2.   Detailed Description
This study uses all analyzed air samples taken at the surface from the NOAA ESRL Cooperative Air Sampling Network available for each year studied, except those flagged for analysis or sampling problems. Assimilated data includes values that are flagged as not representative of typical background conditions. The composition of the network thus varies per week depending on successful sampling and analysis, and each sites' sampling frequency. In addition, we use continuous CO2 time series from five towers: (1) the 396m level of the WLEF tower in Wisconsin, (2) the 107m level of the AMT tower in Argyle, Maine, (3) the 251m level of the KWKT tower in Texas, (4) the 40m level of the tower in Fraserdale, Canada operated by the Meteorological Service Canada (MSC), and (5) the 23m level of the tower at Old Black Spruce, Canada operated by MSC. Other in-situ continuous CO2 time series used are from the NOAA ESRL observatories at Barrow, Mauna Loa, Samoa, and South Pole, and the continuous analyzer at Alert, Canada, operated by MSC. Note that all of these observations are calibrated against the same world CO2 standard (WMO-2005). Also, note that aircraft observations from the NOAA ESRL program were NOT assimilated, but used for independent assessment of the results.

At the continuous sampling sites, we construct one daytime average (12:00-16:00 Local Standard Time) mole fraction for each day from the time series, recognizing that our atmospheric transport model not always captures the continental nighttime stability regime while daytime well-mixed conditions are better matched. This approach is partly based on analysis of TransCom Continuous (TC) results [Law et al., 2007]. Moreover, observations at sub-daily time scales are likely to be strongly correlated and therefore add relatively little independent information to our results. Also based on TC continuous simulations, we decided to move a set of coastal sites by one degree into the ocean to force the model sample to be more representative of the actual site conditions. These sites are labeled for reference in the complete table of sites used in CarbonTracker.

We apply a further selection criterion during the assimilation to exclude non-Marine Boundary Layer (MBL) observations that are very poorly forecasted in our framework. We use the so-called model-data mismatch in this process, which is the random error prescribed to each observation to account for measurement errors as well as modeling errors of that observation. We interpret an observed-minus-forecasted (OmF) mole fraction that exceeds 3 times the prescribed model-data mismatch as an indicator that our modeling framework fails. This can happen for instance when an air sample is representative of local exchange not captured well by our 1x1 degree fluxes, when local meteorological conditions are not captured by our offline transport fields, but also when large-scale CO2 exchange is suddenly changed (e.g. fires, pests, droughts) to an extent that can not be accommodated by our flux modules. This last situation would imply an important change in the carbon cycle and has to be recognized by the researchers when analyzing the results. In accordance with the 3-sigma rejection criterion, ~2% of the observations are discarded through this mechanism in our assimilations. Table 1 gives a summary of the sites used and the assimilation performance.