and modeling (Kort et al., 2008; Zhao et al., 2009) have succeeded in deriving CH4 and N2O emissions at the subnational scale within the United States, although these short, intense campaigns cannot be sustained year-round over much of the globe.

One approach to quantifying regional emissions is to use the relative increase in abundances of several gases during pollution episodes to derive their relative emissions. For example, high-resolution time series of gases have been used to identify European pollution episodes and to quantify the relative emissions of halocarbons and N2O from Western Europe (Prather, 1985), as well as European emissions of dichloromethane, trichloroethene, and tetrachloroethene (Simmonds et al., 2006). This method has also been used to try to separate sources of CO2 by assuming that sulfur hexafluoride (SF6) is co-emitted with fossil-fuel combustion (Rivier et al., 2006; Turnbull et al., 2006). The approach obviates the need for accurate tracer-transport modeling and can define the relative emissions in a polluted air sample to 10 percent or better. However, it is limited to areas where distinct, single-source pollution plumes occur on top of a nearly uniform, clean air background. Surface-based, diurnal column measurements have recently been used to estimate CH4 emissions from the Los Angeles basin (Wunch et al., 2009). Two key criteria need to be met to derive absolute emissions, for example, of fossil-fuel CO2: (1) the absolute emissions of the reference gas must be known to equivalent or better accuracy; and (2) the two gases must be co-emitted in the same ratio over the entire region being sampled, otherwise the much larger errors in tracer-transport inversion dominate. Both criteria fail for SF6, but for 14CO2, the second criterion allows for the separation of fossil-fuel CO2 from biogenic sources (see “14C Measurements” below).

Overall, there are published cases for CH4, N2O, and the synthetic fluorinated gases in which one can distinguish source from sink and other cases in which one cannot. Thus, the committee estimates uncertainties in national emissions of these gases as 50 percent to greater than 100 percent.

CO2Emission Estimates

The long-term, accurate measurements of atmospheric CO2 at Mauna Loa by Keeling (1961) and many others (Figure 1.4) laid the foundation for much of our understanding of the carbon cycle, including the link between fossil-fuel combustion and increases in CO2 (Revelle and Suess, 1957; Pales and Keeling, 1965). Measurements of atmospheric CO2 and other gases are currently made at in situ stations (e.g., surface sites, towers, aircraft profiling) around the world (e.g., Figure 4.1, Appendix C) and from satellites. The combined use of CO2 and oxygen (O2) atmospheric measurements allows the CO2 removed from the atmosphere to be partitioned into land and oceanic carbon sinks (Keeling et al., 1993; Manning and Keeling, 2006; Denman et al., 2007). Observation of isotope analogues of CO2 (including 13C, 14C, 18O), CH4 (13C, 14C, D), and N2O (15N, 18O) as well as O2/N2 ratios have increased our understanding of the global sources and sinks of these greenhouse gases on scales of hemispheres or broad latitude bands. All of these measurements would significantly aid in verification or falsification of reported national emissions if the pattern of emissions being tested is provided and the measurements are made at sufficiently high spatial resolution and at suitable locations (e.g., within or close to the borders of the country whose emissions are being tested).

The mixing time of tracers in the atmosphere ranges from minutes to days between the ground and the tropopause, about two weeks around a latitude circle at midlatitudes, several months to reach through a hemisphere, about one year between the northern and southern hemispheres, and several years to mix through the stratosphere. The density of the observing network, together with the atmospheric mixing times, determines the spatial resolution with which sources and sinks can be inferred using an atmospheric model. In particular, the slow mixing between hemispheres and the preponderance of emissions in the northern hemisphere mean that the signal from anthropogenic emissions is relatively large when hemispheres are compared.

Tans et al. (1990) showed that the interhemispheric gradient in CO2 was smaller than it should have been, given the north-south disparity in fossil-fuel emissions. This implied a “missing sink” of more than one billion tons of carbon per year in the northern hemisphere. Subsequent analyses confirmed this qualitative conclusion but have struggled to improve its spatial resolution. Continued effort has focused on the range of different atmospheric model results and what model error could

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