Proceedings of the National Academy of Sciences of the United States of America, May 8, 2017
High-latitude ecosystems have the capacity to release large amounts of carbon dioxide (CO2) to th... more 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 re...
Proceedings of the National Academy of Sciences of the United States of America, Jan 29, 2016
National-scale emissions of carbon tetrachloride (CCl4) are derived based on inverse modeling of ... more 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 resu...
For more than five years the NOAA/ESRL aircraft project has been collecting flask samples at as m... more For more than five years the NOAA/ESRL aircraft project has been collecting flask samples at as many as 20 sites throughout North America. A compilation of the data demonstrates that vertical profiles are valuable in many ways. In particular, we have demonstrated that it is possible to make continental-scale flux estimates that depend only on well known horizontal re-analysis winds.
Proceedings of the National Academy of Sciences, 2007
We present an estimate of net CO 2 exchange between the terrestrial biosphere and the atmosphere ... more We present an estimate of net CO 2 exchange between the terrestrial biosphere and the atmosphere across North America for every week in the period 2000 through 2005. This estimate is derived from a set of 28,000 CO 2 mole fraction observations in the global atmosphere that are fed into a state-of-the-art data assimilation system for CO 2 called CarbonTracker. By design, the surface fluxes produced in CarbonTracker are consistent with the recent history of CO 2 in the atmosphere and provide constraints on the net carbon flux independent from national inventories derived from accounting efforts. We find the North American terrestrial biosphere to have absorbed −0.65 PgC/yr (1 petagram = 10 15 g; negative signs are used for carbon sinks) averaged over the period studied, partly offsetting the estimated 1.85 PgC/yr release by fossil fuel burning and cement manufacturing. Uncertainty on this estimate is derived from a set of sensitivity experiments and places the sink within a range of −...
Proceedings of the National Academy of Sciences, 2010
Studies diverge substantially on the actual magnitude of the North American carbon budget. This i... more Studies diverge substantially on the actual magnitude of the North American carbon budget. This is due to the lack of appropriate data and also stems from the difficulty to properly model all the details of the flux distribution and transport inside the region of interest. To sidestep these difficulties, we use here a simple budgeting approach to estimate land-atmosphere fluxes across North America by balancing the inflow and outflow of CO 2 from the troposphere. We base our study on the unique sampling strategy of atmospheric CO 2 vertical profiles over North America from the National Oceanic and Atmospheric Administration/Earth System Research Laboratory aircraft network, from which we infer the three-dimensional CO 2 distribution over the continent. We find a moderate sink of 0.5 ± 0.4 PgC y -1 for the period 2004–2006 for the coterminous United States, in good agreement with the forest-inventory-based estimate of the first North American State of the Carbon Cycle Report, and ave...
The sinks are mainly located in the agricultural regions of the Midwest (35%), deciduous forests ... more The sinks are mainly located in the agricultural regions of the Midwest (35%), deciduous forests along the East Coast (25%), and boreal coniferous forests (14%). There also appears to be substantial interannual variation of the sink, correlated with temperature and moisture variations. Our estimates are optimally consistent with measurements of weekly 14,000 air samples from across the world, ~5,000 daily averages of CO2 around the continent and ~ 5,000 daily averages from tall towers at four locations within the continent.
Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ... more Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ecosystem responses to climate change and have implications for the design of carbon monitoring programs. This paper describes changes in the three-dimensional structure of atmospheric CO2 for several representative concentration pathways (RCPs 4.5 and 8.5) using the Community Earth System Model–Biogeochemistry (CESM1-BGC). CO2 simulated for the historical period was first compared to surface, aircraft, and column observations. In a second step, the evolution of spatial and temporal gradients during the twenty-first century was examined. The mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net flux in the Community Land Model (the land component of CESM) was too weak. Consistent with weak summer drawdown in Northern Hemisphere high latitudes, simulated CO2 showed correspondingly weak n...
Journal of Atmospheric and Oceanic Technology, 2010
This work describes the AirCore, a simple and innovative atmospheric sampling system. The AirCore... more This work describes the AirCore, a simple and innovative atmospheric sampling system. The AirCore used in this study is a 150-m-long stainless steel tube, open at one end and closed at the other, that relies on positive changes in ambient pressure for passive sampling of the atmosphere. The AirCore evacuates while ascending to a high altitude and collects a sample of the ambient air as it descends. It is sealed upon recovery and measured with a continuous analyzer for trace gas mole fraction. The AirCore tubing can be shaped into a variety of configurations to accommodate any sampling platform; for the testing done in this work it was shaped into a 0.75-m-diameter coil. Measurements of CO2 and CH4 mole fractions in laboratory tests indicate a repeatability and lack of bias to better than 0.07 ppm (one sigma) for CO2 and 0.4 ppb for CH4 under various conditions. Comparisons of AirCore data with flask data from aircraft flights indicate a standard deviation of differences of 0.3 ppm a...
Proceedings of the National Academy of Sciences of the United States of America, May 8, 2017
High-latitude ecosystems have the capacity to release large amounts of carbon dioxide (CO2) to th... more 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 re...
Proceedings of the National Academy of Sciences of the United States of America, Jan 29, 2016
National-scale emissions of carbon tetrachloride (CCl4) are derived based on inverse modeling of ... more 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 resu...
For more than five years the NOAA/ESRL aircraft project has been collecting flask samples at as m... more For more than five years the NOAA/ESRL aircraft project has been collecting flask samples at as many as 20 sites throughout North America. A compilation of the data demonstrates that vertical profiles are valuable in many ways. In particular, we have demonstrated that it is possible to make continental-scale flux estimates that depend only on well known horizontal re-analysis winds.
Proceedings of the National Academy of Sciences, 2007
We present an estimate of net CO 2 exchange between the terrestrial biosphere and the atmosphere ... more We present an estimate of net CO 2 exchange between the terrestrial biosphere and the atmosphere across North America for every week in the period 2000 through 2005. This estimate is derived from a set of 28,000 CO 2 mole fraction observations in the global atmosphere that are fed into a state-of-the-art data assimilation system for CO 2 called CarbonTracker. By design, the surface fluxes produced in CarbonTracker are consistent with the recent history of CO 2 in the atmosphere and provide constraints on the net carbon flux independent from national inventories derived from accounting efforts. We find the North American terrestrial biosphere to have absorbed −0.65 PgC/yr (1 petagram = 10 15 g; negative signs are used for carbon sinks) averaged over the period studied, partly offsetting the estimated 1.85 PgC/yr release by fossil fuel burning and cement manufacturing. Uncertainty on this estimate is derived from a set of sensitivity experiments and places the sink within a range of −...
Proceedings of the National Academy of Sciences, 2010
Studies diverge substantially on the actual magnitude of the North American carbon budget. This i... more Studies diverge substantially on the actual magnitude of the North American carbon budget. This is due to the lack of appropriate data and also stems from the difficulty to properly model all the details of the flux distribution and transport inside the region of interest. To sidestep these difficulties, we use here a simple budgeting approach to estimate land-atmosphere fluxes across North America by balancing the inflow and outflow of CO 2 from the troposphere. We base our study on the unique sampling strategy of atmospheric CO 2 vertical profiles over North America from the National Oceanic and Atmospheric Administration/Earth System Research Laboratory aircraft network, from which we infer the three-dimensional CO 2 distribution over the continent. We find a moderate sink of 0.5 ± 0.4 PgC y -1 for the period 2004–2006 for the coterminous United States, in good agreement with the forest-inventory-based estimate of the first North American State of the Carbon Cycle Report, and ave...
The sinks are mainly located in the agricultural regions of the Midwest (35%), deciduous forests ... more The sinks are mainly located in the agricultural regions of the Midwest (35%), deciduous forests along the East Coast (25%), and boreal coniferous forests (14%). There also appears to be substantial interannual variation of the sink, correlated with temperature and moisture variations. Our estimates are optimally consistent with measurements of weekly 14,000 air samples from across the world, ~5,000 daily averages of CO2 around the continent and ~ 5,000 daily averages from tall towers at four locations within the continent.
Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ... more Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ecosystem responses to climate change and have implications for the design of carbon monitoring programs. This paper describes changes in the three-dimensional structure of atmospheric CO2 for several representative concentration pathways (RCPs 4.5 and 8.5) using the Community Earth System Model–Biogeochemistry (CESM1-BGC). CO2 simulated for the historical period was first compared to surface, aircraft, and column observations. In a second step, the evolution of spatial and temporal gradients during the twenty-first century was examined. The mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net flux in the Community Land Model (the land component of CESM) was too weak. Consistent with weak summer drawdown in Northern Hemisphere high latitudes, simulated CO2 showed correspondingly weak n...
Journal of Atmospheric and Oceanic Technology, 2010
This work describes the AirCore, a simple and innovative atmospheric sampling system. The AirCore... more This work describes the AirCore, a simple and innovative atmospheric sampling system. The AirCore used in this study is a 150-m-long stainless steel tube, open at one end and closed at the other, that relies on positive changes in ambient pressure for passive sampling of the atmosphere. The AirCore evacuates while ascending to a high altitude and collects a sample of the ambient air as it descends. It is sealed upon recovery and measured with a continuous analyzer for trace gas mole fraction. The AirCore tubing can be shaped into a variety of configurations to accommodate any sampling platform; for the testing done in this work it was shaped into a 0.75-m-diameter coil. Measurements of CO2 and CH4 mole fractions in laboratory tests indicate a repeatability and lack of bias to better than 0.07 ppm (one sigma) for CO2 and 0.4 ppb for CH4 under various conditions. Comparisons of AirCore data with flask data from aircraft flights indicate a standard deviation of differences of 0.3 ppm a...
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