Kirk Thoning

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.

Measurement of fugacity of CO2 in surface water using continuous and discrete sampling methods

Instrumentation and methodology is described which is used for measurement of the fugacity (or partial pressure) of carbon dioxide (fCO2 or pCO2) in surface seawater. Two separate instruments were developed for the measurements. One is an underway system which measures the mixing ratio of CO2, XCO2, in a headspace in equilibrium with surface seawater continuously pumped into a 24 1 equilibration chamber. The other is a discrete system in which 460 ml aliquots of water are equilibrated with a 120 ml headspace. Both systems use a non-dispersive infrared analyzer as detector. In the underway instrument the average XCO2 in the headspace of an equilibration chamber is measured at near in-situ temperature over 20 min each hour. At a cruising speed of 13 knots this translates into a space averaged fCO2 value over 8 km. The underway system is ideally suited for mapping of the surface water fugacity over large geographic regions. Samples from the discrete instrument are analyzed at 20°C. The primary function of the system is for measurement of subsurface fCO2 values. The discrete system is also well suited for determining the relationship between the fugacity of CO2 and other (carbon) parameters sub-sampled from the same aliquot. To calculate the fCO2 in water for in-situ conditions from the mixing ratio in the headspace of the flask of the discrete system, small carbon mass balance and, sometimes significant, temperature corrections have to be applied. Comparison of 100 surface values obtained in the South Atlantic using the underway and discrete systems shows that the average difference of pCO2 values for the two systems ranges from −4.3 μatm to −8.6 μatm, depending on the temperature correction, with a standard deviation of 4 μatm. The differences show scatter of up the 15 μatm which we attribute to a mismatch between the point samples for the discrete system and the integrated samples for the underway system.

A high precision manometric system for absolute calibrations of CO2 in dry air

A high-precision manometric system has been developed for absolute calibrations of CO2-in-air mixture gas. This report describes the principle of the calibration method and evaluates the performance of the manometric apparatus. The test results show that the reproducibility of the manometric system for calibrating CO2-in-air gas mixtures is within ±0.1 μmol mol−1 in the atmospheric CO2 concentration range of 300 to 400 μmol mol−1. Preliminary measurements indicate that the agreement with the absolute World Meteorological Organization mole fraction scale, widely used for atmospheric CO2 monitoring, is also within ±0.1 μmol mol−1 in that same range.

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.

Carbon dioxide sources from Alaska driven by increasing early winter respiration from Arctic tundra

Significance Rising arctic temperatures could mobilize reservoirs of soil organic carbon trapped in permafrost. We present the first quantitative evidence for large, regional-scale early winter respiration flux, which more than offsets carbon uptake in summer in the Arctic. Data from the National Oceanic and Atmospheric Administration’s Barrow station indicate that October through December emissions of CO2 from surrounding tundra increased by 73% since 1975, supporting the view that rising temperatures have made Arctic ecosystems a net source of CO2. It has been known for over 50 y that tundra soils remain unfrozen and biologically active in early winter, yet many Earth System Models do not correctly represent this phenomenon or the associated CO2 emissions, and hence they underestimate current, and likely future, CO2 emissions under climate change. High-latitude ecosystems have the capacity to release large amounts of carbon dioxide (CO2) to the atmosphere in response to increasing temperatures, representing a potentially significant positive feedback within the climate system. Here, we combine aircraft and tower observations of atmospheric CO2 with remote sensing data and meteorological products to derive temporally and spatially resolved year-round CO2 fluxes across Alaska during 2012–2014. We find that tundra ecosystems were a net source of CO2 to the atmosphere annually, with especially high rates of respiration during early winter (October through December). Long-term records at Barrow, AK, suggest that CO2 emission rates from North Slope tundra have increased during the October through December period by 73% ± 11% since 1975, and are correlated with rising summer temperatures. Together, these results imply increasing early winter respiration and net annual emission of CO2 in Alaska, in response to climate warming. Our results provide evidence that the decadal-scale increase in the amplitude of the CO2 seasonal cycle may be linked with increasing biogenic emissions in the Arctic, following the growing season. Early winter respiration was not well simulated by the Earth System Models used to forecast future carbon fluxes in recent climate assessments. Therefore, these assessments may underestimate the carbon release from Arctic soils in response to a warming climate.

An interpretation of trace gas correlations during Barrow, Alaska, winter dark periods, 1986–1997

Positive correlations among CO, CO2, and CH4 during winter–spring have been observed at Barrow, Alaska, for many years. We examine these, as well as negative correlations between O3 and these gases, during the winter dark period. Because biogenic and photochemical processes are limited within this environment, we believe that pollution is driving these relationships. The dearth of mixing processes within the Arctic basin, the strong stability of the winter boundary layer, and lack of sunlight (and hence low OH) contribute to the winter–spring maxima in CO, CO2, and CH4. We hypothesize that the negative correlation of O3 with these gases is the result of O3 titration by NO and NO2 (NOx) in industrial plumes, which leads to the destruction of one to two molecules of O3 per NO emitted. Using the empirical slopes of O3/CO2 and O3/CO determined from 12 years of Barrow data, we derived emission factors, ΔNOx/ΔCO2 and ΔNOx/ΔCO, assuming −1.5 O3/NO because of titration. Comparing these with published emission factors for NOx, CO2, and CO from industrial processes, we found good agreement. This pollution signature is regionally widespread, although air parcels transported from the direction of Siberia have the highest mixing ratios of pollutant gases. Possible scenarios leading to these trace gas relationships are explored.

Atmospheric CO2 variations at Barrow, Alaska, 1973–1982

The first 10 years (1973–1982) of atmospheric CO2 measurements at Barrow, Alaska, by the NOAA/GMCC program are described. The paper updates and extends the Barrow CO2 record presented in Tellus (1982). The data are given in final form, based on recent calibrations of the Scripps Institution of Oceanography, with selected values identified as representative of large, spacescale conditions. Analyses of the data show: (1) a long-term CO2 average increase of 1.3 ppm per year, but with large year-to-year variations in that growth rate; (2) a suggestion, not statistically significant, of a secular increase in the amplitude of the annual cycle, presumably a reflection of global-scale biospheric variability; and (3) good absolute agreement between the Barrow results and those from four neighboring high latitude sites between 50 and 82°N.

Analysis System for Measurement of CO2 Mixing Ratios in Flask Air Samples

Abstract A system for measuring the concentration of CO2 in flask air samples from the NOAA/CMDL worldwide flask sampling network is described. Up to 180 samples per day can he analyzed using a nondispersive infrared CO2 analyzer. All data acquisition and instrument control operations are handled by a Hewlett-Packard Series 300 desktop computer. A noncontaminating diaphragm pump transfers the sample from the flask to the NDIR CO2 analyzer. Tests conducted using flasks filled from tanks of dry air showed either no systematic offsets or a small offset of about 0.1 ppm. The precision of the analysis system is estimated to be better than 0.1 ppm.

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%.

Continued emissions of carbon tetrachloride from the United States nearly two decades after its phaseout for dispersive uses

Significance Global-scale observations suggest large unexplained emissions of the ozone-depleting chemical carbon tetrachloride (CCl4) despite stringent limits on its production for dispersive uses for many years. Identifying the sources of continued CCl4 emission is necessary before steps can be taken to accelerate the emission decline and limit future ozone depletion. Results from an extensive air sampling network over the United States indicate continued emission of CCl4 with a similar distribution but much larger magnitude than industrial facilities reporting emissions to the US Environmental Protection Agency. If these emissions are attributable to chlorine production and processing and are indicative of release rates of CCl4 from these industries worldwide, a large fraction of ongoing global emissions of CCl4 can be explained. National-scale emissions of carbon tetrachloride (CCl4) are derived based on inverse modeling of atmospheric observations at multiple sites across the United States from the National Oceanic and Atmospheric Administration’s flask air sampling network. We estimate an annual average US emission of 4.0 (2.0–6.5) Gg CCl4 y−1 during 2008–2012, which is almost two orders of magnitude larger than reported to the US Environmental Protection Agency (EPA) Toxics Release Inventory (TRI) (mean of 0.06 Gg y−1) but only 8% (3–22%) of global CCl4 emissions during these years. Emissive regions identified by the observations and consistently shown in all inversion results include the Gulf Coast states, the San Francisco Bay Area in California, and the Denver area in Colorado. Both the observation-derived emissions and the US EPA TRI identified Texas and Louisiana as the largest contributors, accounting for one- to two-thirds of the US national total CCl4 emission during 2008–2012. These results are qualitatively consistent with multiple aircraft and ship surveys conducted in earlier years, which suggested significant enhancements in atmospheric mole fractions measured near Houston and surrounding areas. Furthermore, the emission distribution derived for CCl4 throughout the United States is more consistent with the distribution of industrial activities included in the TRI than with the distribution of other potential CCl4 sources such as uncapped landfills or activities related to population density (e.g., use of chlorine-containing bleach).