Patricia M. Lang

The growth rate and distribution of atmospheric methane

Methane was measured in air samples collected approximately weekly from a globally distributed network of sites from 1983 to 1992. Sites range in latitude from 90°S to 82°N. All samples were analyzed by gas chromatography, with flame ionization detection at the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory in Boulder, Colorado, and the measurements were referenced against a single calibration scale. The estimated precision of the measurements is ±0.2%. Samples which had clear sampling or analytical errors, or which appeared to be contaminated by local CH4 sources, were identified and excluded from the data analysis. The data reveal a strong north-south gradient in methane with an annual mean difference of about 140 ppb between the northernmost and southernmost sampling sites. Methane time series from the high southern latitude sites have a relatively simple seasonal cycle with a minimum during late summer-early fall, almost certainly dominated by the seasonality in its photochemical destruction. Typical seasonal cycle amplitudes there are about 30 ppb. Seasonal cycles at sites in the northern hemisphere are complex when compared to sites in the southern hemisphere due to the interaction among CH4 sources and sinks, and atmospheric transport. Seasonal cycle amplitudes in the high north are about twice those observed in the high southern hemisphere. Annual mean methane mixing ratios were ∼1% lower at 3397 m than at sea level on the island of Hawaii. Trends were determined at each site in the network and globally. The average increase in the globally averaged methane mixing ratio over the period of these measurements is (11.1±0.2) ppb yr−1. Globally, the growth rate for methane decreased from approximately 13.5 ppb yr−1 in 1983 to about 9.3 ppb yr−1 in 1991. The growth rate of methane in the northern hemisphere during 1992 was near zero. Various possibilities for the long-term, slow decrease in the methane growth rate over the last decade and the rapid change in growth rate in the northern hemisphere in 1992 are given. The most likely explanation is a change in a methane source influenced directly by human activities, such as fossil fuel production.

Molecular hydrogen in the troposphere: Global distribution and budget

Molecular hydrogen (H2) has been measured since 1989 in air samples collected using a globally distributed sampling network. Time series from 50 locations are used to better define the distribution and recent changes of H2 in the remote lower troposphere. These data show that the globally averaged H2 mixing ratio between 1991 and 1996 was about 531±6 parts per billion (ppb). Hydrogen exhibited well-defined seasonal cycles in each hemisphere, with similar seasonal maxima (530–550 ppb). However, in the Northern Hemisphere the seasonal minimum was 70 ppb deeper than in the Southern Hemisphere (∼450 and 520 ppb, respectively), resulting in ∼3% more H2 in the south than in the north. With these data we have reevaluated the global H2 budget. Methane oxidation is the largest source of H2 to the troposphere, and soil uptake accounts for much of its sink. The global annual turnover is estimated as ∼75 Tg H2 yr−1. The annual turnover, combined with a calculated tropospheric burden of 155 Tg, indicates a lifetime of ∼2 years. While our understanding of the global distribution of the sources and sinks of H2 is still incomplete, the lower annual minimum in the north may be reasonably attributed to hemispheric asymmetry in uptake by soils. The seasonal cycles in the two hemispheres show unusual similarities: the northern and the southern seasonal maxima and minima were offset by only a few months. We suggest that the seasonal cycle in the Southern Hemisphere is dominated by H2 emissions from biomass burning.

Recent changes in atmospheric carbon monoxide

Measurements of carbon monoxide (CO) in air samples collected from 27 locations between 71�N and 41�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.

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

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 Agencys 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.

Atmospheric methane at Mauna Loa and Barrow observatories: Presentation and analysis of in situ measurements

In situ methane (CH4) measurement techniques and data from the NOAA Climate Monitoring and Diagnostics Laboratory observatories at Mauna Loa, Hawaii, and Barrow, Alaska, are presented. For Mauna Loa, the data span the time period April 1987 to April 1994. At Barrow the measurements cover the period January 1986 to January 1994. Sixty air samples per day were measured with a fully automated gas chromatograph using flame ionization detection. Details of the experimental methods and procedures are given. Data are presented and assessed over various timescales. The average peak to peak seasonal cycle amplitudes obtained from four harmonics fitted to the detrended data were 25.1 ppb at Mauna Loa and 47.2 ppb at Barrow. When the seasonal cycle amplitude during each calendar year was determined as the difference between the maximum and minimum value from a smooth curve fitted to the data, the average amplitudes were (30.6±4.2) ppb at Mauna Loa and (57.5±11.4) ppb at Barrow. A discrepancy exists between these two methods due to the temporal variability in the positions of the seasonal maxima. The average trend at Mauna Loa was 9.7 ppb yr−1, but this trend was observed to decrease at a rate of 1.5 ppb yr−2. For Barrow the average trend was 8.5 ppb yr−1, and the rate of decrease in the trend was 2.1 ppb yr−2. At Mauna Loa, a diurnal cycle was sometimes observed with an amplitude of up to 10 ppb when averaged over 1 month.

Variations in atmospheric methane at Mauna Loa Observatory related to long-range transport

Methane measurements, radon measurements, and air mass trajectories calculated for Mauna Loa Observatory (MLO) are examined to determine relationships among methane source/sink regions, flow patterns for MLO, and methane variations on the synoptic-to-seasonal scale. We present evidence that the methane seasonal cycle observed at MLO is in large part driven by seasonal variations in transport. Furthermore, the variability in methane mixing ratio at MLO is higher in winter than in summer because of greater variability in flow patterns. Ten-day back trajectories are classified according to wind speed and direction using cluster analysis to determine six typical transport regimes. The methane data are then grouped according to transport cluster. The median methane mixing ratio corresponding to tradewind flow was 17.2 ppbv (parts per billion by volume) lower than that corresponding to strong westerly flow. This difference is attributed to transport from source/sink regions, flow across the methane latitudinal gradient, and seasonality of flow patterns. Case studies utilizing individual trajectories and radon measurements to determine probable air parcel origins illustrate the effects of long-range transport on the methane mixing ratio at MLO. Changes in flow pattern from sink to source origins can result in a 50 ppbv rise in methane mixing ratio over a period of a few days. During winter, alternation of westerly winds, tradewinds and anticyclonically curving flows contributes to the large variability in the methane mixing ratio. During summer this variability is reduced with the cessation of strong westerly flows from methane source regions. In July and August, air parcels originate far from methane source regions and in the area of highest modeled OH concentration. At the same time, methane mixing ratios decrease to the lowest values for the year. In this way, the seasonality of flow patterns exerts a major influence on the observed seasonal cycle of methane at MLO.

Seasonal climatology of CO2 across North America from aircraft measurements in the NOAA/ESRL Global Greenhouse Gas Reference Network

Seasonal spatial and temporal gradients for the CO2 mole fraction over North America are examined by creating a climatology from data collected 2004–2013 by the NOAA/ESRL Global Greenhouse Gas Reference Network Aircraft Program relative to trends observed for CO2 at the Mauna Loa Observatory. The data analyzed are from measurements of air samples collected in specially fabricated flask packages at frequencies of days to months at 22 sites over continental North America and shipped back to Boulder, Colorado, for analysis. These measurements are calibrated relative to the CO2 World Meteorological Organization mole fraction scale. The climatologies of CO2 are compared to climatologies of CO, CH4, SF6, N2O (which are also measured from this sampling program), and winds to understand the dominant transport and chemical and biological processes driving changes in the spatial and temporal mole fractions of CO2 as air passes over continental North America. The measurements show that air masses coming off the Pacific on the west coast of North America are relatively homogeneous with altitude. As air masses flow eastward, the lower section from the surface to 4000 m above sea level (masl) becomes distinctly different from the 4000–8000 masl section of the column. This is due in part to the extent of the planetary boundary layer, which is directly impacted by continental sources and sinks, and to the vertical gradient in west-to-east wind speeds. The slowdown and southerly shift in winds at most sites during summer months amplify the summertime drawdown relative to what might be expected from local fluxes. This influence counteracts the dilution of summer time CO2 drawdown (known as the “rectifier effect”) as well as changes the surface influence “footprint” for each site. An early start to the summertime drawdown, a pronounced seasonal cycle in the column mean (500 to 8000 masl), and small vertical gradients in CO2, CO, CH4, SF6, and N2O at high-latitude western sites such as Poker Flat, Alaska, suggest recent influence of transport from southern latitudes and not local processes. This transport pathway provides a significant contribution to the large seasonal cycle observed in the high latitudes at all altitudes sampled. A sampling analysis of the NOAA/ESRL CarbonTracker model suggests that the average sampling resolution of 22 days is sufficient to get a robust estimate of mean seasonal cycle of CO2 during this 10 year period but insufficient to detect interannual variability in emissions over North America.

Impact of equatorial and continental airflow on primary greenhouse gases in the northern South China Sea

Four-year ground-level measurements of the two primary greenhouse gases (carbon dioxide (CO2) and methane (CH4)) were conducted at Dongsha Island (DSI), situated in the northern South China Sea (SCS), from March 2010 to February 2014. Their mean mixing ratios are calculated to be 396.3 ± 5.4 ppm and 1863.6 ± 50.5 ppb, with an annual growth rate of +2.19 ± 0.5 ppm yr–1 and +4.70 ± 4.4 ppb yr–1 for CO2 and CH4, respectively, over the study period. Our results suggest that the Asian continental outflow driven by the winter northeast monsoon could have brought air pollutants into the northern SCS, as denoted by significantly elevated levels of 6.5 ppm for CO2 and 59.6 ppb for CH4, which are greater than the marine boundary layer references at Cape Kumukahi (KUM) in the tropical northern Pacific in January. By contrast, the summertime CH4 at DSI is shown to be lower than that at KUM by 19.7 ppb, whereas CO2 is shown to have no differences (<0.42 ppm in July) during the same period. Positive biases of the Greenhouse Gases Observing Satellite (GOSAT) L4B data against the surface measurements are estimated to be 2.4 ± 3.4 ppm for CO2 and 43.2 ± 36.8 ppb for CH4. The satellite products retrieved from the GOSAT showed the effects of anthropogenic emissions and vegetative sinks on land on a vertical profiling basis. The prevailing southeasterly winds originating from as far south as the equator or Southern Hemisphere pass through the lower troposphere in the northern SCS, forming a tunnel of relatively clean air masses as indicated by the low CH4 mixing ratios observed on the DSI in summer.