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  1. Abstract. The continued warming of the Arctic could release vast stores of carbon into the atmosphere from high-latitude ecosystems, especially from thawingpermafrost. Increasing uptake of carbon dioxide (CO2) by vegetation during longer growing seasons may partially offset such release of carbon. However, evidence of significant net annual release of carbon from site-level observations and model simulations across tundra ecosystems has been inconclusive. To address this knowledge gap, we combined top-down observations of atmospheric CO2 concentration enhancements from aircraft and a tall tower, which integrate ecosystem exchange over large regions, with bottom-up observed CO2 fluxes from tundraenvironments and found that the Alaska North Slope is not a consistent net source nor net sink of CO2 to the atmosphere (ranging from −6 to+6 Tg C yr−1 for 2012–2017). Our analysis suggests that significant biogenic CO2 fluxes from unfrozen terrestrial soils, and likely inland waters, during the early cold season (September–December) are major factors in determining the net annual carbon balance of the North Slope, implying strong sensitivity to the rapidly warming freeze-up period. At the regional level, we find no evidence of the previously reported large late-cold-season (January–April) CO2 emissions to the atmosphere during the study period. Despite the importance of the cold-season CO2 emissionsmore »to the annual total, the interannual variability in the net CO2 flux is driven by the variability in growing season fluxes. During the growing season, the regional net CO2 flux is also highly sensitive to the distribution of tundra vegetation types throughout the North Slope. This study shows that quantification and characterization of year-round CO2 fluxes from the heterogeneous terrestrial and aquatic ecosystems in the Arctic using both site-level and atmospheric observations are important to accurately project the Earth system response to future warming.« less
  2. The Southern Ocean plays an important role in determining atmospheric carbon dioxide (CO 2 ), yet estimates of air-sea CO 2 flux for the region diverge widely. In this study, we constrained Southern Ocean air-sea CO 2 exchange by relating fluxes to horizontal and vertical CO 2 gradients in atmospheric transport models and applying atmospheric observations of these gradients to estimate fluxes. Aircraft-based measurements of the vertical atmospheric CO 2 gradient provide robust flux constraints. We found an annual mean flux of –0.53 ± 0.23 petagrams of carbon per year (net uptake) south of 45°S during the period 2009–2018. This is consistent with the mean of atmospheric inversion estimates and surface-ocean partial pressure of CO 2 ( P co 2 )–based products, but our data indicate stronger annual mean uptake than suggested by recent interpretations of profiling float observations.
  3. Abstract. We apply airborne measurements across three seasons(summer, winter and spring 2017–2018) in a multi-inversion framework toquantify methane emissions from the US Corn Belt and Upper Midwest, a keyagricultural and wetland source region. Combing our seasonal results withprior fall values we find that wetlands are the largest regional methanesource (32 %, 20 [16–23] Gg/d), while livestock (enteric/manure; 25 %,15 [14–17] Gg/d) are the largest anthropogenic source. Naturalgas/petroleum, waste/landfills, and coal mines collectively make up theremainder. Optimized fluxes improve model agreement with independentdatasets within and beyond the study timeframe. Inversions reveal coherentand seasonally dependent spatial errors in the WetCHARTs ensemble meanwetland emissions, with an underestimate for the Prairie Pothole region butan overestimate for Great Lakes coastal wetlands. Wetland extent andemission temperature dependence have the largest influence on predictionaccuracy; better representation of coupled soil temperature–hydrologyeffects is therefore needed. Our optimized regional livestock emissionsagree well with the Gridded EPA estimates during spring (to within 7 %) butare ∼ 25 % higher during summer and winter. Spatial analysisfurther shows good top-down and bottom-up agreement for beef facilities (withmainly enteric emissions) but larger (∼ 30 %) seasonaldiscrepancies for dairies and hog farms (with > 40 % manureemissions). Findings thus support bottom-up enteric emission estimates butsuggest errors for manure; we propose that the latter reflects inadequatetreatment of managementmore »factors including field application. Overall, ourresults confirm the importance of intensive animal agriculture for regionalmethane emissions, implying substantial mitigation opportunities throughimproved management.« less
  4. Abstract

    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. H2O2mixing 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 H2O2distribution is the influence of biomass burning emissions in the Atlantic Ocean basin, encountered in all four seasons, where the highest H2O2mixing 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 H2O2by a factor of 1.3–2.2, onmore »average, in the Atlantic compared with the Pacific Ocean basin.

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  5. Abstract

    We present airborne observations of the vertical gradient of atmospheric oxygen (δ(O2/N2)) and carbon dioxide (CO2) through the atmospheric boundary layer (BL) over the Drake Passage region of the Southern Ocean, during the O2/N2Ratio and CO2Airborne Southern Ocean Study, from 15 January to 29 February 2016. Gradients were predominately anticorrelated, with excesses ofδ(O2/N2) and depletions of CO2found within the boundary layer, relative to a mean reference height of 1.7 km. Through analysis of the molar ratio of the gradients (GR), the behavior of other trace gases measured in situ, and modeling experiments with the Community Earth System Model, we found that the main driver of gradients was air‐sea exchange of O2and CO2driven by biological processes, more so than solubility effects. An exception to this was in the eastern Drake Passage, where positive GRs were occasionally observed, likely due to the dominance of thermal forcing on the air‐sea flux of both species. GRs were more spatially consistent than the magnitudes of the gradients, suggesting that GRs can provide integrated process constraints over broad spatial scales. Based on the model simulation within a domain bounded by 45°S, 75°S, 100°W, and 45°W, we show that the sampling density of the campaign was suchmore »that the observed mean GR (± standard error), −4.0± 0.8 mol O2per mol CO2, was a reasonable proxy for both the mean GR and the mean molar ratio of air‐sea fluxes of O2and CO2during the O2/N2Ratio and CO2Airborne Southern Ocean Study.

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  6. Abstract

    Carbon monoxide (CO) is an ozone precursor, oxidant sink, and widely used pollution tracer. The importance of anthropogenic versus other CO sources in the US is uncertain. Here, we interpret extensive airborne measurements with an atmospheric model to constrain US fossil and nonfossil CO sources. Measurements reveal a low bias in the simulated CO background and a 30% overestimate of US fossil CO emissions in the 2016 National Emissions Inventory. After optimization we apply the model for source partitioning. During summer, regional fossil sources account for just 9%–16% of the sampled boundary layer CO, and 32%–38% of the North American enhancement—complicating use of CO as a fossil fuel tracer. The remainder predominantly reflects biogenic hydrocarbon oxidation plus fires. Fossil sources account for less domain‐wide spatial variability at this time than nonfossil and background contributions. The regional fossil contribution rises in other seasons, and drives ambient variability downwind of urban areas.

  7. Abstract

    Acetone is one of the most abundant oxygenated volatile organic compounds (VOCs) in the atmosphere. The oceans impose a strong control on atmospheric acetone, yet the oceanic fluxes of acetone remain poorly constrained. In this work, the global budget of acetone is evaluated using two global models: CAM‐chem and GEOS‐Chem. CAM‐chem uses an online air‐sea exchange framework to calculate the bidirectional oceanic acetone fluxes, which is coupled to a data‐oriented machine‐learning approach. The machine‐learning algorithm is trained using a global suite of seawater acetone measurements. GEOS‐Chem uses a fixed surface seawater concentration of acetone to calculate the oceanic fluxes. Both model simulations are compared to airborne observations from a recent global‐scale, multiseasonal campaign, the NASA Atmospheric Tomography Mission (ATom). We find that both CAM‐chem and GEOS‐Chem capture the measured acetone vertical distributions in the remote atmosphere reasonably well. The combined observational and modeling analysis suggests that (i) the ocean strongly regulates the atmospheric budget of acetone. The tropical and subtropical oceans are mostly a net source of acetone, while the high‐latitude oceans are a net sink. (ii) CMIP6 anthropogenic emission inventory may underestimate acetone and/or its precursors in the Northern Hemisphere. (iii) The MEGAN biogenic emissions model may overestimatemore »acetone and/or its precursors, and/or the biogenic oxidation mechanisms may overestimate the acetone yields. (iv) The models consistently overestimate acetone in the upper troposphere‐lower stratosphere over the Southern Ocean in austral winter. (v) Acetone contributes up to 30–40% of hydroxyl radical production in the tropical upper troposphere/lower stratosphere.

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