[1] Atmospheric CO 2 gradients are usually dominated by the signal from net terrestrial biological fluxes, despite the fact that fossil fuel combustion fluxes are larger in the annual mean. Here, we use a six year long series of 14 CO 2 and CO 2 measurements obtained from vertical profiles at two northeast U.S. aircraft sampling sites to partition lower troposphere CO 2 enhancements (and depletions) into terrestrial biological and fossil fuel components (C bio and C ff ). Mean C ff is 1.5 ppm, and 2.4 ppm when we consider only planetary boundary layer samples. However, we find that the contribution of C bio to CO 2 enhancements is large throughout the year, and averages 60% in winter. Paired observations of C ff and the lower troposphere enhancements (D gas ) of 22 other anthropogenic gases (CH 4 , CO, halo-and hydrocarbons and others) measured in the same samples are used to determine apparent emission ratios for each gas. We then scale these ratios by the well known U.S. fossil fuel CO 2 emissions to provide observationally based estimates of national emissions for each gas and compare these to "bottom up" estimates from inventories. Correlations of D gas with C ff for almost all gases are statistically significant with median r 2 for winter, summer and the entire year of 0.59, 0.45, and 0.42, respectively. Many gases exhibit statistically significant winter:summer differences in ratios that indicate seasonality of emissions or chemical destruction. The variability of ratios in a given season is not readily attributable to meteorological or geographic variables and instead most likely reflects real, short-term spatiotemporal variability of emissions.

This article presents results from the first 3 rounds of an international intercomparison of measurements of  14 CO 2 in liter-scale samples of whole air by groups using accelerator mass spectrometry (AMS). The ultimate goal of the intercomparison is to allow the merging of  14 CO 2 data from different groups, with the confidence that differences in the data are geophysical gradients and not artifacts of calibration. Eight groups have participated in at least 1 round of the intercomparison, which has so far included 3 rounds of air distribution between 2007 and 2010. The comparison is intended to be ongoing, so that: a) the community obtains a regular assessment of differences between laboratories; and b) individual laboratories can begin to assess the long-term repeatability of their measurements of the same source air. Air used in the intercomparison was compressed into 2 high-pressure cylinders in 2005 and 2006 at Niwot Ridge, Colorado (USA), with one of the tanks "spiked" with fossil CO 2 , so that the 2 tanks span the range of  14 CO 2 typically encountered when measuring air from both remote background locations and polluted urban ones. Three groups show interlaboratory comparability within 1‰ for ambient level  14 CO 2. For high CO 2 /low  14 CO 2 air, 4 laboratories showed comparability within 2‰. This approaches the goals set out by the World Meteorological Organization (WMO) CO 2 Measurements Experts Group in 2005. One important observation is that single-sample precisions typically reported by the AMS community cannot always explain the observed differences within and between laboratories. This emphasizes the need to use long-term repeatability as a metric for measurement precision, especially in the context of long-term atmospheric monitoring.