Here we analyze decadal records of CH 4 (2006–2015) and C 3 H 8 (2008–2015) from 11 sites where CH 4 vertical profiles are collected from aircraft (Sweeney et al., 2015 ) and 9 surface sites (including tall towers; Andrews et al., 2014 ) in the National Oceanic and Atmospheric Administration Global Greenhouse Gas Reference Network (GGGRN; see Figure S2 for site locations). The majority of the sites are designed to capture air masses that are well mixed and thus represent influences of emissions from large areas. According to our footprint and inventory (Maasakkers et al., 2016 ) analysis in which the sensitivity of observed mole fraction changes to emissions from different sources are estimated (see Figure S3 for footprints), five GGGRN sites are substantially influenced by ONG activities (DND, CAR, SGP, SGP‐s, and WKT‐s; ‘‐s’ following a site code indicates surface site; site codes are given in Table S1 in the supporting information ). Thus, these five sites are defined as ‘ONG sites’ in the following text. The other sites are defined as ‘non‐ONG sites’, although some of them are moderately influenced by ONG emissions. Please refer to Supporting Information (SI) for details of measurement and statistical analysis.

Atmospheric CH 4 is a well‐mixed greenhouse gas with the second largest increase in radiative forcing after carbon dioxide (Butler & Montzka, 2017 ). It can be released from natural (e.g., wetlands, wild animals, and termites) and anthropogenic sources (e.g., oil and natural gas [ONG] operations, landfills, and agriculture; United States Environmental Protection Agency [EPA], 2017 , Saunois et al., 2016 ). The global atmospheric CH 4 abundance was nearly stable from 1999 through 2006 (Bousquet et al., 2006 ; Dlugokencky et al., 2003 ; Dlugokencky et al., 2009 ), but since then has significantly increased (Dlugokencky, 2018 ; Nisbet et al., 2016 ). ONG activities are a large source of atmospheric CH 4 and alkanes such as ethane (C 2 H 6 ), propane (C 3 H 8 ), and others. In the past decade, natural gas production has increased by ~46% in the United States (Figure S1 in the supporting information ) due to the development of horizontal drilling and hydraulic‐fracturing techniques (U.S. Energy Information Administration, 2016 ). Several studies conclude that there are substantial increases in ONG CH 4 emissions (Franco et al., 2016 ; Hausmann et al., 2016 ; Helmig et al., 2016 ; Turner et al., 2016 ), with some (Hausmann et al., 2016 ; Turner et al., 2016 ) further suggesting that these have substantially contributed to the global CH 4 increases after 2007. Ethane/methane emission ratios have been used to argue for large increases in ONG CH 4 emissions (Franco et al., 2016 ; Hausmann et al., 2016 ; Helmig et al., 2016 ), since C 2 H 6 and CH 4 are coemitted from ONG activities, although Helmig et al. ( 2016 ) notes an inconsistency with measurements of CH 4 and its isotopic ratios (δ 13 CH 4 ). Measured changes of δ 13 CH 4 lead to the hypothesis that the global CH 4 rise after 2007 is dominated by biogenic emissions (Nisbet et al., 2016 ; Schaefer et al., 2016 ; Schwietzke et al., 2016 ). A recent study using an ensemble of global atmospheric inversions constrained by surface observations, with some including satellite retrievals of column‐averaged CH 4 , finds no statistically significant increase in total North American CH 4 emissions during 2000–2014 (Bruhwiler et al., 2017 ).

2 Trends in CH 4 and CH 4 Vertical Gradients (ΔCH 4 )

Atmospheric methane trends from GGGRN sites demonstrate clear increases after 2006 (e.g., Figures 1a and 1b), similar to the global background CH 4 trend. Excluding the five ONG sites, CH 4 trends in the United States (contiguous 48 states, ‘CONUS’) are indistinguishable within 1σ uncertainty with an average increase rate of 6.12 ± 0.11 ppb/year (0.33 ± 0.01%/year). This is similar to the trends in the marine boundary layer reference (Dlugokency et al., 2015) of 6.11 ± 0.07 ppb/year for the 25–55°N zonal average and of 6.08 ± 0.04 ppb/year for the global average. For the five ONG sites, a significantly larger trend is measured in CH 4 mole fractions, which is 7.65 ± 0.31 ppb/year (0.40 ± 0.02%/year).

Figure 1 Open in figure viewer PowerPoint 4 vertical gradients (ΔCH 4 ) and trend fits to those data. (e and f) C 3 H 8 vertical gradients (ΔC 3 H 8 ) and trend fits to those data. Trend fits are performed to annual means and are weighted by the standard errors of the mean (blue error bars; SI section 10 scale (see SI section Methane and propane data and trend fits. Left column shows data from Southern Great Plains, Oklahoma (SGP‐s, an ONG site), and right column shows data from Worcester, Massachusetts (NHA, an east coast outflow site; see Figure S2 for average wind pattern). (a and b) Deseasonalized data and trend fits to those data. For the aircraft site (NHA), those data are the averages below 2.5 km above sea level. (c and d) CHvertical gradients (ΔCH) and trend fits to those data. (e and f) Cvertical gradients (ΔC) and trend fits to those data. Trend fits are performed to annual means and are weighted by the standard errors of the mean (blue error bars; SI section 2 ). The left axes are in logscale (see SI section 2 for details); labels on the right axes show corresponding values of the ticks on the left axes. Trends are presented in Figures 2 and S6

To reveal trends in CONUS CH 4 emissions, we assess CH 4 mole fraction enhancements (‘Δ’) after removing appropriate background mole fractions, because the background contributes the largest part of CH 4 in ambient air, while enhancements due to regional and local emissions are relatively small. A trend in CH 4 mole fractions (see above) without subtracting the background signals cannot represent the trend in local and regional emissions. We use the midtroposphere (3.5–5.5 km above sea level) as representative of background condition and investigate the trends in ΔCH 4 , which are, in this case, also vertical gradients (e.g., Figures 1c and 1d; see SI section 3 for calculation). A lower boundary of 3.5 km ensures free troposphere air, and the upper boundary ensures that the background air masses are not completely detached from the surface since measurements of CH 4 and its trend at high altitudes may considerably lag surface CH 4 in time and be influenced by the stratosphere. Free troposphere measurements have been used as background references in previous studies of CO 2 (including its isotopic ratios), which also has a large background signal (Ballantyne et al., 2010; Miller et al., 2012). We do not use west coast sites as background reference for all CONUS sites since a large fraction of the air masses we measure are not from the western sector directly (see Figure S2 for average wind pattern). The vertical gradient of CH 4 has been proposed as a more sensitive indicator of surface emissions than horizontal gradients (Bruhwiler et al., 2017). When CH 4 is emitted at the surface, the enhanced CH 4 is mostly retained within the planetary boundary layer (typically below 2.5 km above sea level) for several days, while the free troposphere receives a considerably smaller local influence and mostly represents well‐mixed background air (Sweeney et al., 2015). East coast sites that are downwind of CONUS emissions show much larger vertical gradients (39.3 ppb for the average of the NHA and SCA sites; see Table S2) than the west coast inflow site (12.0 ppb at ESP).

We expect to detect trends in vertical gradients if there are increases in surface emissions from locations upwind of a measurement site and no significant change in transport patterns. Our footprint analysis finds that all of the GGGRN sites have similar patterns of surface influences during the first and second halves of the past decade (Figure S4). However, we find no significant trends, meaning the estimated trend is less than1σ uncertainty of the trend, in the CH 4 vertical gradients (ΔCH 4 ) at most non‐ONG sites (Figures 2 and S6). The average trend in ΔCH 4 is ‐0.21 ± 0.10 ppb/year (‐1.00 ± 0.36%/year) for non‐ONG sites. For the five ONG sites, the average trend is 1.14 ± 0.30 ppb/year (2.05 ± 0.58%/year). The average trend for east coast sites is ‐0.12 ± 0.16 ppb/year (‐0.10 ± 0.50%/year). Note that trends in ΔCH 4 are contributed by both anthropogenic and natural sources. However, we find strong correlations between ΔCH 4 and ΔC 3 H 8 (a tracer for ONG emissions) in winter even for non‐ONG sites (Figure S5), which is likely due to the reduction in natural CH 4 emissions during wintertime. Despite the increased wintertime correlation between ΔCH 4 and ΔC 3 H 8 and the steeper vertical gradients due to the reduced vertical transport, there is no evidence of an increased trend in ΔCH 4 during winter (Figure S7) as would be expected if there were significant increases in ONG CH 4 emissions.

Figure 2 Open in figure viewer PowerPoint 4 , C 3 H 8 , and C 2 H 6 enhancements (‘Δ’) over North America in recent years (2006‐2015 for CH 4 and 2008‐2015 for C 2 H 6 and C 3 H 8 for most sites; see Table 4 or in ppt/year for C 2 H 6 and C 3 H 8 . The error bars show 1σ uncertainty. For CH 4 and C 3 H 8 , enhancements are relative to midtroposphere measurements (thus, the trends are for vertical gradients, also see 2 H 6 , enhancements are relative to the Marine Boundary Layer background (SI section Trends in CH, C, and Cenhancements (‘Δ’) over North America in recent years (2006‐2015 for CHand 2008‐2015 for Cand Cfor most sites; see Table S1 .). The green squares and black dots show ONG and non‐ONG sites, respectively. ‘‐s’ following a site code indicates surface site. For all bar charts each tick increment is 2%/year and the horizontal axis crosses at 0%/year (e.g., ETL and DND); ‘%/year’ means increase of Δ relative to previous year; see Table S2 for values, and trends in ppb/year for CHor in ppt/year for Cand C. The error bars show 1σ uncertainty. For CHand C, enhancements are relative to midtroposphere measurements (thus, the trends are for vertical gradients, also see Figure S6 ). For C, enhancements are relative to the Marine Boundary Layer background (SI section 3 ).

The GGGRN sites are sensitive to ONG emissions because their footprint areas include major ONG production basins in CONUS (Figure S3). However, we find moderate increases in ΔCH 4 from three out of five ONG sites (DND, SGP‐s, and WKT‐s). If we use the ONG trends from these three sites to represent the ONG emissions trend in the United States (see SI section 5 for calculation), the average (weighted by upwind ONG CH 4 emissions) annual growth rate would be 3.4 ± 1.4%/year or 0.3 ± 0.1 Tg/year2 as a long‐term average (note that the relative trend in %/year is independent of inventory estimates of emissions). This estimate is about an order of magnitude lower than estimates from several previous studies showing 2.1–4.4 Tg/year2 increase (Turner et al., 2016, Franco et al., 2016, Hausmann et al., 2016, Helmig et al., 2016; note different but overlapping study periods). Nevertheless, a few studies suggest an underestimate in the magnitude of CH 4 emissions in inventories (e.g., Alvarez et al., 2018; Brandt et al., 2014; Miller et al., 2013); our study does not address the magnitude of emissions but only focuses on the trend in emissions that can be estimated directly from atmospheric observations. This relatively small trend in ONG emissions (3.4 ± 1.4%/year) is challenging for the east coast sites to detect.

How much do CH 4 emissions need to increase to be detected by the GGGRN? Since the relative trend in vertical gradients is equal to the relative trend in total regional emissions (both as %/year changes) considering no significant secular changes in atmospheric transport/mixing (as shown in Figure S4), the uncertainty of the trend in vertical gradients can serve as an indicator for the detectability of the trend in emissions. Note that the variability in midtroposphere background has been accounted for in estimating the trend uncertainty (SI section 3). We find that more than half of the sites have trend uncertainties (1σ) smaller than 1.3%/year (Table S2), which suggests at least half of GGGRN sites can likely detect a relative change of total CH 4 emissions greater than 1.3%/year (averaged over the 10‐year period). The detectability thresholds across four east coast sites (NHA, SCA, SCT‐s, and AMT‐s) range from 0.7 to 1.2 %/year. Our estimated ONG emission trend of 3.4 ± 1.4%/year corresponds to a 0.7 ± 0.3%/year increase in total U.S. emissions assuming that ONG emissions account for 20% of total emissions (United States EPA, 2017, Saunois et al., 2016). Thus, these east coast sites are not sensitive enough to clearly capture the relatively small ONG emissions trend (meaning 0.7 ± 0.3%/year is not significantly higher than detectability thresholds for east coast sites). Increasing the numbers of vertical profiles and sampling sites would help to decrease uncertainty in trends and better monitor changes in ONG emissions. However, if the large trends of ONG emissions proposed by previous studies exist (Franco et al., 2016; Hausmann et al., 2016; Helmig et al., 2016; Turner et al., 2016), which correspond to trends in total CH 4 emissions of 5.2–11.0%/year, the east coast sites are capable of detecting them. Such large trends are not apparent in our measurement data from east coast sites making the suggested large increases in U.S. ONG emissions highly unlikely. Note that our trend and detectability are directly calculated from long‐term observations, and influences of temporal coverage are fully considered during calculations (SI section 3). Thus, they are more definitive than model results (Bruhwiler et al., 2017) that are subject to transport and other errors.

In principle, a significant decrease in agricultural CH 4 emissions (mainly from livestock) could have cancelled the increases in ONG emissions given that both emissions are of similar magnitude (United States EPA, 2017). The footprint regions of two ONG sites (SGP‐s and WKT‐s) overlap with important cattle production regions. To evaluate the potential impact of a livestock emission trend on estimated ONG trends, we impose an agriculture emission trend (‐1.7%/year for 2006‐2013; Wolf et al., 2017) in calculating the new ONG trends for these two ONG sites (SI section 5). The new ONG trends are 3.2 ± 1.3%/year and 4.8± 1.0%/year for SGP‐s and WKT‐s, respectively, which are within the uncertainty ranges of our previous estimates. This is expected because both sites are dominated by ONG emissions and non‐ONG emissions account for less than 15% of the CH 4 enhancements (Wolf et al., 2017).