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Constraints on emissions of carbon monoxide, methane, and a suite of hydrocarbons in the Colorado Front Range using observations of (CO2)-C-14

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Abstract

Atmospheric radiocarbon (14CO) represents an important observational constraint on emissions of fossil-fuel derived carbon into the atmosphere due to the absence of 14CO in fossil fuel reservoirs. The high sensitivity and precision that accelerator mass spectrometry (AMS) affords in atmospheric 14CO analysis has greatly increased the potential for using such measurements to evaluate bottom-up emissions inventories of fossil fuel CO2 (CO2ff), as well as those for other co-emitted species. Here we use observations of 14CO2 and a series of hydrocarbons and combustion tracers from discrete air samples collected between June 2009 and September 2010 at the National Oceanic and Atmospheric Administration Boulder Atmospheric Observatory (BAO; Lat: 40.050° N, Lon: 105.004° W) to derive emission ratios of each species to CO2ff. From these emission ratios, we estimate emissions of these species by using the Vulcan CO2ff high resolution data product as a reference. The species considered in this analysis are carbon monoxide (CO), methane (CH4), acetylene (C2H2), benzene (C6H6), and C3-C5 alkanes. Comparisons of top-down emissions estimates are made to existing inventories of these species for Denver and adjacent counties, as well as to previous efforts to estimate emissions from atmospheric observations over the same area. We find that CO is overestimated in the 2008 National Emissions Inventory (NEI, 2008) by a factor of ~2. A close evaluation of the inventory suggests that the ratio of CO emitted per unit fuel burned from on-road gasoline vehicles is likely over-estimated by a factor of 2.5. The results also suggest that while the oil and gas sector is the largest contributor to the CH4 signal in air arriving from the north and east, it is very likely that other sources, including agricultural sources, contribute to this signal and must be accounted for when attributing these signals to oil and gas industry activity from a top-down perspective. Our results are consistent with ~60% of the total CH4 emissions from regions to the north and east of the BAO tower stemming from the oil and gas industry, equating to ~70 Gg yr-1 or ~1.7% of total natural gas production in the region.
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... attributes 64% of the total point source VOC emissions in Colorado to O&NG extraction activities, while in Weld County the O&NG contributions account for 90% of the total point source VOC emissions. Several studies conducted within the DJB since 2011 have found a clear O&NG signature in atmospheric VOCs, with enhancement in the alkanes and aromatics without corresponding increases in combustion tracers[Pétron et al., 2012;Gilman et al., 2013;LaFranchi et al., 2013;Swarthout et al., 2013;Thompson et al., 2014]. O&NG-related VOC sources include leaks from infrastructure and transport, fugitive emissions, and deliberate venting and flaring[Adgate et al., 2014;Vinciguerra et al., 2015]. ...
... O&NG-related VOC sources include leaks from infrastructure and transport, fugitive emissions, and deliberate venting and flaring[Adgate et al., 2014;Vinciguerra et al., 2015]. Increased concentrations of methane, light alkanes, and aromatics in the DJB have been connected to O&NG development[Pétron et al., 2012;Gilman et al., 2013;LaFranchi et al., 2013;Thompson et al., 2014;Eisele et al., 2016]. In addition to degrading air quality, high VOC concentrations are also a concern from a public health perspective. ...
... Instead, our observations suggest a stationary CO source and a separate, temporally variable benzene source, both located in the same general region to the SW of PAO. In Colorado CO emissions have been attributed to gasoline vehicles (66%) and nonroad gasolinepowered equipment (26%)[LaFranchi et al., 2013]. The remaining emissions are attributed to a variety of processes, including industrial activity and natural gas processing. ...
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High time resolution measurements of volatile organic compounds (VOCs) were collected using a proton-transfer-reaction quadrupole mass spectrometry (PTR-QMS) instrument at the Platteville Atmospheric Observatory (PAO) in Colorado to investigate how oil and natural gas (O&NG) development impacts air quality within the Wattenburg Gas Field (WGF) in the Denver-Julesburg Basin. The measurements were carried out in July and August 2014 as part of NASA's DISCOVER-AQ field campaign. The PTR-QMS data were supported by pressurized whole air canister samples and airborne vertical and horizontal surveys of VOCs. Unexpectedly high benzene mixing ratios were observed at PAO at ground level (mean benzene = 0.53 ppbv, maximum benzene = 29.3 ppbv), primarily at night (mean nighttime benzene = 0.73 ppbv). These high benzene levels were associated with southwesterly winds. The airborne measurements indicate that benzene originated from within the WGF, and typical source signatures detected in the canister samples implicate emissions from O&NG activities rather than urban vehicular emissions as primary benzene source. This conclusion is backed by a regional toluene-to-benzene ratio analysis which associated southerly flow with vehicular emissions from the Denver area. Weak benzene-to-CO correlations confirmed that traffic emissions were not responsible for the observed high benzene levels. Previous measurements at the Boulder Atmospheric Observatory (BAO) and our data obtained at PAO allow us to locate the source of benzene enhancements between the two atmospheric observatories. Fugitive emissions of benzene from O&NG operations in the Platteville area are discussed as the most likely causes of enhanced benzene levels at PAO.
... Our flask-based R CO estimate is similar to but slightly lower than with other recent top-down observational studies in U.S. cities and regions (observations from 2004 to 2013), which have obtained R CO values ranging from 9 to 14 ppb/ppm [Turnbull et al., 2006; Graven et al., 2009; Wunch et al., 2009; Turnbull et al., 2011a; Miller et al., 2012; LaFranchi et al., 2013]. Our slightly lower values than observed previously are likely a consequence of two factors. ...
... 3.3.2. Winter R CO Comparison With Bottom-Up Inventory-Based Estimates Using R CO , and assuming that the emission flux of CO 2 ff is well constrained from the Hestia bottom-up data product, we can infer the CO emission flux [e.g., Turnbull et al., 2011a; Miller et al., 2012; LaFranchi et al., 2013]. This method relies on CO and CO 2 ff being coemitted and requires an estimate of the spatial extent of the tower footprint. ...
... The trend in USEPA NEI CO estimates through time suggests a possible decrease in CO emissions of a few percent from 2011 to 2012 (although 2012 data are not available), insufficient to explain the difference. A similar apparent overestimate of the CO emission flux from U.S. cities in the USEPA CO inventories has been observed in a number of studies in several other U.S. regions [Turnbull et al., 2006; Graven et al., 2009; Miller et al., 2012; Parrish, 2006; Hudman et al., 2008; Brioude et al., 2011; Kim et al., 2013] and is suspected to be due primarily to an overestimate of the on-road mobile emissions sector CO emissions [LaFranchi et al., 2013; Kota et al., 2014]. The USEPA NEI2011 also provides a breakdown of CO emissions by source sector, reporting that on-road vehicle (gasoline and diesel) emissions account for 69% of Marion County CO emissions or 114,000 tCO yr À1 (Table 3). ...
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The Indianapolis Flux Experiment (INFLUX) aims to develop and assess methods for quantifying urban greenhouse gas emissions. Here we use CO2, 14CO2 and CO measurements from tall towers around Indianapolis, USA to determine urban total CO2, the fossil fuel derived CO2 component (CO2ff) and CO enhancements relative to background measurements. When a local background directly upwind of the urban area is used, the wintertime total CO2 enhancement over Indianapolis can be entirely explained by urban CO2ff emissions. Conversely, when a continental background is used, CO2ff enhancements are larger and account for only half the total CO2 enhancement, effectively representing the combined CO2ff enhancement from Indianapolis and the wider region. In summer, we find that diurnal variability in both background CO2 mole fraction and co-varying vertical mixing make it difficult to use a simple upwind-downwind difference for a reliable determination of total CO2 urban enhancement. We use characteristic CO2ff source sector CO:CO2ff emission ratios to examine the contribution of the CO2ff source sectors to total CO2ff emissions. This method is strongly sensitive to the mobile sector, which produces most CO. We show that the inventory-based emission product (“bottom-up”) and atmospheric observations (“top-down”) can be directly compared throughout the diurnal cycle using this ratio method. For Indianapolis, the top-down observations are consistent with the bottom-up Hestia data product emission sector patterns for most of the diurnal cycle, but disagree during the nighttime hours. Further examination of both the top-down and bottom-up assumptions is needed to assess the exact cause of the discrepancy.
... Another study reports on radiocarbon measurements in California (Riley et al., 2008). Graven et al. (2011) andLaFranchi et al. (2013) use radiocarbon observations from an aircraft and a tall tower, respectively, to estimate the contribution of anthropogenic and biogenic CO 2 emissions in Colorado. Beyond these studies, radiocarbon measurements are not widely used in regional-or continental-scale inversions. ...
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This review paper explores recent efforts to estimate state- and national-scale carbon dioxide (CO2) and methane (CH4) emissions from individual anthropogenic source sectors in the US. Nearly all state and national climate change regulations in the US target specific source sectors, and detailed monitoring of individual sectors presents a greater challenge than monitoring total emissions. We particularly focus on opportunities to synthesize disparate types of information on emissions, including emission inventory data and atmospheric greenhouse gas data. We find that inventory estimates of sector-specific CO2 emissions are sufficiently accurate for policy evaluation at the national scale but that uncertainties increase at state and local levels. CH4 emission inventories are highly uncertain for all source sectors at all spatial scales, in part because of the complex, spatially variable relationships between economic activity and CH4 emissions. In contrast to inventory estimates, top-down estimates use measurements of atmospheric mixing ratios to infer emissions at the surface; thus far, these efforts have had some success identifying urban CO2 emissions and have successfully identified sector-specific CH4 emissions in several opportunistic cases. We also describe a number of forward-looking opportunities that would aid efforts to estimate sector-specific emissions: fully combine existing top-down datasets, expand intensive aircraft measurement campaigns and measurements of secondary tracers, and improve the economic and demographic data (e.g., activity data) that drive emission inventories. These steps would better synthesize inventory and top-down data to support sector-specific emission reduction policies.
... (Brandt et al., 2014). While there are an increasing number of field studies addressing methane leakage (Allen et al., 2013; Karion et al., 2013; Miller et al., 2013; Phillips et al., 2013; Caulton et al., 2014b; Karion et al., 2015; Lan et al., 2015; Lavoie et al., 2015; Peischl et al., 2015; Yacovitch et al., 2015; Zimmerle et al., 2015), other field studies have addressed simultaneously emitted higher hydrocarbon emissions as a result of UOG exploration (Petron et al., 2012; Gilman et al., 2013; LaFranchi et al., 2013; Peischl et al., 2013; Swarthout et al., 2013; Edwards et al., 2014; Helmig et al., 2014; Macey et al., 2014; Petron et al., 2014; Thompson et al., 2014; Zavala-Araiza et al., 2014; Smith et al., 2015; Townsend-Small et al., 2015; Yuan et al., 2015). Ethane has been used as an indicator of the UOG exploration methane source (Simpson et al., 2012; Smith et al., 2015; Townsend-Small et al., 2015; Vinciguerra et al., 2015), and the large variety of volatile, non-methane hydrocarbons (NMHCs) associated with oil and gas and their potential impacts on atmospheric ozone formation have been investigated in detail (Kemball-Cook et al., 2010; Carter and Seinfeld, 2012; Edwards et al., 2014; Helmig et al., 2014; Ahmadov et al., 2015; Field et al., 2015; Koss et al., 2015). ...
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Abstract Within the last decade, unconventional oil and gas exploration in the US has become a new source of atmospheric hydrocarbons. Although a geographically dispersed source, field measurements in and downwind of a number of shale basins demonstrate the impact exploration activities have on ambient levels of hydrocarbons. Due to concerns related to ozone production, regulatory agencies are adding monitoring stations to better understand the potential influence of emissions from areas with increased oil and gas related activities. The Eagle Ford shale in south Texas is a rapidly developing shale play producing both oil and natural gas, providing 10% and 5% of US domestic oil and gas production, respectively, in 2013. We analyzed the first year of measurements from a newly established monitoring site at its central north edge. The data reveal median ethane mixing ratios—used as a marker for oil and gas exploration related emissions—at five times its typical clean air background. Ethane mixing ratios above ten times the background occurred regularly. Saturated hydrocarbons with likely origin in oil and gas exploration explain half of the data set’s variability. They dominate OH radical reactivity at levels both similar to other shale areas and similar to Houston’s ship channel area a decade ago. Air advecting slowly across the shale area from east-southeast and southwest directions shows the most elevated hydrocarbon concentrations, and evidence is presented linking elevated alkene abundances to flaring in the shale area. A case study is presented linking high emissions from an upwind facility to hydrocarbon plumes observed at the monitor.
... This is false and represents a misunderstanding of a decadeslong effort within the biogeochemistry and climate-change research communities to validate anthropogenic emissions inventories. As described in a series of peer-reviewed articles (e.g., see Cambaliza et al. 2015;Gurney 2014;Duren and Miller 2011;LaFranchi et al. 2013;Hutyra et al. 2014;Turnbull et al. 2011Turnbull et al. , 2015Brioude et al. 2012;McKain et al. 2012;Weiss and Prinn 2011), the approach taken by the Hestia Project is specifically constructed to best leverage atmospheric concentration measurements as validation in a "top-down/ bottom-up" procedure. Atmospheric concentration measurements have been used for decades in this way, with a recent review and recommendation of continued research by the U.S. National Research Council (NRC), among others (NRC 2010). ...
... Carbon monoxide can be used to separate anthropogenic CO2 from biotic sources (Vogel et al., 2010;Turnbull et al., 2011;LaFranchi et al., 2013), provided the emission ratios of the two gases is known. ...
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Chapter
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