The rise of hydraulic fracturing techniques has fostered rapid growth of oil and natural gas (O&NG) extraction in areas across the United States. In the Denver-Julesburg Basin (DJB), which mostly ...overlaps with Weld County in the Northern Colorado Front Range (NCFR) north of the City of Denver Metropolitan Area (DMA), the well drilling has increasingly approached, and in many instances moved into urban residential areas. During the same time, the region has also experienced steady population growth. The DMA – NCFR has been in exceedance of the ozone U.S. National Ambient Air Quality Standard (NAAQS) and was designated a non-attainment area of the standard in 2007. Despite State efforts to curb precursors, ozone has consistently remained above the standard. A growing number of atmospheric studies has provided an ever increasing body of literature for assessing influences from O&NG industry emissions on air quality in the DMA-NCFR. This paper provides 1. An overview of available literature on O&NG influences on the regional air quality, 2. A summary of the pertinent findings presented in these works, 3. An assessment of the most important pollutants and air quality impacts, 4. Identification of knowledge and monitoring gaps, and 5. Recommendations for future research and policy.
Anthropogenic activities have led to large-scale mercury (Hg) pollution in the Arctic. It has been suggested that sea-salt-induced chemical cycling of Hg (through 'atmospheric mercury depletion ...events', or AMDEs) and wet deposition via precipitation are sources of Hg to the Arctic in its oxidized form (Hg(ii)). However, there is little evidence for the occurrence of AMDEs outside of coastal regions, and their importance to net Hg deposition has been questioned. Furthermore, wet-deposition measurements in the Arctic showed some of the lowest levels of Hg deposition via precipitation worldwide, raising questions as to the sources of high Arctic Hg loading. Here we present a comprehensive Hg-deposition mass-balance study, and show that most of the Hg (about 70%) in the interior Arctic tundra is derived from gaseous elemental Hg (Hg(0)) deposition, with only minor contributions from the deposition of Hg(ii) via precipitation or AMDEs. We find that deposition of Hg(0)-the form ubiquitously present in the global atmosphere-occurs throughout the year, and that it is enhanced in summer through the uptake of Hg(0) by vegetation. Tundra uptake of gaseous Hg(0) leads to high soil Hg concentrations, with Hg masses greatly exceeding the levels found in temperate soils. Our concurrent Hg stable isotope measurements in the atmosphere, snowpack, vegetation and soils support our finding that Hg(0) dominates as a source to the tundra. Hg concentration and stable isotope data from an inland-to-coastal transect show high soil Hg concentrations consistently derived from Hg(0), suggesting that the Arctic tundra might be a globally important Hg sink. We suggest that the high tundra soil Hg concentrations might also explain why Arctic rivers annually transport large amounts of Hg to the Arctic Ocean.
The high reactivity and low vapor pressure of many biogenic volatile organic compounds (BVOC) make it difficult to measure whole-canopy fluxes of BVOC species using common analytical techniques. The ...most appropriate approach for estimating these BVOC fluxes is to determine emission rates from dynamic vegetation enclosure measurements. After scaling leaf- and branch-level emission rates to the canopy level, these fluxes can then be used in models to determine BVOC influences on atmospheric chemistry and aerosol processes. Previously published reports from enclosure measurements show considerable variation among procedures with limited guidelines or standard protocols to follow. This article reviews this literature and describes the variety of enclosure types, materials, and analysis techniques that have been used to determine BVOC emission rates. The current review article is followed by a companion paper which details a comprehensive enclosure technique that incorporates both recommendations from the literature as well as insight gained from theoretical calculations and practical experiences. These methods have yielded new BVOC emission data for highly reactive monoterpenes (MT) and sesquiterpenes (SQT) from a variety of vegetation species.
The radiative properties of soot particles depend on their morphology and mixing state, but their evolution during transport is still elusive. Here we report observations from an electron microscopy ...analysis of individual particles transported in the free troposphere over long distances to the remote Pico Mountain Observatory in the Azores in the North Atlantic. Approximately 70% of the soot particles were highly compact and of those 26% were thinly coated. Discrete dipole approximation simulations indicate that this compaction results in an increase in soot single scattering albedo by a factor of ≤2.17. The top of the atmosphere direct radiative forcing is typically smaller for highly compact than mass‐equivalent lacy soot. The forcing estimated using Mie theory is within 12% of the forcing estimated using the discrete dipole approximation for a high surface albedo, implying that Mie calculations may provide a reasonable approximation for compact soot above remote marine clouds.
Key Points
Long‐range transported soot in the free troposphere is very compact
Soot compaction increases single scattering albedo and reduces direct forcing
Mie model might provide a reasonable approximation in remote marine atmosphere
Dry deposition of ozone is an important sink of ozone in near‐surface air. When dry deposition occurs through plant stomata, ozone can injure the plant, altering water and carbon cycling and reducing ...crop yields. Quantifying both stomatal and nonstomatal uptake accurately is relevant for understanding ozone's impact on human health as an air pollutant and on climate as a potent short‐lived greenhouse gas and primary control on the removal of several reactive greenhouse gases and air pollutants. Robust ozone dry deposition estimates require knowledge of the relative importance of individual deposition pathways, but spatiotemporal variability in nonstomatal deposition is poorly understood. Here we integrate understanding of ozone deposition processes by synthesizing research from fields such as atmospheric chemistry, ecology, and meteorology. We critically review methods for measurements and modeling, highlighting the empiricism that underpins modeling and thus the interpretation of observations. Our unprecedented synthesis of knowledge on deposition pathways, particularly soil and leaf cuticles, reveals process understanding not yet included in widely used models. If coordinated with short‐term field intensives, laboratory studies, and mechanistic modeling, measurements from a few long‐term sites would bridge the molecular to ecosystem scales necessary to establish the relative importance of individual deposition pathways and the extent to which they vary in space and time. Our recommended approaches seek to close knowledge gaps that currently limit quantifying the impact of ozone dry deposition on air quality, ecosystems, and climate.
Plain Language Summary
The removal of tropospheric ozone at Earth's surface (often called dry deposition) is important for our understanding of air pollution, ecosystem health, and climate. Several processes contribute to dry deposition of ozone. While we have basic knowledge of these processes, we lack the ability to robustly estimate changes in ozone dry deposition through time and from one place to another. Here we review ozone deposition processes, measurements, and modeling and propose steps necessary to close gaps in understanding. A major conclusion revealed by our review is that most deposition processes can be fairly well described from a theoretical standpoint, but the relative importance of the various processes remains uncertain. We suggest that progress can be made by establishing multiyear measurements of ozone dry deposition at a limited set of sites around the world and coordinating these measurements with laboratory and field experiments that can be integrated with theory through carefully designed modeling studies.
Key Points
Ozone dry deposition through pathways other than plant stomata is critical for describing the total terrestrial ozone sink
Process‐level knowledge of ozone deposition pathways is missing from the models used to quantify deposition impacts on the Earth system
Long‐term ozone flux and related measurements are key for establishing relative importance of individual pathways
The deposition of ozone to seawater is an important ozone sink. Despite constituting as much as a third of the total ozone deposition, it receives significantly less attention than the deposition to ...terrestrial ecosystems. Models have typically calculated the deposition rate based on a resistance-in-series model with a uniform waterside resistance. This leads to models having an essentially uniform deposition velocity of approximately 0.05 cm s−1 to seawater, which is significantly higher than the limited observational dataset. Following from Luhar et al. (2018) we include a representation of the oceanic deposition of ozone in the GEOS-Chem model of atmospheric chemistry and transport based on its reaction with sea-surface iodide. The updated scheme halves the calculated annual area-weighted mean deposition velocity to water from 0.0464 cm s−1 (25th and 75th percentiles of 0.0461 cm s−1 and 0.0471 cm s−1 respectively) to 0.0231 cm s−1 (25th and 75th percentiles of 0.0121 cm s−1 and 0.0303 cm s−1 respectively). The calculated ozone deposition velocity varies from 0.009 cm s−1 in polar waters to 0.040 cm s−1 at the tropics. This improves comparisons to observations. The variability is driven mainly by the temperature-dependent rate constant for the reaction between iodide and ozone, the temperature dependence of the solubility, and variations in the ocean iodide concentration. The calculated annual deposition flux of ozone to the ocean is reduced from 222 to 122 Tg yr−1, and overall deposition of ozone to all surface types reduces from 862 to 758 Tg yr−1. Tropospheric ozone burdens and global mean OH increase from 324 to 328 Tg, and from 1.17×106 to 1.18×106 molec.cm-3, respectively. A total of 34 % of surface grid boxes experience a 10 % or greater increase in ozone concentration. Comparisons between observations of surface ozone and the model are improved with the new parameterization notably around the Southern Ocean. Process-level representation of oceanic deposition of ozone thus appears essential for representing the concentration of surface ozone over the planet.
Emissions of methane (CH4) from oil and natural gas (O&G) operations in the most densely drilled area of the Denver‐Julesburg Basin in Weld County located in northeastern Colorado are estimated for 2 ...days in May 2012 using aircraft‐based CH4 observations and planetary boundary layer height and ground‐based wind profile measurements. Total top‐down CH4 emission estimates are 25.8 ± 8.4 and 26.2 ± 10.7 t CH4/h for the 29 and 31 May flights, respectively. Using inventory data, we estimate the total emissions of CH4 from non‐O&G gas‐related sources at 7.1 ± 1.7 and 6.3 ± 1.0 t CH4/h for these 2 days. The difference in emissions is attributed to O&G sources in the study region, and their total emission is on average 19.3 ± 6.9 t/h, close to 3 times higher than an hourly emission estimate based on Environmental Protection Agency's Greenhouse Gas Reporting Program data for 2012. We derive top‐down emissions estimates for propane, n‐butane, i‐pentane, n‐pentane, and benzene from our total top‐down CH4 emission estimate and the relative hydrocarbon abundances in aircraft‐based discrete air samples. Emissions for these five nonmethane hydrocarbons alone total 25.4 ± 8.2 t/h. Assuming that these emissions are solely originating from O&G‐related activities in the study region, our results show that the state inventory for total volatile organic compounds emitted by O&G activities is at least a factor of 2 too low for May 2012. Our top‐down emission estimate of benzene emissions from O&G operations is 173 ± 64 kg/h, or 7 times larger than in the state inventory.
Key Points
Emissions from an oil and gas basin are estimated using airborne measurements
Inventories underestimate hydrocarbon emissions by a factor of 2 or more
After methane, ethane is the most abundant hydrocarbon in the remote atmosphere. It is a precursor to tropospheric ozone and it influences the atmosphere's oxidative capacity through its reaction ...with the hydroxyl radical, ethane's primary atmospheric sink. Here we present the longest continuous record of global atmospheric ethane levels. We show that global ethane emission rates decreased from 14.3 to 11.3 teragrams per year, or by 21 per cent, from 1984 to 2010. We attribute this to decreasing fugitive emissions from ethane's fossil fuel source--most probably decreased venting and flaring of natural gas in oil fields--rather than a decline in its other major sources, biofuel use and biomass burning. Ethane's major emission sources are shared with methane, and recent studies have disagreed on whether reduced fossil fuel or microbial emissions have caused methane's atmospheric growth rate to slow. Our findings suggest that reduced fugitive fossil fuel emissions account for at least 10-21 teragrams per year (30-70 per cent) of the decrease in methane's global emissions, significantly contributing to methane's slowing atmospheric growth rate since the mid-1980s.
In 2016, the Denver Metro Area (DMA)/Northern Colorado Front Range (NCFR) was reclassified from a Marginal to a Moderate O3 Non-Attainment Area due to the prevalence of high summer ozone (O3) ...occurrences. Hourly surface O3 data collected during 2000–2015 from a total of 80 monitoring sites in the State of Colorado were investigated for geographical features in O3 behavior and O3 changes over time. We particularly focus on summer O3 (June, July, August), which is the time when most exceedances of the O3 National Ambient Air Quality Standard (NAAQS) have been recorded. Variables investigated include the statistical (5th, 50th (median), and 95th percentile) distribution of O3 mixing ratios, diurnal amplitudes, and their trends. Trend analyses were conducted for 20 site records that had at least ten years of data. The majority of Colorado ozone monitoring sites show an increase of the 5th (16 total; 11 of these are statistically significant (p-value ≤ 0.05) trends) and 50th (15 total; 4 statistically significant trends) percentile values. Changes for the 95th percentile values were smaller and less consistent. One site showed a statistically significant declining trend, and one site an increasing trend; the majority of other sites had slightly negative, albeit not statistically significant declining O3. Ozone changes at the two highest elevations sites (>2500 m asl) are all negative, contrasting increasing O3 at U.S. West Coast sites. NCFR urban sites do not show the rate of decreasing higher percentile O3 as seen for the majority of urban areas across the U.S. during the past 1–2 decades. The amplitudes of diurnal O3 cycles were studied as a proxy for nitrogen oxides (NOx) emissions and the diurnal O3 production chemistry. The majority of sites show a decrease in the median summer O3 diurnal amplitude (19 total/10 statistically significant). This is mostly driven by the increase in nighttime O3 minima, which is most likely a sign for a declining rate of nighttime O3 loss from titration with nitric oxide (NO), indicating a change in O3 behavior from declining NOx emissions. Since median and upper percentile surface O3 values in the DMA have not declined at the rates seen in other western U.S. regions, thus far the reduction in NOx has had a more pronounced effect on the lower percentile O3 distribution than on high O3 occurrences that primarily determine air quality. An assessment of the influence of oil and gas emissions on Colorado, and in particular DMA O3, is hampered by the sparsity of monitoring within oil and gas basins. Continuous, long-term, high quality, and co-located O3, NOx, and VOC monitoring are recommended for elucidating the geographical heterogeneity of O3 precursors, their changing emissions, and for evaluation of the effectiveness of O3 air quality regulations.
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