Intensive atmospheric sampling field programs are envisioned as a key component of integrated research programs such as the North American Carbon Program (NACP) [Sarmiento and Wofsy, 1999; Wofsy and Harriss, 2002].  The intensive sampling provides unique information about the spatial distribution of CO₂ as well as imposes tight constraints on regional budgets that are difficult to obtain from other means. We summarize what we have learned from the numerous COBRA (CO₂ Budget and Rectification Airborne study) experiments [Gerbig et al., 2003a] that have taken place in 2000, 2003, and 2004.  We present the observed spatial variability of CO₂ [Gerbig et al., 2003a; Lin et al., 2004a] and regional budgets derived from regional air parcel-following experiments [Lin et al., 2004b].  These observations are also used as a critical testbed for modeling frameworks [Gerbig et al., 2003b]. We draw conclusions about ways to maximize the value of intensive atmospheric sampling experiments and the role that such experiments should play within programs like the NACP.

There are two basic approaches for inferring surface-atmosphere exchange for trace gases on regional scales: a bottom-up approach, in which local process knowledge is scaled up, and a top-down approach, in which the larger-scale constraint from atmospheric concentration measurements is applied in combination with transport models. Here we combine the two approaches, and assess the information content added by boundary layer concentration data. More specifically, we analyze the potential for inferring spatially resolved surface fluxes from atmospheric tracer observations within the mixed layer, such as from monitoring towers, using a receptor oriented transport model (Stochastic Time-Inverted Lagrangian Transport [STILT] model, [Lin et al., 2003]) coupled to a simple biosphere in which CO2 fluxes are represented as functional responses to environmental drivers (radiation and temperature, [Gerbig et al., 2003]). Transport and fluxes are coupled on a dynamic grid using a polar projection with high horizontal resolution (~20 km) in near field, and low resolution far away (as coarse as 2000 km), reducing the number of surface pixels without significant loss of information. To test the system, and to evaluate the errors associated with the retrieval of fluxes from atmospheric observations, a pseudo data experiment was performed. A large number of realizations of measurements (pseudo data) and a priori fluxes was generated, and for each case spatially resolved fluxes were retrieved. Results indicate strong potential for high resolution retrievals based on a network of tall towers, subject to the requirement of correctly specifying the a priori uncertainty covariance, especially the off diagonal elements that control spatial correlations.

To examine vertical distributions of the O₂/N₂ ratio in the stratosphere, air samples were collected using a cryogenic sampler over Sanriku, Japan and Syowa, Antarctica. It was clearly seen that d(O₂/N₂), as well as simultaneously measured d¹⁵N of N₂ and d¹⁸O of O₂, decreased gradually with increasing height in the stratosphere. The observed profiles of stratospheric ï€ d¹⁵N and d¹⁸O were in good agreement with those calculated using a steady state 1-dimensional eddy-diffusion/molecular-diffusion model suggesting that the upward decrease of stratospheric d(O₂/N₂) is caused by O₂ and N₂ molecules fractionated differently by gravity. The stratospheric d(O₂/N₂) corrected for the gravitational separation indicated that the average value at heights above 20-25 km over Sanriku was always higher than the upper tropospheric d(O₂/N₂) value over Japan at the corresponding time, and that it has decreased secularly, as was found in the troposphere.

Regular vertical profiles over Europe were set up in 2001 as part of the AEROCARB and Carboeurope-IP projects at five locations: Griffin (56°36'N, 3°47'W, Scotland), Orléans (47°50'N, 2°30'E, France), Schauinsland (47°55'N, 7°55'E, Germany), Hegyhatsal (46°57'N, 16°39'E, Hungary), and Bialystok (53.20°N, 22.75°E, Poland). The objective of the program is to measure CO₂, CH₄, N₂O, SF₆, CO, ¹³C and ¹⁸O in CO₂ vertical profiles at a bi-weekly frequency using air samples taken up at several levels from 100m up to 3000 m above the ground surface. One liter flasks are sampled on board small aircraft using a standardised protocol. The samples are analysed at three laboratories (LSCE, MPI-BGC, IUP-UHEI) which are linked through regular intercomparison exercises. We have characterised for each site the CO₂ seasonal cycles within the atmospheric boundary layer (ABL: 14 to 20 ppm) and the free troposphere (FT: 10 to 13 ppm). From these signals we have calculated the difference between ABL and FT, known as the CO₂ 'jump', which will be compared to the simulations from atmospheric transport models. We have also calculated the offset between each airborne sampling site and the time series from Mace Head observatory, used as a maritime reference. For CO₂, the wintertime offsets at the lowest level of the average vertical profiles are ranging from 0 ppm in Scotland up to 10 ppm in all continental sites. Depending of the site the positive offset due to emissions from anthropogenic and biospheric processes may extend up to 300 to 1500 m agl. In summertime we observe a negative gradient in most of the sites with a typical decrease of 5 ppm between 2000m and 100m agl. The average vertical gradients will be compared to the ouput of atmospheric models, and will be analysed with regards to the other trace gas (CO, CH₄, and CO₂ isotopes).

In order to elucidate seasonal and interannual variations of oceanic CO₂ uptake in the Greenland Sea and the Barents Sea, partial pressures of CO₂ in the surface ocean (pCO₂^sea) were measured from 1992 to 2001. The values of pCO₂^sea were lower than the partial CO₂ pressures in the atmosphere (pCO₂^air) throughout the year, and the annual net air-sea CO₂ fluxes in the Greenland Sea and the Barents Sea were evaluated to be 52 ± 31 and 46 ± 27 gC m^-2 yr^-1, respectively, yielding a total oceanic CO₂ uptake of 0.050 ± 0.030 GtC yr^-1. We also found that the annual mean CO₂ uptake was positively correlated with the North Atlantic Oscillation Index (NAOI) via wind strength, but was negatively correlated with DpCO₂ (pCO₂^air-pCO₂^sea) and the sea ice coverage. The results also indicate that the wind speed and sea ice coverage play a major role in determining the interannual variation of CO₂ uptake, with DpCO₂ playing a minor role.

Variations in atmospheric oxygen (O₂) are a sensitive indicator of biogeochemical processes involved in the global carbon cycle.  To improve our understanding of these processes, we developed a system for continuous high precision measurements of atmospheric O₂ and CO₂ that is suitable for shipboard use.  This system was employed on two voyages in the Western Pacific sector of the Southern Ocean, in February 2003 and April 2004.  Elevated O₂ concentrations were observed south of New Zealand and across the Chatham Rise suggesting that these regions of ocean are outgassing O₂ in late summer to autumn.

Measurements of the partial pressure of CO₂ in surface seawater (pCO₂^w) have been made frequently and extensively in the western North Pacific (3-35°N, 132-142°E) since 1990. Based on the time series analysis of pCO₂^w data, we obtained a “climatological view” of seasonal variation in pCO₂^w in the western North Pacific. We have examined the relationship between pCO₂^w and sea surface temperature (SST). The pCO₂^w–SST relationship varies spatially and temporally. The pCO₂^w showed an average growth rate of 1.6 µatm yr^-1 (nearly equal to that of the air, pCO₂^a) with large variability (±8.9µatm yr^-1). In 1998, larger growth rates of pCO₂^w occurred in the subtropical gyre and the western equatorial Pacific, which was probably associated with the 1997/98 El Niño phenomena. To know processes affecting long-term variations in pCO₂^w, we have examined seasonal variation in growth rate of pCO₂^w. The linear growth rate of pCO₂^w during the winter season ranged from 1.3±0.2 to 2.1±0.2µatm yr^-1 with an average of 1.7±0.2µatm yr^-1. During spring/summer seasons, the average growth rate of pCO₂^w was larger than 2µatm yr^-1 north of 27°N, and within the range from 0 to 1µatm yr^-1 in the North Equatorial Current. These increases were mostly caused by the oceanic uptake of anthropogenic CO₂, and to some extent, other processes controlling the pCO₂^w change: thermodynamic effect, lateral transport and vertical mixing, and biological activity.

High-frequency atmospheric CO₂ measurements should become increasingly available by the end of this decade from a variety of sources, including low-Earth orbiting satellites. If of sufficient accuracy, these should allow the functioning of the global carbon cycle to be monitored at fine time/space resolutions using atmospheric transport inversions. Since traditional direct inversion methods (e.g., Bayesian synthesis) become computationally infeasible at these resolutions, we use an approximate method, variational data assimilation, to estimate surface CO₂ fluxes at spatial resolutions ranging from 10x10 degrees to 1x1 degrees and at time resolutions ranging from 2 weeks to 1 hour. We assess its performance using simulated data, including the effects of realistic transport and data errors.

Seven CARIOCA lagrangian buoys drifted in the Subantarctic Zone, SAZ, of the Indian and Pacific Ocean between 2001 and 2005. Measurements indicate that pCO₂ in sea water is undersaturated with respect to the atmospheric value and consequently the subantartic zone of the Southern Ocean acts as a sink for atmospheric CO₂ during all seasons. Large observed pCO₂ variability is associated with mixing close to the subantarctic front, with biological activity and local warming. These variabilities are higher than the seasonal cycle in the studied areas.

CO₂ and methane are important greenhouse gases, both contributing in increasing amounts towards positive radiative forcing. It is hence important to gain maximum understanding of the carbon cycle in the atmosphere, and the scale of carbon trace gas sources and sinks, not only globally but also on a more regional level. The Orbiting Carbon Observatory (OCO) satellite, scheduled for launch in 2008, is designed for dedicated global mapping of CO₂. In order to investigate the usefulness of a variety of methods, including retrievals from satellite mapping, some preliminary inverse modelling using a Bayesian synthesis technique is performed using pseudodata generated to represent possible future measurement regimes. This study will focus on the ability of in-situ measurements within Australia to reduce the uncertainties in Australian continental CO₂ flux estimates. The specific measurements investigated include a Ghan railway transect between Adelaide (34.9°S, 138.6°E) and Darwin (12.5°S, 130.9°E), and a number of continuous permanent sites. The reduction in flux uncertainties from additional measurements compared to a background inversion is examined, from which it is concluded that measuring on the Ghan railway is potentially worthwhile for reducing uncertainties associated with flux estimates.