2.   Detailed Description
This study uses measurements of air samples collected at surface sites in the NOAA ESRL Cooperative Global Air Sampling Network and the CSIRO Air Sampling Network available for each year studied, except those flagged for analysis or sampling problems, or those thought to be influenced by local sources. The sites for which data are available thus varies each week depending on successful sampling and analysis, and each site's sampling frequency. In addition, we use in situ quasi-continuous CO2 time series from six towers: (1) the 396m level of the LEF tower in Wisconsin; (2) the 107m level of the AMT tower in Argyle, Maine; (3) the 251m level of the WKT tower in Texas; (4) the 40m level of the tower in Fraserdale, Canada (FRD) operated by Environment Canada (EC); (5) the 30m level of the tower at Candle Lake (CDL, formerly Old Black Spruce), Canada operated by EC; and (6) the 60m level of the tower at the Atmospheric Radiation and Monitoring (ARM) program Southern Great Plains, Oklahoma site (SGP). Other in situ quasi-continuous CO2 time series used are from the NOAA ESRL observatories at Barrow (BRW), Mauna Loa (MLO), Samoa (SMO), and South Pole (SPO); the EC Canadian sites at Alert, Nunavut (ALT), Sable Island, Nova Scotia (SBL) and Egbert, Ontario (EGB); and the NCAR sites at Niwot Ridge, Colorado (NWR) and Storm Peak Laboratory, Colorado (SPL). 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 CarbonTracker results.

For most of the quasi-continuous sampling sites, we construct an afternoon daytime average mole fraction for each day from the time series, recognizing that our atmospheric transport model does not always capture the continental nighttime stability regime while daytime well-mixed conditions are better matched. At mountain-top sites (MLO, NWR, and SPL), we use an average of nighttime hours as this tends to be the most stable time period and avoids periods of upslope flows that contain local vegetative and/or anthropogenic influence. 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 Transcom 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. Table 1 summarizes how data from the different measurement programs are preprocessed for this study.

The preprocessed data used in CarbonTracker are freely available for download. Preprocessed data are not the original measurement data! Users are encouraged to review the literature and contact the measurement labs directly for details about and access to the actual observations.

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 ascribed 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 2 gives a summary of the observing sites used in CarbonTracker and the assimilation performance. Model-data-mismatch ("r") is a value assigned to a given site that is meant to quantify our expected ability to simulate observations there. This value is principally determined from the limitations of the atmospheric transport model. It is part of the standard deviation used to interpret the difference between a simulation first guess ("Hx") of an observation and the actual measured value ("z"). The other component, HPH^T is a measure of the ability of the ensemble Kalman filter to improve its simulated value for this observation by adjusting fluxes. These elements together form the innovation χ statistic for the site: χ = (z-Hx)/√(HPH^T+r²). The innovation χ² reported above is the mean of all squared χ values for a given site. An average χ² below 1.0 indicates that the √(HPH^T+r²) values are too large. Conversely, values above 1.0 mean that this standard deviation is underestimated. The bias and SE columns are statistics of the posterior residuals (final modeled values - measured values). The bias is the mean of these residuals; the SE is the standard error of those residuals.

Table 2: Summary of observing sites used in CarbonTracker and assimilation performance.

Site code	Lab.	Location	Latitude	Longitude	Elev. (m ASL)	No. Obs. Avail.	No. Obs. Used	No. Obs. Rej.	r (μmol mol-1)	Innov. χ2	Bias (μmol mol-1)	SE (μmol mol-1)
ALT_01D0	ESRL	Alert, Nunavut, Canada	82.45°N	62.52°W	200	385	385	0	1.50	0.47	0.11	0.87
ALT_06C0	EC	Alert, Nunavut, Canada	82.45°N	62.51°W	200	2597	2597	0	2.50	0.22	0.21	1.02
AMT_01C3	ESRL	Argyle, Maine, United States	45.03°N	68.68°W	50	1368	1355	13	3.00	1.04	0.49	3.30
ASC_01D0	ESRL	Ascension Island, United Kingdom	7.92°S	14.42°W	54	677	677	0	0.75	0.97	-0.07	0.68
ASK_01D0	ESRL	Assekrem, Algeria	23.17°N	5.42°E	2728	354	354	0	1.50	0.39	0.09	0.93
AZR_01D0	ESRL	Terceira Island, Azores, Portugal	38.75°N	27.20°W	40	262	256	6	1.50	1.24	0.52	1.56
BAL_01D0	ESRL	Baltic Sea, Poland	55.43°N	16.93°E	3	652	652	0	7.50	0.41	-1.06	4.62
BKT_01D0	ESRL	Bukit Kototabang, Indonesia	0.20°S	100.31°E	864	139	139	0	7.50	0.72	5.39	3.51
BME_01D0	ESRL	St. Davids Head, Bermuda, United Kingdom	32.37°N	64.65°W	30	225	216	9	1.50	2.00	0.80	1.98
BMW_01D0	ESRL	Tudor Hill, Bermuda, United Kingdom	32.27°N	64.87°W	30	260	259	1	1.50	1.14	0.64	1.40
BRW_01C0	ESRL	Barrow, Alaska, United States	72.32°N	156.60°W	11	2218	2202	1	2.50	0.32	0.24	1.16
Site code	Lab.	Location	Latitude	Longitude	Elev. (m ASL)	No. Obs. Avail.	No. Obs. Used	No. Obs. Rej.	r (μmol mol-1)	Innov. χ2	Bias (μmol mol-1)	SE (μmol mol-1)
BRW_01D0	ESRL	Barrow, Alaska, United States	72.32°N	156.59°W	11	360	358	2	1.50	0.84	0.24	1.15
BSC_01D0	ESRL	Black Sea, Constanta, Romania	44.17°N	29.67°E	3	300	298	2	7.50	0.91	-2.55	6.78
CBA_01D0	ESRL	Cold Bay, Alaska, United States	54.20°N	162.72°W	21	612	590	22	1.50	1.78	-0.19	1.92
CDL_06C3	EC	Candle Lake, Saskatchewan, Canada	53.99°N	105.12°W	600	1733	1728	5	3.00	0.68	0.46	2.31
CFA_02D0	CSIRO	Cape Ferguson, Queensland, Australia	19.28°S	147.06°E	2	142	142	0	2.50	0.42	-0.87	1.15
CGO_01D0	ESRL	Cape Grim, Tasmania, Australia	40.67°S	143.67°E	94	317	317	0	0.75	0.19	-0.06	0.36
CGO_02D0	CSIRO	Cape Grim, Tasmania, Australia	40.68°S	143.68°E	94	311	311	0	0.75	0.19	-0.11	0.35
CHR_01D0	ESRL	Christmas Island, Republic of Kiribati	1.70°N	157.17°W	3	309	309	0	0.75	1.30	-0.59	0.53
CRZ_01D0	ESRL	Crozet Island, France	46.45°S	51.85°E	120	275	275	0	0.75	0.25	-0.09	0.35
CYA_02D0	CSIRO	Casey, Antarctica, Australia	66.28°S	110.52°E	51	149	149	0	0.75	0.48	-0.40	0.29
EGB_06C3	EC	Egbert, Ontario, Canada	44.23°N	79.78°W	226	900	858	42	3.00	1.84	1.36	4.19
Site code	Lab.	Location	Latitude	Longitude	Elev. (m ASL)	No. Obs. Avail.	No. Obs. Used	No. Obs. Rej.	r (μmol mol-1)	Innov. χ2	Bias (μmol mol-1)	SE (μmol mol-1)
EIC_01D0	ESRL	Easter Island, Chile	27.14°S	109.45°W	50	187	187	0	7.50	0.03	0.81	0.98
FRD_06C3	EC	Fraserdale, Canada	49.88°N	81.57°W	210	2637	2618	19	3.00	0.74	0.33	2.78
GMI_01D0	ESRL	Mariana Islands, Guam	13.42°N	144.77°E	1	562	562	0	1.50	0.39	0.09	0.93
HBA_01D0	ESRL	Halley Station, Antarctica, United Kingdom	75.57°S	26.50°W	30	369	369	0	0.75	0.20	-0.17	0.26
HUN_01D0	ESRL	Hegyhatsal, Hungary	46.95°N	16.64°E	248	369	367	2	7.50	0.49	0.01	5.12
ICE_01D0	ESRL	Storhofdi, Vestmannaeyjar, Iceland	62.29°N	20.21°W	118	355	354	1	1.50	0.56	0.07	1.03
IZO_01D0	ESRL	Tenerife, Canary Islands, Spain	28.30°N	16.47°W	2360	295	293	2	1.50	1.18	1.02	1.30
KEY_01D0	ESRL	Key Biscayne, Florida, United States	25.67°N	79.20°W	3	250	250	0	2.50	0.36	0.18	1.45
KUM_01D0	ESRL	Cape Kumukahi, Hawaii, United States	19.52°N	154.82°W	3	359	359	0	1.50	0.41	0.17	0.94
KZD_01D0	ESRL	Sary Taukum, Kazakhstan	44.45°N	75.57°E	412	372	371	1	2.50	1.48	0.63	2.53
KZM_01D0	ESRL	Plateau Assy, Kazakhstan	43.25°N	77.87°E	2519	327	326	1	2.50	1.22	0.32	2.76
Site code	Lab.	Location	Latitude	Longitude	Elev. (m ASL)	No. Obs. Avail.	No. Obs. Used	No. Obs. Rej.	r (μmol mol-1)	Innov. χ2	Bias (μmol mol-1)	SE (μmol mol-1)
LEF_01C3	ESRL	Park Falls, Wisconsin, United States	45.93°N	90.27°W	472	4977	2465	35	3.00	0.91	0.29	3.53
MAA_02D0	CSIRO	Mawson Station, Antarctica, Australia	67.62°S	62.87°E	32	161	161	0	0.75	0.40	-0.36	0.26
MHD_01D0	ESRL	Mace Head, County Galway, Ireland	53.32°N	10.90°W	25	301	301	0	2.50	0.22	0.02	1.13
MID_01D0	ESRL	Sand Island, Midway, United States	28.21°N	177.37°W	4	360	360	0	1.50	0.70	0.59	1.10
MKN_01D0	ESRL	Mt. Kenya, Kenya	0.05°S	37.29°E	3897	75	75	0	2.50	1.13	1.82	1.92
MLO_01C0	ESRL	Mauna Loa, Hawaii, United States	19.54°N	155.58°W	3397	2519	2519	0	0.75	0.88	0.17	0.68
MLO_01D0	ESRL	Mauna Loa, Hawaii, United States	19.53°N	155.57°W	3397	415	415	0	1.50	0.23	0.13	0.69
MQA_02D0	CSIRO	Macquarie Island, Australia	54.48°S	158.97°E	12	197	197	0	0.75	0.32	-0.10	0.41
NMB_01D0	ESRL	Gobabeb, Namibia	23.57°S	15.02°E	456	48	48	0	2.50	0.18	-0.14	1.05
NWR_01D0	ESRL	Niwot Ridge, Colorado, United States	40.04°N	105.57°W	3523	356	354	2	1.50	0.93	0.55	1.38
NWR_03C3	NCAR	Niwot Ridge, Colorado, United States	40.05°N	105.58°W	3523	1364	682	0	3.00	0.33	-0.08	1.64
Site code	Lab.	Location	Latitude	Longitude	Elev. (m ASL)	No. Obs. Avail.	No. Obs. Used	No. Obs. Rej.	r (μmol mol-1)	Innov. χ2	Bias (μmol mol-1)	SE (μmol mol-1)
OBN_01D0	ESRL	Obninsk, Russia	55.10°N	36.60°E	183	123	118	5	7.50	2.71	-3.00	12.72
OXK_01D0	ESRL	Ochsenkopf, Germany	50.03°N	11.80°E	1022	81	73	8	2.50	3.29	0.31	3.71
PAL_01D0	ESRL	Pallas-Sammaltunturi, GAW Station, Finland	67.96°N	24.11°E	560	236	234	2	2.50	0.88	0.05	2.10
POC_01D1	ESRL	Pacific Ocean, N/A	0.33°S	130.05°W	10	1668	1666	0	0.75	1.08	-0.12	0.70
PSA_01D0	ESRL	Palmer Station, Antarctica, United States	64.92°S	64.00°W	10	380	380	0	0.75	0.40	-0.25	0.34
PTA_01D0	ESRL	Point Arena, California, United States	38.95°N	124.72°W	17	251	251	0	7.50	0.40	-2.56	3.95
RPB_01D0	ESRL	Ragged Point, Barbados	13.17°N	59.42°W	45	366	366	0	1.50	0.79	0.20	1.31
SBL_06C3	EC	Sable Island, Nova Scotia, Canada	49.93°N	60.02°W	5	1517	1477	40	3.00	1.27	0.10	3.39
SEY_01D0	ESRL	Mahe Island, Seychelles	4.67°S	55.17°E	3	348	348	0	0.75	1.00	-0.05	0.74
SGP_01D0	ESRL	Southern Great Plains, Oklahoma, United States	36.70°N	97.49°W	314	575	559	16	2.50	1.27	0.06	2.81
SGP_64C3	LBNL	Southern Great Plains, Oklahoma, United States	36.80°N	97.50°W	314	705	697	8	3.00	1.26	0.32	3.25
Site code	Lab.	Location	Latitude	Longitude	Elev. (m ASL)	No. Obs. Avail.	No. Obs. Used	No. Obs. Rej.	r (μmol mol-1)	Innov. χ2	Bias (μmol mol-1)	SE (μmol mol-1)
SHM_01D0	ESRL	Shemya Island, Alaska, United States	52.72°N	174.09°E	40	294	293	1	2.50	0.86	0.09	2.26
SMO_01C0	ESRL	Tutuila, American Samoa	14.24°S	170.57°W	42	2578	2554	0	0.75	0.54	-0.01	0.53
SMO_01D0	ESRL	Tutuila, American Samoa	14.24°S	170.57°W	42	421	421	0	1.50	0.15	-0.02	0.54
SPL_03C3	NCAR	Storm Peak Laboratory (Desert Research Institute), United States	40.45°N	106.73°W	3210	1504	751	0	3.00	0.44	-0.31	1.78
SPO_01C0	ESRL	South Pole, Antarctica, United States	89.98°S	24.80°W	2810	2780	2779	0	0.75	0.17	-0.13	0.28
SPO_01D0	ESRL	South Pole, Antarctica, United States	89.97°S	24.80°W	2810	392	392	0	1.50	0.04	-0.05	0.28
STM_01D0	ESRL	Ocean Station M, Norway	66.00°N	2.00°E	0	684	678	3	1.50	0.79	0.28	1.20
SUM_01D0	ESRL	Summit, Greenland	72.57°N	38.47°W	3238	281	281	0	1.50	0.50	0.36	0.93
SYO_01D0	ESRL	Syowa Station, Antarctica, Japan	69.00°S	39.57°E	11	186	186	0	0.75	0.36	-0.34	0.25
TAP_01D0	ESRL	Tae-ahn Peninsula, Republic of Korea	35.72°N	125.12°E	20	275	273	2	7.50	0.63	1.79	5.58
TDF_01D0	ESRL	Tierra Del Fuego, Ushuaia, Argentina	54.87°S	68.47°W	20	107	107	0	0.75	0.41	-0.13	0.44
Site code	Lab.	Location	Latitude	Longitude	Elev. (m ASL)	No. Obs. Avail.	No. Obs. Used	No. Obs. Rej.	r (μmol mol-1)	Innov. χ2	Bias (μmol mol-1)	SE (μmol mol-1)
THD_01D0	ESRL	Trinidad Head, California, United States	41.03°N	125.15°W	107	233	209	24	2.50	3.59	-2.48	4.08
UTA_01D0	ESRL	Wendover, Utah, United States	39.89°N	114.72°W	1320	339	339	0	2.50	0.60	0.27	1.82
UUM_01D0	ESRL	Ulaan Uul, Mongolia	44.45°N	111.09°E	914	369	367	2	2.50	1.02	-0.21	2.50
WIS_01D0	ESRL	Sede Boker, Negev Desert, Israel	31.12°N	34.87°E	400	392	391	1	2.50	0.72	0.07	2.07
WKT_01C3	ESRL	Moody, Texas, United States	31.32°N	97.33°W	251	2406	1198	11	3.00	0.95	0.55	3.05
WLG_01D0	ESRL	Mt. Waliguan, Peoples Republic of China	36.28°N	100.91°E	3810	260	254	6	1.50	1.44	-0.14	1.83
ZEP_01D0	ESRL	Ny-Alesund, Svalbard, Norway and Sweden	78.90°N	11.87°E	475	448	445	3	1.50	0.87	0.53	1.10
All-site summary			-	-	-	47417	47119	298	-	0.72	0.12	2.39

3.   Further Reading

• ESRL Carbon Cycle Program
• WMO/GAW Report No. 168, 2006 [Note: Requires a few minutes to load]