Constraints on emissions of carbon monoxide, methane, and a suite of hydrocarbons in the Colorado Front Range using observations of <sup>14</sup>CO<sub>2</sub>

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

Simulating estimation of California fossil fuel and biosphere carbon dioxide exchanges combining in situ tower and satellite column observations

“…Propagating the above uncertainties (which are dominated by uncertainty in measurement of 14 CO 2 ) through equation yields an uncertainty in ffCO 2 of ~1.3 ppm, which we round up to 1.5 ppm in this work. We note that this estimate is similar to uncertainties reported in recent studies by using flask measurements of CO 2 and Δ 14 C in the Western U.S. (1.0–1.3 ppm, 1 σ ) [ LaFranchi et al , ; Turnbull et al , ]. As described in the section on regional inversions, we conduct some sensitivity tests to estimate the effect of systematic offset and gain biases in estimated fossil fuel CO 2 .…”

A new look at methane and nonmethane hydrocarbon emissions from oil and natural gas operations in the Colorado Denver‐Julesburg Basin

“…The aircraft and BAO discrete air samples discussed here were all analyzed by NOAA ESRL GMD for CH 4 using a gas chromatography GC‐flame ionization detector [ Dlugokencky et al ., ], for CO using resonance fluorescence at ~150 nm with a repeatability of ± 0.4 ppb [ Novelli et al ., ] and for 43 other compounds including the six nonmethane hydrocarbons mentioned above using a GC‐mass spectrometry (MS) [ Montzka et al ., ; Pétron et al ., ; Lafranchi et al ., ]. The GMD analyses of NMHCs in aircraft and BAO samples are reported on the same calibration scale: C 6 H 6 on NOAA‐2006 and all other hydrocarbons (besides CH 4 ) on NOAA‐2008.…”

“…Atmospheric chemical measurements conducted throughout the northern Colorado Front Range between 2007 and 2010 showed elevated levels of several hydrocarbons found in natural gas and oil, including CH 4 and other light alkanes (ethane (C 2 H 6 ), propane (C 3 H 8 ), i ‐butane ( i C 4 H 10 ), n ‐butane ( n C 4 H 10 ), i ‐pentane ( i C 5 H 12 ), and n ‐pentane ( n C 5 H 12 )), and sometimes aromatics including the carcinogen benzene (C 6 H 6 ) [ Eisele et al ., ; Pétron et al ., ; Lafranchi et al ., ]. These measurements showed similar relative enhancements of nonmethane hydrocarbons (NMHCs) as those observed in the early 1990s by Goldan et al .…”

Atmospheric measurement of point source fossil CO<sub>2</sub> emissions

“…Unfortunately, the emission ratio CO : CO 2 ff is imperfectly known, variable by combustion efficiency and method, and large power plants typically emit little or no CO (USEPA, 2012). Other correlate tracers have been considered, including sulphur hexafluoride (Turnbull et al, 2006), perchloroethylene , and acetylene (LaFranchi et al, 2013), but most of these tracers are only indirectly associated with CO 2 ff combustion sources, so are likely not appropriate for monitoring of individual point source emissions. Further, the amount of correlate trace gases emitted directly from the point source may vary widely depending on the fuel used, combustion process, and "scrubbing" of pollutant gases before they are emitted into the atmosphere.…”