecedented resolution Fire emissions inventories struggle to capture the emissions rate characteristics of individual fires but may perform well in the aggregate Bottom‐up inventories suffer from major uncertainty in key variables, while top‐down inventories may have bias from imperfect assumptions

To define the structural gene for ornithine decarboxylase (ODC) in Neurospora crassa, we sought mutants with kinetically altered enzyme. Four mutants, PE4, PE7, PE69, and PE85, were isolated. They were able to grow slowly at 25 degrees C on minimal medium but required putrescine or spermidine supplementation for growth at 35 degrees C. The mutants did not complement with one another or with ODC-less spe-1 mutants isolated in earlier studies. In all of the mutants isolated to date, the mutations map at the spe-1 locus on linkage group V. Strains carrying mutations PE4, PE7, and PE85 displayed a small amount of residual ODC activity in extracts. None of them had a temperature-sensitive enzyme. The enzyme of the PE85 mutant had a 25-fold higher Km for ornithine (5mM) than did the enzyme of wild-type or the PE4 mutant (ca. 0.2 mM). The enzyme of this mutant was more stable to heat than was the wild-type enzyme. These characteristics were normal in the mutant carrying allele PE4. The mutant carrying PE85 was able to grow well at 25 degrees C and weakly at 35 degrees C with ornithine supplementation. This mutant and three ODC-less mutants isolated previously displayed a polypeptide corresponding to ODC in Western immunoblots with antibody raised to purified wild-type ODC. We conclude that spe-1 is the structural gene for the ODC.

We wished to identify metabolic signals governing changes in ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) activity in Neurospora crassa. By manipulations of the ornithine supply and by the use of inhibitors of the polyamine pathway, we found that spermidine negatively governs formation of active ornithine decarboxylase and that putrescine promotes inactivation of the enzyme. Direct addition of putrescine or spermidine to cycloheximide-treated cells confirmed the role of putrescine in enzyme inactivation and showed that spermidine had no effect on this process. Increases in ornithine decarboxylase activity caused by blocking spermidine synthesis occurred prior to a significant decrease in the spermidine pool. This is consistent with our previous finding that only 10-20% of the spermidine pool is freely diffusible within N. crassa cells. We presume that only this small fraction of the pool is active in regulation.

Ornithine decarboxylase, a highly regulated enzyme of the polyamine pathway, was purified 670-fold from mycelia of Neurospora crassa that were highly augmented for enzyme activity. The enzyme is significantly different from those reported from three other lower eucaryotic organisms: Saccharomyces cerevisiae, Physarum polycephalum, and Tetrahymena pyriformis. Instead, the enzyme closely resembles the enzymes from mammals. The Mr = 110,000 enzyme is a dimer of 53,000 Da subunits, with a specific activity of 2,610 mumol per h per mg of protein. Antisera were raised to the purified enzyme and were rendered highly specific by cross-absorption with extracts of a mutant strain lacking ornithine decarboxylase protein. With the antisera, we show that the inactivation of the enzyme in response to polyamines is proportional to the loss of ornithine decarboxylase protein over almost 2 orders of magnitude. This is similar to the inactivation process in certain mammalian tissues, and different from the process in S. cerevisiae and P. polycephalum, in which enzyme modification, without proportional loss of antigen, accompanies enzyme inactivation. The N. crassa enzyme is therefore suitable as a microbial model for studies of the molecular regulation of the mammalian enzyme.

The largest share in the climate impact of aviation results from contrail cirrus clouds. Here, the dependence of microphysical contrail ice properties and extinction on temperature and humidity is investigated. Contrail measurements were performed at various altitudes during the 2018 ECLIF II/NDMAX campaign with the NASA DC‐8 chasing the DLR A320. Ice number concentrations and contrail extinction coefficients are largest at altitudes near 9.5 km, typical for short‐ and medium‐range air traffic. At higher altitudes near 11.5 km, low ambient water vapor concentrations lead to smaller contrail particle sizes and lower extinction coefficients. In addition, contrails were detected below 8.2 km near the Schmidt‐Appleman contrail formation threshold temperature. Here, only a small fraction (<15%) of the emitted soot particles were activated into ice. Our observations enhance the understanding of contrail formation near the formation threshold and give a glimpse on the altitude dependence of climate‐relevant contrail properties. Aviation has an impact on our climate and of all components, contrails are the main contributor. In January 2018, we measured contrails during the ECLIF II/NDMAX campaign. The NASA research aircraft DC‐8 flew closely behind the DLR's A320. With a wide range of instruments, we measured trace gases and soot particles emitted by the A320, as well as contrail ice crystals. The data presented here are the most comprehensive contrail data set ever collected during a single flight campaign. The contrail properties are strongly influenced by ambient parameters as well as aircraft and fuel properties. Here, we focus on the influence of environmental conditions such as temperature and humidity on the formation and growth of contrail ice crystals. Therefore, we present and compare ice measurements at three different flight altitudes. For 1–2‐min old contrails, we find a decrease in extinction with increasing altitude. Extinction describes the interaction between contrails and atmospheric radiation. We also find that at the lowest and warmest altitudes only a small part of the emitted soot particles is activated into contrail ice crystals. In the end, our data help to better understand contrail formation and will contribute to the research on sustainable aviation. The dependence of contrail ice microphysics and extinction on ambient temperature and humidity is investigatedSoot activation fractions in contrails formed at varying ambient temperatures are derived of soot and apparent ice emission indicesAt altitudes below 8.2 km, we observed contrails formed under incomplete soot activation near the contrail formation threshold temperature The dependence of contrail ice microphysics and extinction on ambient temperature and humidity is investigated Soot activation fractions in contrails formed at varying ambient temperatures are derived of soot and apparent ice emission indices At altitudes below 8.2 km, we observed contrails formed under incomplete soot activation near the contrail formation threshold temperature

Earth's atmosphere oxidizes the greenhouse gas methane and other gases, thus determining their lifetimes and oxidation products. Much of this oxidation occurs in the remote, relatively clean free troposphere above the planetary boundary layer, where the oxidation chemistry is thought to be much simpler and better understood than it is in urban regions or forests. The NASA airborne Atmospheric Tomography study (ATom) was designed to produce cross sections of the detailed atmospheric composition in the remote atmosphere over the Pacific and Atlantic Oceans during four seasons. As part of the extensive ATom data set, measurements of the atmosphere's primary oxidant, hydroxyl (OH), and hydroperoxyl (HO2) are compared to a photochemical box model to test the oxidation chemistry. Generally, observed and modeled median OH and HO2agree to within combined uncertainties at the 2σconfidence level, which is ~±40%. For some seasons, this agreement is within ~±20%below 6‐km altitude. While this test finds no significant differences, OH observations increasingly exceeded modeled values at altitudes above 8 km, becoming ~35% greater, which is near the combined uncertainties. Measurement uncertainty and possible unknown measurement errors complicate tests for unknown chemistry or incorrect reaction rate coefficients that would substantially affect the OH and HO2abundances. Future analysis of detailed comparisons may yield additional discrepancies that are masked in the median values. Chemistry in the vast, remote atmosphere destroys methane and other greenhouse gases. This chemistry is thought to be simple and well understood compared to that in polluted cities or in forests. From the NASA airborne Atmospheric Tomography study over remote oceans, comparisons of observed and modeled reactive gases show that the chemistry is generally understood to within the uncertainties of the measurement and model. However, for the atmosphere's primary reactive gas, hydroxyl, measured values exceed modeled values in the upper troposphere, pointing to errors in probably the measurements but possibly the model chemistry. Free tropospheric hydroxyl and hydroperoxyl radical chemistry appears to be understood to within measurement and model uncertainty of ±40%Observed hydroxyl often exceeded modeled values in the upper troposphere, but measurement uncertainty masks any model chemistry errorsA hydroxyl measurement interference found in forests for this instrument does not exist throughout the free troposphere, even near convection

We examine the distribution and fate of nitrogen oxides (NOx) in the lower troposphere over the Northeast United States (NE US) using aircraft observations from the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign in February–March 2015, as well as the GEOS‐Chem chemical transport model and concurrent ground‐based observations. We find that the National Emission Inventory from the U.S. Environmental Protection Agency is consistent with WINTER observations of total reactive nitrogen (TNOy) to within 10% on average, in contrast to the significant overestimate reported in past studies under warmer conditions. Updates to the dry deposition scheme and dinitrogen pentoxide (N2O5) reactive uptake probability, ɣ(N2O5), result in an improved simulation of gas‐phase nitric acid (HNO3) and submicron particulate nitrate (pNO3−), reducing the longstanding factor of 2–3 overestimate in wintertime HNO3+ pNO3−to a 50% positive bias. We find a NOxlifetime against chemical loss and deposition of 22 hr in the lower troposphere over the NE US. Chemical loss of NOxis dominated by N2O5hydrolysis (58% of loss) and reaction with OH (33%), while 7% of NOxleads to the production of organic nitrates. Wet and dry deposition account for 55% and 45% of TNOydeposition over land, respectively. We estimate that 42% of the NOxemitted is exported from the NE US boundary layer during winter, mostly in the form of HNO3+ pNO3−(40%) and NOx(38%). Nitrogen oxides are a key family of pollutants emitted by cars, electric utilities, and industry. The fate of nitrogen oxides remains poorly understood especially during the winter season, when low sunlight leads to their persistence in the atmosphere. We analyze comprehensive aircraft observations of nitrogen oxides and their atmospheric products over the Northeast United States during winter 2015. This detailed chemical information allows to resolve a long‐standing overestimate of the oxidation products of nitrogen oxides and places new constraints on their deposition to land ecosystems and export to the global atmosphere. Existing anthropogenic NOxinventory is consistent with aircraft and ground‐based observations over Northeast United States during winterNOxhas a 22 hr lifetime, with half of NOypresent as NOx, 37% as HNO3and pNO3−, and remaining 13% mostly as PANModel reproduces NOypartitioning and predicts a 42% NOxexport efficiency in winter, with a 55–45% split between wet and dry deposition