Kenneth A. Masarie

Evidence for interannual variability of the carbon cycle from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory global air sampling network

The distribution and variations of atmospheric CO2 from 1981 to 1992 were determined by measuring CO2 mixing ratios in samples collected weekly at a cooperative global air sampling network. The results constitute the most geographically extensive, carefully calibrated, internally consistent CO2 data set available. Analysis of the data reveals that the global CO2 growth rate has declined from a peak of approximately 2.5 ppm/yr in 1987-1988 to approximately 0.6 ppm/yr in 1992. In 1992 we find no increase in atmospheric CO2 from 30 deg to 90 deg N. Variations in fossil fuel CO2 emissions cannot explain this result. The north pole-south pole CO2 difference increased from approximately 3 ppm during 1981-1987 to approximately 4 ppm during 1988-1991. In 1992 the difference was again approximately 3 ppm. A two-dimensional model analysis of the data indicates that the low CO2 growth rate in 1992 is mainly due to an increase in the northern hemisphere CO2 sink from 3.9 Gt C/yr in 1991 to 5.0 Gt C/yr in 1992. The increase in the north pole-south pole CO2 difference appears to result from an increase in the southern hemisphere CO2 sink from approximately 0.5 to approximately 1.5 Gt C/yr.

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.

TransCom 3 inversion intercomparison: Impact of transport model errors on the interannual variability of regional CO2 fluxes, 1988–2003

Monthly CO2 fluxes are estimated across 1988–2003 for 22 emission regions using data from 78 CO2 measurement sites. The same inversion (method, priors, data) is performed with 13 different atmospheric transport models, and the spread in the results is taken as a measure of transport model error. Interannual variability (IAV) in the winds is not modeled, so any IAV in the measurements is attributed to IAV in the fluxes. When both this transport error and the random estimation errors are considered, the flux IAV obtained is statistically significant at P ≤ 0.05 when the fluxes are grouped into land and ocean components for three broad latitude bands, but is much less so when grouped into continents and basins. The transport errors have the largest impact in the extratropical northern latitudes. A third of the 22 emission regions have significant IAV, including the Tropical East Pacific (with physically plausible uptake/release across the 1997–2000 El Nino/La Nina) and Tropical Asia (with strong release in 1997/1998 coinciding with large-scale fires there). Most of the global IAV is attributed robustly to the tropical/southern land biosphere, including both the large release during the 1997/1998 El Nino and the post-Pinatubo uptake.

Extension and integration of atmospheric carbon dioxide data into a globally consistent measurement record

Atmospheric transport models are used to constrain sources and sinks of carbon dioxide by requiring that the modeled spatial and temporal concentration patterns are consistent with the observations. Serious obstacles to this approach are the sparsity of sampling sites and the lack of temporal continuity among observations at different locations. A procedure is presented that attempts to extend the knowledge gained during a limited period of measurements beyond the period itself resulting in records containing measurement data and extrapolated and interpolated values. From limited measurements we can define trace gas climatologies that describe average seasonal cycles, trends, and changes in trends at individual sampling sites. A comparison of the site climatologies with a reference defined over a much longer period of time constitutes the framework used in the development of the data extension procedure. Two extension methods are described. The benchmark trend method uses a deseasonalized long-term trend from a single site as a reference to individual site climatologies. The latitude reference method utilizes measurements from many sites in constructing a reference to the climatologies. Both methods are evaluated and the advantages and limitations of each are discussed. Data extension is not based on any atmospheric models but entirely on the data themselves. The methods described here are relatively straightforward and reproducible and result in extended records that are model independent. The cooperative air sampling network maintained by the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory in Boulder, Colorado, provides a test bed for the development of the data extension method; we intend to integrate and extend CO2 measurement records from other laboratories providing a globally consistent atmospheric CO2 database to the modeling community.

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.

Monitoring the isotopic composition of atmospheric CO2: Measurements from the NOAA Global Air Sampling Network

The stable isotopic composition of atmospheric CO2 is being monitored via measurements made at the University of Colorado-Institute of Arctic and Alpine Research, using air samples collected weekly by the Global Air Sampling Network of the NOAA Climate Monitoring and Diagnostics Laboratory. These measurements, in concert with the monitoring of atmospheric CO2 mixing ratios, offer the potential to characterize quantitatively the mechanisms operating in the global carbon cycle, by recording the isotopic signatures imparted to CO2 as it moves among the atmosphere, biosphere, and oceans. This data set increases the number of measurements of atmospheric CO2 isotopes by nearly an order of magnitude over those previously available. We describe the analytical techniques used to obtain and calibrate these data and report measurements from 25 land-based sites, and two ships in the Pacific Ocean, from samples collected during 1990–1993. The typical precision of our mass spectrometric technique is 0.03‰ for δ13C and 0.05‰ for δ18O. Collecting the flask samples without drying leads to loss of δ18O information at many sites. The seasonal cycle in δ13C at sites in the northern hemisphere is highly correlated with that of the CO2 mixing ratio, with amplitudes approaching 1‰ at high latitudes. The seasonal cycle in δ18O is of similar amplitude, though variable from year to year and lags the other species by 2–4 months. Interhemispheric differences of the 1992 and 1993 means of the isotopic tracers are in strong contrast: the north pole-south pole difference for δ13C is −0.20‰, which though highly quantitatively significant is dwarfed by the −2‰ difference for δ18O. In contrast to the record of atmospheric δ13C during the 1980s we observe no significant temporal trend in annual mean δ13C during 1990–1993.

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.

U.S. emissions of HFC-134a derived for 2008–2012 from an extensive flask-air sampling network

U.S. national and regional emissions of HFC-134a are derived for 2008-2012 based on atmospheric observations from ground and aircraft sites across the U.S. and a newly developed regional inverse model. Synthetic data experiments were first conducted to optimize the model assimilation design and to assess model-data mismatch errors and prior flux error covariances computed using a maximum likelihood estimation technique. The synthetic data experiments also tested the sensitivity of derived national and regional emissions to a range of assumed prior emissions, with the goal of designing a system that was minimally reliant on the prior. We then explored the influence of additional sources of error in inversions with actual observations, such as those associated with background mole fractions and transport uncertainties. Estimated emissions of HFC-134a range from 52 to 61 Gg yr(-1) for the contiguous U.S. during 2008-2012 for inversions using air transport from Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model driven by the 12km resolution meteorogical data from North American Mesoscale Forecast System (NAM12) and all tested combinations of prior emissions and background mole fractions. Estimated emissions for 2008-2010 were 20% lower when specifying alternative transport from Stochastic Time-Inverted Lagrangian Transport (STILT) model driven by the Weather Research and Forecasting (WRF) meteorology. Our estimates (for HYSPLIT-NAM12) are consistent with annual emissions reported by U.S. Environmental Protection Agency for the full study interval. The results suggest a 10-20% drop in U.S. national HFC-134a emission in 2009 coincident with a reduction in transportation-related fossil fuel CO2 emissions, perhaps related to the economic recession. All inversions show seasonal variation in national HFC-134a emissions in all years, with summer emissions greater than winter emissions by 20-50%.