Hydrocarbon Tracers Suggest Methane Emissions from Fossil Sources Occur Predominately Before Gas Processing and That Petroleum Plays Are a Significant Source

We use global airborne observations of propane (C₃H₈) and ethane (C₂H₆) from the Atmospheric Tomography (ATom) and HIAPER Pole-to-Pole Observations (HIPPO), as well as U.S.-based aircraft and tower observations by NOAA and from the NCAR FRAPPE campaign as tracers for emissions from oil and gas operations. To simulate global mole fraction fields for these gases, we update the default emissions' configuration of C₃H₈ used by the global chemical transport model, GEOS-Chem v13.0.0, using a scaled C₂H₆ spatial proxy. With the updated emissions, simulations of both C₃H₈ and C₂H₆ using GEOS-Chem are in reasonable agreement with ATom and HIPPO observations, though the updated emission fields underestimate C₃H₈ accumulation in the arctic wintertime, pointing to additional sources of this gas in the high latitudes (e.g., Europe). Using a Bayesian hierarchical model, we estimate global emissions of C₂H₆ and C₃H₈ from fossil fuel production in 2016-2018 to be 13.3 ± 0.7 (95% CI) and 14.7 ± 0.8 (95% CI) Tg/year, respectively. We calculate bottom-up hydrocarbon emission ratios using basin composition measurements weighted by gas production and find their magnitude is higher than expected and is similar to ratios informed by our revised alkane emissions. This suggests that emissions are dominated by pre-processing activities in oil-producing basins.

Enhanced North American carbon uptake associated with El Niño

Long-term atmospheric CO₂ mole fraction and δ¹³CO₂ observations over North America document persistent responses to the El Niño-Southern Oscillation. We estimate these responses corresponded to 0.61 (0.45 to 0.79) PgC year^-1 more North American carbon uptake during El Niño than during La Niña between 2007 and 2015, partially offsetting increases of net tropical biosphere-to-atmosphere carbon flux around El Niño. Anomalies in derived North American net ecosystem exchange (NEE) display strong but opposite correlations with surface air temperature between seasons, while their correlation with water availability was more constant throughout the year, such that water availability is the dominant control on annual NEE variability over North America. These results suggest that increased water availability and favorable temperature conditions (warmer spring and cooler summer) caused enhanced carbon uptake over North America near and during El Niño.

Increased water-use efficiency and reduced CO2 uptake by plants during droughts at a continental-scale

Severe droughts in the Northern Hemisphere cause widespread decline of agricultural yield, reduction of forest carbon uptake, and increased CO2 growth rates in the atmosphere. Plants respond to droughts by partially closing their stomata to limit their evaporative water loss, at the expense of carbon uptake by photosynthesis. This trade-off maximizes their water-use efficiency, as measured for many individual plants under laboratory conditions and field experiments. Here we analyze the 13C/12C stable isotope ratio in atmospheric CO2 (reported as δ13C) to provide new observational evidence of the impact of droughts on the water-use efficiency across areas of millions of km2 and spanning one decade of recent climate variability. We find strong and spatially coherent increases in water-use efficiency along with widespread reductions of net carbon uptake over the Northern Hemisphere during severe droughts that affected Europe, Russia, and the United States in 2001-2011. The impact of those droughts on water-use efficiency and carbon uptake by vegetation is substantially larger than simulated by the land-surface schemes of six state-of-the-art climate models. This suggests that drought induced carbon-climate feedbacks may be too small in these models and improvements to their vegetation dynamics using stable isotope observations can help to improve their drought response.

Ratios of greenhouse gas emissions observed over the Yellow Sea and the East China Sea

During a cruise of the survey vessel Dongfanghong II on the Yellow Sea and the East China Sea in the spring of 2017 we performed accurate measurements of the mole fractions of carbon dioxide (CO₂), methane (CH₄), carbon monoxide (CO) and nitrous oxide (N₂O) using two types of Cavity Ring-Down Spectrometers (CRDS). The spatial variations of the mole fraction of the four trace gases were very similar. The emission sources of these gases were divided into several regions by using the NOAA HYSPLIT model. Then we analyzed the variations of the ratios of the mole fraction enhancements between every pair of trace gases downwind of these source areas. The ratios showed that the distributions of these trace gases over the Yellow Sea and the East China Sea in the spring were mainly caused by the emissions from Eastern China. The much higher enhancement ratio of ΔCO/ΔCO₂ and the lower ratio of ΔCH₄/ΔCO observed in the air parcels from big cities like Beijing and Shanghai indicated high CO emission from the cities during our time of observation. Compared with the values of NOAA's Marine Boundary Layer (MBL), the ratios of the averages in the air coming from the Northern sector (Russia) were on average closer to the MBL, and the air that stayed over the Yellow Sea and the East China Sea was a mixture of emissions from wide regional areas. The highly variable N₂O data of the air from Qingdao and Shanghai showed much more fluctuation.

Improved Mechanistic Understanding of Natural Gas Methane Emissions from Spatially Resolved Aircraft Measurements

Divergence in recent oil and gas related methane emission estimates between aircraft studies (basin total for a midday window) and emissions inventories (annualized regional and national statistics) indicate the need for better understanding the experimental design, including temporal and spatial alignment and interpretation of results. Our aircraft-based methane emission estimates in a major U.S. shale gas basin resolved from west to east show (i) similar spatial distributions for 2 days, (ii) strong spatial correlations with reported NG production (R² = 0.75) and active gas well pad count (R² = 0.81), and (iii) 2× higher emissions in the western half (normalized by gas production) despite relatively homogeneous dry gas and well characteristics. Operator reported hourly activity data show that midday episodic emissions from manual liquid unloadings (a routine operation in this basin and elsewhere) could explain ∼1/3 of the total emissions detected midday by the aircraft and ∼2/3 of the west-east difference in emissions. The 22% emission difference between both days further emphasizes that episodic sources can substantially impact midday methane emissions and that aircraft may detect daily peak emissions rather than daily averages that are generally employed in emissions inventories. While the aircraft approach is valid, quantitative, and independent, our study sheds new light on the interpretation of previous basin scale aircraft studies, and provides an improved mechanistic understanding of oil and gas related methane emissions.

A Large Northern Hemisphere Terrestrial CO2 Sink Indicated by the 13C/12C Ratio of Atmospheric CO2

Measurements of the concentrations and carbon-13/carbon-12 isotope ratios of atmospheric carbon dioxide can be used to quantify the net removal of carbon dioxide from the atmosphere by the oceans and terrestrial plants. A study of weekly samples from a global network of 43 sites defined the latitudinal and temporal patterns of the two carbon sinks. A strong terrestrial biospheric sink was found in the temperate latitudes of the Northern Hemisphere in 1992 and 1993, the magnitude of which is roughly half that of the global fossil fuel burning emissions for those years. The challenge now is to identify those processes that would cause the terrestrial biosphere to absorb carbon dioxide in such large quantities.

Weak Northern and Strong Tropical Land Carbon Uptake from Vertical Profiles of Atmospheric CO2

Measurements of midday vertical atmospheric CO2 distributions reveal annual-mean vertical CO2 gradients that are inconsistent with atmospheric models that estimate a large transfer of terrestrial carbon from tropical to northern latitudes. The three models that most closely reproduce the observed annual-mean vertical CO2 gradients estimate weaker northern uptake of -1.5 petagrams of carbon per year (Pg C year(-1)) and weaker tropical emission of +0.1 Pg C year(-1) compared with previous consensus estimates of -2.4 and +1.8 Pg C year(-1), respectively. This suggests that northern terrestrial uptake of industrial CO2 emissions plays a smaller role than previously thought and that, after subtracting land-use emissions, tropical ecosystems may currently be strong sinks for CO2.

Evaluation of solid adsorbent materials for cryogen-free trapping—gas chromatographic analysis of atmospheric C2–C6 non-methane hydrocarbons

Nine commercial solid adsorbent materials (in order of decreasing surface area: Carboxen 1000, Carbosieve S III, molecular sieve 5A, molecular sieve 4A, silica gel, Carboxen 563, activated alumina, Carbotrap and Carboxen 1016) were investigated for their ability to trap and release C2-C6 non-methane hydrocarbons (NMHCs) in atmospheric samples for subsequent thermal desorption gas chromatography-flame ionization detection analysis (GC-FID). Recovery rates for 23 NMHCs and methyl chloride (CH3Cl) were determined. A microtrap filled with the three adsorbents Carbosieve S III, Carboxen 563 and Carboxen 1016 was found to allow for the analysis of the widest range of target analytes. A detection limit of approximately 3pptC [parts per trillion (carbon)] in a 1l air sample and a linear response over a wide range of volatilities and sample volumes was determined for this configuration. Water vapor in the sample air was found to causes interference in trapping and subsequent chromatographic analysis of light NMHCs. A Peltier-cooled, regenerable water trap inserted into the sample flow path was found to mitigate these problems and to allow quantitative and reproducible results for all analytes at all tested humidity conditions.

Stable isotopic analysis of atmospheric methane by infrared spectroscopy by use of diode laser difference-frequency generation

An infrared absorption spectrometer has been constructed to measure the stable isotopic composition of atmospheric methane samples. The spectrometer employs periodically poled lithium niobate to generate 15 microW of tunable difference-frequency radiation from two near-infrared diode lasers that probe the nu3 rotational-vibrational band of methane at 3.4 microm. To enhance the signal, methane is extracted from 25 l of air by use of a cryogenic chromatographic column and is expanded into the multipass cell for analysis. A measurement precision of 12 per thousand is demonstrated for both delta13C and deltaD.

Regional changes in carbon dioxide fluxes of land and oceans since 1980

We have applied an inverse model to 20 years of atmospheric carbon dioxide measurements to infer yearly changes in the regional carbon balance of oceans and continents. The model indicates that global terrestrial carbon fluxes were approximately twice as variable as ocean fluxes between 1980 and 1998. Tropical land ecosystems contributed most of the interannual changes in Earth's carbon balance over the 1980s, whereas northern mid- and high-latitude land ecosystems dominated from 1990 to 1995. Strongly enhanced uptake of carbon was found over North America during the 1992-1993 period compared to 1989-1990.

Global carbon sinks and their variability inferred from atmospheric O2 and delta13C

Recent time-series measurements of atmospheric O2 show that the land biosphere and world oceans annually sequestered 1.4 +/- 0.8 and 2.0 +/- 0.6 gigatons of carbon, respectively, between mid-1991 and mid-1997. The rapid storage of carbon by the land biosphere from 1991 to 1997 contrasts with the 1980s, when the land biosphere was approximately neutral. Comparison with measurements of delta13CO2 implies an isotopic flux of 89 +/- 21 gigatons of carbon per mil per year, in agreement with model- and inventory-based estimates of this flux. Both the delta13C and the O2 data show significant interannual variability in carbon storage over the period of record. The general agreement of the independent estimates from O2 and delta13C is a robust signal of variable carbon uptake by both the land biosphere and the oceans.

Recent Changes in Atmospheric Carbon Monoxide

Measurements of carbon monoxide (CO) in air samples collected from 27 locations between 71 degrees N and 41 degrees S show that atmospheric levels of this gas have decreased worldwide over the past 2 to 5 years. During this period, CO decreased at nearly a constant rate in the high northern latitudes. In contrast, in the tropics an abrupt decrease occurred beginning at the end of 1991. In the Northern Hemisphere, CO decreased at a spatially and temporally averaged rate of 7.3 (+/-0.9) parts per billion per year (6.1 percent per year) from June 1990 to June 1993, whereas in the Southern Hemisphere, CO decreased 4.2 (+/-0.5) parts per billion per year (7.0 percent per year). This recent change is opposite a long-term trend of a 1 to 2 percent per year increase inferred from measurements made in the Northern Hemisphere during the past 30 years.

Observational Contrains on the Global Atmospheric Co2 Budget

Observed atmospheric concentrations of CO2 and data on the partial pressures of CO2 in surface ocean waters are combined to identify globally significant sources and sinks of CO2. The atmospheric data are compared with boundary layer concentrations calculated with the transport fields generated by a general circulation model (GCM) for specified source-sink distributions. In the model the observed north-south atmospheric concentration gradient can be maintained only if sinks for CO2 are greater in the Northern than in the Southern Hemisphere. The observed differences between the partial pressure of CO2 in the surface waters of the Northern Hemisphere and the atmosphere are too small for the oceans to be the major sink of fossil fuel CO2. Therefore, a large amount of the CO2 is apparently absorbed on the continents by terrestrial ecosystems.