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We present an analysis of a 39-year record of continuous atmospheric CO2 observations made at Baring Head, New Zealand, filtered for steady background CO2 mole fractions during southerly wind conditions. We discuss relationships between variability in the filtered CO2 time series and regional to global carbon cycling. Baring Head is well situated to sample air that has been isolated from terrestrial influences over the Southern Ocean, and experiences extended episodes of strong southerly winds with low CO2 variability. The filtered Baring Head CO2 record reveals an average seasonal cycle with amplitude of 0.95 ppm that is 13% smaller and 3 weeks earlier in phase than that at the South Pole. Seasonal variations in a given year are sensitive to the timing and magnitude of the combined influences of Southern Ocean CO2 fluxes and terrestrial fluxes from both hemispheres. The amplitude of the seasonal cycle varies throughout the record, but we find no significant long-term seasonal changes with respect to the South Pole. Interannual variations in CO2 growth rate in the Baring Head record closely match the El Niño-Southern Oscillation, reflecting the global reach of CO2 mole fraction anomalies associated with this cycle. We use atmospheric transport model results to investigate contributions to seasonal and annual-mean components of the observed CO2 record. Long-term trends in mean gradients between Baring Head and other stations are predominately due to increases in Northern Hemisphere fossil-fuel burning and Southern Ocean CO2 uptake, for which there remains a wide range of future estimates. We find that the postulated recent reduction in the efficiency of Southern Ocean anthropogenic CO2 uptake, as a result of increased zonal winds, is too small to be detectable as significant differences in atmospheric CO2 between mid to high latitude Southern Hemisphere observing stations.

Thermal infrared radiances from the Tropospheric Emission Spectrometer (TES) between 10 and 15 μm contain significant carbon dioxide (CO2) information, however the CO2 signal must be separated from radiative interference from temperature, surface and cloud parameters, water, and other trace gases. Validation requires data sources spanning the range of TES CO2 sensitivity, which is approximately 2.5 to 12 km with peak sensitivity at about 5 km and the range of TES observations in latitude (40° S to 40° N) and time (2005–2011). We therefore characterize Tropospheric Emission Spectrometer (TES) CO2 version 5 biases and errors through comparisons to ocean and land-based aircraft profiles and to the CarbonTracker assimilation system. We compare to ocean profiles from the first three Hiaper Pole-to-Pole Observations (HIPPO) campaigns between 40° S and 40° N with measurements between the surface and 14 km and find that TES CO2 estimates capture the seasonal and latitudinal gradients observed by HIPPO CO2 measurements. Actual errors range from 0.8–1.8 ppm, depending on the campaign and pressure level, and are approximately 1.6–2 times larger than the predicted errors. The bias of TES versus HIPPO is within 1 ppm for all pressures and datasets; however, several of the sub-tropical TES CO2 estimates are lower than expected based on the calculated errors. Comparisons to land aircraft profiles from the United States Southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) between 2005 and 2011 measured from the surface to 5 km to TES CO2 show good agreement with an overall bias of −0.3 ppm to 0.1 ppm and standard deviations of 0.8 to 1.0 ppm at different pressure levels. Extending the SGP aircraft profiles above 5 km using AIRS or CONTRAIL measurements improves comparisons with TES. Comparisons to CarbonTracker (version CT2011) show a persistent spatially dependent bias pattern and comparisons to SGP show a time-dependent bias of −0.2 ppm yr−1. We also find that the predicted sensitivity of the TES CO2 estimates is too high, which results from using a multi-step retrieval for CO2 and temperature. We find that the averaging kernel in the TES product corrected by a pressure-dependent factor accurately reflects the sensitivity of the TES CO2 product.

We present descriptions of the in situ instrumentation, calibration procedures, intercomparison efforts, and data filtering methods used in a 39-yr record of continuous atmospheric carbon dioxide (CO2) observations made at Baring Head, New Zealand. Located on the southern coast of the North Island, Baring Head is exposed to extended periods of strong air flow from the south with minimal terrestrial influence resulting in low CO2 variability. The site is therefore well suited for sampling air masses that are representative of the Southern Ocean region. Instrumental precision is better than 0.015 ppm (1-σ) on 1-Hz values. Comparisons to over 600 co-located flask samples, as well as laboratory based flask and cylinder comparison exercises, suggest that over recent decades compatibility with respect to the Scripps Institution of Oceanography (SIO) and World Meteorological Organisation (WMO) CO2 scales has been 0.3 ppm or better.

International agreements to limit greenhouse gas emissions require verification to ensure that they are effective and fair. Verification based on direct observation of atmospheric greenhouse gas concentrations will be necessary to demonstrate that estimated emission reductions have been actualized in the atmosphere. Here we assess the capability of ground-based observations and a high-resolution (1.3 km) mesoscale atmospheric transport model to determine a change in greenhouse gas emissions over time from a metropolitan region. We test the method with observations from a network of CO 2 surface monitors in Salt Lake City. Many features of the CO 2 data were simulated with excellent fidelity, although data-model mismatches occurred on hourly timescales due to inadequate simulation of shallow circulations and the precise timing of boundary-layer stratification and destratification. Using two optimization procedures, monthly regional fluxes were constrained to sufficient precision to detect an increase or decrease in emissions of approximately 15% at the 95% confidence level. We argue that integrated column measurements of the urban dome of CO 2 from the ground and/or space are less sensitive than surface point measurements to the redistribution of emitted CO 2 by small-scale processes and thus may allow for more precise trend detection of emissions from urban regions.

There is a widely recognized need to improve our understanding of biosphere-atmosphere carbon exchanges in areas of complex terrain including the United States Mountain West. CO2 fluxes over mountainous terrain are often difficult to measure due to unusual and complicated influences associated with atmospheric transport. Consequently, deriving regional fluxes in mountain regions with carbon cycle inversion of atmospheric CO2 mole fraction is sensitive to filtering of observations to those that can be represented at the transport model resolution. Using five years of CO2 mole fraction observations from the Regional Atmospheric Continuous CO2 Network in the Rocky Mountains (Rocky RACCOON), five statistical filters are used to investigate a range of approaches for identifying regionally representative CO2 mole fractions. Test results from three filters indicate that subsets based on short-term variance and local CO2 gradients across tower inlet heights retain nine-tenths of the total observations and are able to define representative diel variability and seasonal cycles even for difficult-to-model sites where the influence of local fluxes is much larger than regional mole fraction variations. Test results from two other filters that consider measurements from previous and following days using spline fitting or sliding windows are overly selective. Case study examples showed that these windowing-filters rejected measurements representing synoptic changes in CO2, which suggests that they are not well suited to filtering continental CO2 measurements. We present a novel CO2 lapse rate filter that uses CO2 differences between levels in the model atmosphere to select subsets of site measurements that are representative on model scales. Our new filtering techniques provide guidance for novel approaches to assimilating mountain-top CO2 mole fractions in carbon cycle inverse models.

We describe CO2 concentration measurement systems based on relatively inexpensive single-cell non-dispersive infrared CO2 sensors. The systems utilize signal averaging to obtain precision (1-σ in 100 s) of 0.1 parts per million dry air mole fraction (ppm), frequent calibrations and sample drying in order to achieve state-of-the-art compatibility, and can run autonomously for months at a time. Laboratory tests indicate compatibility among four to six systems to be ±0.1 ppm (1-σ), and field measurements of known reference-gases yield median errors of 0.01 to 0.17 ppm with 1-σ variance of ±0.1 to 0.2 ppm. From May to August 2007, a system co-located with a NOAA-ESRL dual-cell NDIR system at the WLEF tall tower in Wisconsin measured daytime-only daily averages of CO2 that differ by 0.26 ± 0.15 ppm (median ± 1 σ), and from August 2005 to April 2011 a system co-located with weekly NOAA-ESRL network flask collection at Niwot Ridge, Colorado measured coincident CO2 concentrations that differed by −0.06 ± 0.30 ppm (n = 585). Data from these systems are now supporting a wide range of analyses and this approach may be applicable in future studies where accuracy and initial cost of the sensors are priorities.

The bioethical metaphor The Tragedy of the Commons shows how individual interests can lead to overexploitation of a shared resource [1]. In the example of a communal village pasture, the villagers ...

Air temperature T a , specific humidity q, CO2 mole fraction χ c , and three-dimensional winds were measured in mountainous terrain from five tall towers within a 1 km region encompassing a wide range of canopy densities. The measurements were sorted by a bulk Richardson number Ri b . For stable conditions, we found vertical scalar differences developed over a “transition” region between 0.05 < Ri b < 0.5. For strongly stable conditions (Ri b > 1), the vertical scalar differences reached a maximum and remained fairly constant with increasing stability. The relationships q and χ c have with Ri b are explained by considering their sources and sinks. For winds, the strong momentum absorption in the upper canopy allows the canopy sublayer to be influenced by pressure gradient forces and terrain effects that lead to complex subcanopy flow patterns. At the dense-canopy sites, soil respiration coupled with wind-sheltering resulted in CO2 near the ground being 5–7 μmol mol−1 larger than aloft, even with strong above-canopy winds (near-neutral conditions). We found Ri b -binning to be a useful tool for evaluating vertical scalar mixing; however, additional information (e.g., pressure gradients, detailed vegetation/topography, etc.) is needed to fully explain the subcanopy wind patterns. Implications of our results for CO2 advection over heterogenous, complex terrain are discussed.

A significant fraction of Earth consists of mountainous terrain. However, the question of how to monitor the surface–atmosphere carbon exchange over complex terrain has not been fully explored. This article reports on studies by a team of investigators from U.S. universities and research institutes who carried out a multiscale and multidisciplinary field and modeling investigation of the CO2 exchange between ecosystems and the atmosphere and of CO2 transport over complex mountainous terrain in the Rocky Mountain region of Colorado. The goals of the field campaign, which included ground and airborne in situ and remote-sensing measurements, were to characterize unique features of the local CO2 exchange and to find effective methods to measure regional ecosystem–atmosphere CO2 exchange over complex terrain. The modeling effort included atmospheric and ecological numerical modeling and data assimilation to investigate regional CO2 transport and biological processes involved in ecosystem–atmosphere carbon exch...

A single particle soot photometer (SP2) was operated on the NCAR C-130 during the MIRAGE campaign (part of MILAGRO), sampling black carbon (BC) over Mexico. The highest BC concentrations were measured over Mexico City (sometimes as much as 2 μg/m3) and over hill-fires to the south of the city. The age of plumes outside of Mexico City was determined using a combination of HYSPLIT trajectories, WRF-FLEXPART modeling and CMET balloon tracks. As expected, older, diluted air masses had lower BC concentrations. A comparison of carbon monoxide (CO) and BC suggests a CO background of around 65 ppbv, and a background-corrected BC/COnet ratio of 2.89±0.89 (ng/m3-STP)/ppbv (average ± standard deviation). This ratio is similar for fresh emissions over Mexico City, as well as for aged airmasses. Comparison of light absorption measured with a particle soot absorption photometer (PSAP) and the SP2 BC suggests a BC mass-normalized absorption cross-section (MAC) of 10.9±2.1 m2/g at 660 nm (or 13.1 m2/g @ 550 nm, assuming MAC is inversely dependent on wavelength). This appears independent of aging and similar to the expected absorption cross-section for aged BC, but values, particularly in fresh emissions, could be biased high due to instrument artifacts. SP2-derived BC coating indicators show a prominent thinly-coated BC mode over the Mexico City Metropolitan Area (MCMA), while older air masses show both thinly-coated and thickly-coated BC. Some 2-day-old plumes do not show a prominent thickly-coated BC mode, possibly due to preferential wet scavenging of the likely-hydrophilic thickly-coated BC.

Observations of CO2 concentration at a mountaintop in the Colorado Rockies in summer show a large diurnal variability with minimum CO2 concentrations found between 10:00 and 18:00 MST. Simulations are performed with a mesoscale model to examine the effects of atmospheric structure and large-scale flows on the diurnal variability. In the simulations initialized without large-scale winds, the CO2 minimum occurs earlier compared to the observations. Upslope flows play an important role in the presence of this early (pre-noon) minimum while the timing and magnitude of the minimum depend only weakly on the temperature structure. An increase in large-scale flow has a noticeable impact on the diurnal variability with a more gradual decrease in daytime CO2 concentration, similar to summer-averaged observations. From the idealized simulations and a case study, it is concluded that multi-scale flows and their interactions have a large influence on the observed diurnal variability.

The construction and deployment of a portable trace-gas measurement system (TGaMS) is described. The air-collection system (dubbed HYDRA) collects air samples from 18 different locations and was connected to either one or two LI-COR LI-7000 gas analyzers to measure CO2. An in situ “field calibration” method, that uses four calibration gases with an uncertainty on the order of ±0.1 μmol mol−1 relative to the WMO CO2 mole fraction scale, revealed CO2 output from the LI-7000 had a slightly nonlinear relationship relative to the CO2 concentration of the calibration gases. The sensitivity of the field-calibrated CO2 to different forms of the field-calibration equation is investigated. To evaluate TGaMS performance, CO2 from collocated inlets, portable gas cylinders, and nearby independent CO2 instruments are compared. Results are as follows: 1) CO2 measurements from HYDRA multiple inlets are feasible with a reproducibility of ±0.4 μmol mol−1 (based on the standard deviation of the CO2 difference between collocated inlets when HYDRA was operating with two LI-7000s); 2) CO2 differences among the various field-calibration equations were on the order of ±0.3 μmol mol−1; and 3) comparison of midday hourly CO2 measurements at 30 m AGL between TGaMS and an independent high-accuracy CO2 measurement system (within 300 m of TGaMS) had a median difference and standard deviation of 0.04 ± 0.81 μmol mol−1 over two months.

The goal of this study was to test if Asian long-range transport (ALRT) of pollutants leads to enhanced levels of atmospheric mercury in the Colorado Rocky Mountains in the United States. Storm Peak Laboratory, a high-elevation (3200 m asl) mountaintop research facility, is located on top of the Continental Divide ideally exposed to the prevailing westerly winds 1500 km inland from the Pacific where near-coastal stations previously measured significant atmospheric mercury enhancements due to ALRT. Measurements of gaseous elemental mercury (GEM) from October 2006 to May 2007 averaged 1.51 � 0.12 ng m �3 at this laboratory, with minimum and maximum concentrations of 1.06 ng m �3 and 2.15 ng m �3 , respectively. GEM showed diel patterns with peak concentrations during daytime and lowest concentrations during late night and early morning hours. In fall and winter, these patterns closely followed diel fluctuations of water vapor, aerosol number concentration, and ozone. The patterns are hence attributed to transitions of daytime boundary layer and nighttime air masses with regional representation as daytime surface heating causes air masses enhanced with air pollutants to rise from the valley floor to the laboratory. In spring, diel patterns of GEM and carbon monoxide (CO) occurred time-shifted in respect to water vapor, aerosol number, and ozone concentrations, indicating additional or different sources or sinks. A large GEM enhancement (2.15 ng m

CO2 transport processes relevant for estimating net ecosystem exchange (NEE) at the Niwot Ridge AmeriFlux site in the front range of the Rocky Mountains, Colorado, USA, were investigated during a pilot experiment. We found that cold, moist, and CO2rich air was transported downslope at night and upslope in the early morning at this forest site situated on a � 5% east-facing slope. We found that CO2 advection dominated the total CO2 transport in the NEE estimate at night although there are large uncertainties because of partial cancellation of horizontal and vertical advection. The horizontal CO2 advection captured not only the CO2 loss at night, but also the CO2 uptake during daytime. We found that horizontal CO2 advection was significant even during daytime especially whenturbulentmixingwas notsignificant, such as inearly morningand evening transitionperiodsand withinthe canopy. Similar processes can occur anywhere regardless of whether flow is generated by orography, synoptic pressure gradients, or surface heterogeneity as long as CO2 concentration is not well mixed by turbulence. The long-term net effect of all the CO2 budget terms on estimates of NEE needs to be investigated. # 2007 Elsevier B.V. All rights reserved.

A commercially available differential fuel-cell analyzer has been adapted to make field-based ppm-level measurements of atmospheric O2 variations. With the implementation of rapid calibrations and active pressure and flow control, the analysis system described here has a 1σ precision of ±2.5 per meg (≈0.5 ppm) for a 2-min measurement. Allowing for system stabilization after switching inlet lines, a 6-min measurement with a precision of ±1.4 per meg (≈0.3 ppm) every 20 min is obtained. The elimination of biases in any atmospheric O2 measurement depends critically on careful gas-handling procedures, and after screening for known sources of bias a comparability of ±10 per meg (≈2 ppm) with the present setup is estimated. In comparison to existing techniques, the relatively small size, low cost, fast response, motion insensitivity, and ease of implementation of the fuel-cell analyzer make it particularly useful for a wide range of unattended field applications. This system has been used to measure atmospheric O2 concentrations at the WLEF tall-tower research site in northern Wisconsin semicontinuously from June 2000 to December 2003. These measurements represent the first extended O2 record in and above a forest ecosystem, and are being used to investigate global carbon budgeting, plant physiology, continental boundary layer mixing and synoptic transport, and potential means of industrial emission verification. In this paper, the measurement technique is described in detail and several weeks of data are presented to illustrate its performance.

Feedbacks from the terrestrial carbon cycle significantly affect future climate change. The CO2 concentration dependence of global terrestrial carbon storage is one of the largest and most uncertain feedbacks. Theory predicts the CO2 effect should have a tropical maximum, but a large terrestrial sink has been contradicted by analyses of atmospheric CO2 that do not show large tropical uptake. Our results, however, show significant tropical uptake and, combining tropical and extratropical fluxes, suggest that up to 60% of the present-day terrestrial sink is caused by increasing atmospheric CO2. This conclusion is consistent with a validated subset of atmospheric analyses, but uncertainty remains. Improved model diagnostics and new space-based observations can reduce the uncertainty of tropical and temperate zone carbon flux estimates. This analysis supports a significant feedback to future atmospheric CO2 concentrations from carbon uptake in terrestrial ecosystems caused by rising atmospheric CO2 concentrations. This feedback will have substantial tropical contributions, but the magnitude of future carbon uptake by tropical forests also depends on how they respond to climate change and requires their protection from deforestation.

▪ Abstract Agriculture and industrial development have led to inadvertent changes in the natural carbon cycle. As a consequence, concentrations of carbon dioxide and other greenhouse gases have increased in the atmosphere and may lead to changes in climate. The current challenge facing society is to develop options for future management of the carbon cycle. A variety of approaches has been suggested: direct reduction of emissions, deliberate manipulation of the natural carbon cycle to enhance sequestration, and capture and isolation of carbon from fossil fuel use. Policy development to date has laid out some of the general principles to which carbon management should adhere. These are summarized as: how much carbon is stored, by what means, and for how long. To successfully manage carbon for climate purposes requires increased understanding of carbon cycle dynamics and improvement in the scientific capabilities available for measurement as well as for policy needs. The specific needs for scientific information to underpin carbon cycle management decisions are not yet broadly known. A stronger dialogue between decision makers and scientists must be developed to foster improved application of scientific knowledge to decisions. This review focuses on the current knowledge of the carbon cycle, carbon measurement capabilities (with an emphasis on the continental scale) and the relevance of carbon cycle science to carbon sequestration goals.

Observations of seasonal variations in the atmospheric O-2/N-2 ratio are reported at nine baseline sites in the northern and southern hemispheres. Concurrent CO2 measurements are used to correct for the effects of land biotic exchanges of O-2 on the O-2/N-2 cycles thus allowing the residual component of the cycles due to oceanic exchanges of O-2 and N-2 to be calculated. The residual oceanic cycles in the northern hemisphere are nearly diametrically out of phase with the cycles in the southern hemisphere. The maxima in both hemispheres occur in summer. In both hemispheres, the middle-latitude sea level stations show the cycles with largest amplitudes and earliest phasing. Somewhat smaller amplitudes are observed at the high-latitude stations, and much smaller amplitudes are observed at the tropical stations. A model for simulating the oceanic component of the atmospheric O-2/N-2 cycles is presented consisting of the TM2 atmospheric tracer transport model [Heimann, 1995] driven at the lower boundary by O-2 fluxes derived from observed O-2 saturation anomalies in surface waters and by N-2 fluxes derived from the net air-sea heat flux. The model is optimized to fit the observed atmospheric O-2/N-2 cycles by adjusting the air-sea gas-exchange velocity, which relates O-2 anomaly to O-2 flux. The optimum fit corresponds to spatially and temporally averaged exchange velocities of 24+/-6 cm/hr for the oceans north of 31 degrees N and 29+/-12 cm/hr for the oceans south of 31 degrees S. These velocities agree to within the uncertainties with the gas-exchange velocities expected from the Wanninkhof [1992] formulation of the air-sea gas-exchange velocity combined with European Centre for Medium-Range Weather Forecasts winds [Gibson et al., 1997] but are larger than the exchange velocities expected from the Liss and Merlivat [1986] relation using the same winds. The results imply that the gas-exchange velocity for O-2, like that of CO2, may be enhanced in the open ocean by processes that were not systematically accounted for in the experiments used to derive the Liss and Merlivat relation. [References: 57]

We measured soil respiration during two winters in three different ecotypes of the BOREAS northern study area. The production of CO2 was continuous throughout the winter and, when totaled for the winter of 1993-1995, was equivalent to the release of similar to 40-55 g C/m(2) from the soil surface. As soils cooled in the early winter, the CO2 production rate decreased in a manner that appeared to be exponentially related to shallow soil temperatures. This exponential relationship was not observed when soils began to warm, possibly indicating that there may be additional or different processes responsible for increased CO2 production during winter warming events. We also measured CO2 concentrations in soil gas and the Delta(14)C of the soil CO2. These measurements show that the CO2 produced in winter is not simply the return to the atmosphere Of the carbon fixed during the previous growing season. We suggest that the wintertime production of CO2 originates, at least in part, from the decomposition of old organic carbon stored at depth in the soil.

Downs and Ups Every spring, the concentration of CO 2 in the atmosphere of the Northern Hemisphere decreases as terrestrial vegetation grows, and every fall, CO 2 rises as vegetation dies and rots. Climate change has destabilized the seasonal cycle of atmospheric CO 2 such that Graven et al. (p. 1085 , published online 8 August; see the Perspective by Fung ) have found that the amplitude of the seasonal cycle has exceeded 50% at some latitudes. The only way to explain this increase is if extratropical land ecosystems are growing and shrinking more than they did half a century ago, as a result of changes in the structure and composition of northern ecosystems.

Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ecosystem responses to climate change and have implications for the design of carbon monitoring programs. This paper describes changes in the three-dimensional structure of atmospheric CO2 for several representative concentration pathways (RCPs 4.5 and 8.5) using the Community Earth System Model–Biogeochemistry (CESM1-BGC). CO2 simulated for the historical period was first compared to surface, aircraft, and column observations. In a second step, the evolution of spatial and temporal gradients during the twenty-first century was examined. The mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net flux in the Community Land Model (the land component of CESM) was too weak. Consistent with weak summer drawdown in Northern Hemisphere high latitudes, simulated CO2 showed correspondingly weak north–south and vertical gradients during the summer. In the simulations of the twenty-first century, CESM predicted increases in the mean annual cycle of atmospheric CO2 and larger horizontal gradients. Not only did the mean north–south gradient increase due to fossil fuel emissions, but east–west contrasts in CO2 also strengthened because of changing patterns in fossil fuel emissions and terrestrial carbon exchange. In the RCP8.5 simulation, where CO2 increased to 1150 ppm by 2100, the CESM predicted increases in interannual variability in the Northern Hemisphere midlatitudes of up to 60% relative to present variability for time series filtered with a 2–10-yr bandpass. Such an increase in variability may impact detection of changing surface fluxes from atmospheric observations.

Amid a worldwide increase in tree mortality, mountain pine beetles (Dendroctonus ponderosae Hopkins) have led to the death of billions of trees from Mexico to Alaska since 2000. This is predicted to have important carbon, water and energy balance feedbacks on the Earth system. Counter to current projections, we show that on a decadal scale, tree mortality causes no increase in ecosystem respiration from scales of several square metres up to an 84 km(2) valley. Rather, we found comparable declines in both gross primary productivity and respiration suggesting little change in net flux, with a transitory recovery of respiration 6-7 years after mortality associated with increased incorporation of leaf litter C into soil organic matter, followed by further decline in years 8-10. The mechanism of the impact of tree mortality caused by these biotic disturbances is consistent with reduced input rather than increased output of carbon.

We have developed an instrument for making continuous, field-based, part-per-million (ppm) level measurements of atmospheric oxygen concentration, and have implemented it on research cruises in the equatorial Pacific and Southern Oceans. The instrument detects changes in oxygen by the absorption of vacuum ultraviolet (VUV) radiation as it passes through a flowing gas stream, and has a precision comparable to existing laboratory techniques. Here we describe the VUV instrument and present atmospheric O 2 and CO 2 data collected from the NOAA ship Ka'imimoana in the equatorial Pacific during April and May of 1998, and from the NSF ship Lawrence M. Gould in the Southern Ocean during October 1998. These data represent the first field-based measurements of atmospheric O 2 , and significant additions to the O 2 datasets in these regions. Our boreal-springtime equatorial measurements reveal significant short-term variations in atmospheric O 2 , resulting from variations in atmospheric mixing relative to the strong interhemispheric gradient. Our austral-springtime Southern Ocean observations confirm the low O 2 concentrations seen in flask samples from this region, allow the separate identification of oceanic and industrial influences on CO 2 , and provide evidence of a Southern Ocean source for CO 2 at this time of year. These shipboard VUV observations do not provide any evidence to support coupled ocean–atmosphere model predictions of a large decreasing atmospheric O 2 gradient between equatorial and high-southern latitudes. DOI: 10.1046/j.1435-6935.2003.00075.x

[1] High-elevation forests represent a large fraction of potential carbon uptake in North America, but this uptake is not well constrained by observations. Additionally, forests in the Rocky Mountains have recently been severely damaged by drought, fire, and insect outbreaks, which have been quantified at local scales but not assessed in terms of carbon uptake at regional scales. The Airborne Carbon in the Mountains Experiment was carried out in 2007 partly to assess carbon uptake in western U.S. mountain ecosystems. The magnitude and seasonal change of carbon uptake were quantified by (1) paired upwind-downwind airborne CO2 observations applied in a boundary layer budget, (2) a spatially explicit ecosystem model constrained using remote sensing and flux tower observations, and (3) a downscaled global tracer transport inversion. Top-down approaches had mean carbon uptake equivalent to flux tower observations at a subalpine forest, while the ecosystem model showed less. The techniques disagreed on temporal evolution. Regional carbon uptake was greatest in the early summer immediately following snowmelt and tended to lessen as the region experienced dry summer conditions. This reduction was more pronounced in the airborne budget and inversion than in flux tower or upscaling, possibly related to lower snow water availability in forests sampled by the aircraft, which were lower in elevation than the tower site. Changes in vegetative greenness associated with insect outbreaks were detected using satellite reflectance observations, but impacts on regional carbon cycling were unclear, highlighting the need to better quantify this emerging disturbance effect on montane forest carbon cycling.

[1] A multiple-box model was developed to determine how meteorological, anthropogenic, and biological processes combine to produce diel cycles of carbon dioxide (CO2) mole fraction during each of the four calendar-based seasons within Salt Lake Valley, Utah, USA. The model was forced by observed winds, sounding-derived mixing depths, an anthropogenic CO2 emissions inventory, and net biological flux estimates based on temperature, solar radiation, and ecosystem type. The model was validated using hourly CO2 mole fractions measured at five sites in the urban Salt Lake Valley (uSLV) area for 2005–2009 (spatial average of observations denoted by Cobs). The model accounted for 53% of Cobs on an hourly basis, and accounted for 90–94% of the mean diel cycle of Cobs depending on season. The multiple-box model results indicated that CO2 change rates within the uSLV mean diel cycles were largely the result of imbalances between anthropogenic processes adding CO2 and meteorological processes removing or diluting CO2. Removal by wind (advection) was the most important CO2 reduction process on average, but dilution of CO2 by entrainment of air from above the mixing height overtook advection in importance between sunrise and midday. During summer mornings, CO2 reduction attributable to photosynthesis below shallow mixing heights was of intermediate importance between advection and entrainment, but the overall net effect of biological processes was the least important influence on CO2 change rates during each of the four seasons.

New observations of the vertically integrated CO2 mixing ratio, ⟨CO2⟩, from ground-based remote sensing show that variations in CO2⟩ are primarily determined by large-scale flux patterns. They therefore provide fundamentally different information than observations made within the boundary layer, which reflect the combined influence of large-scale and local fluxes. Observations of both ⟨CO2⟩ and CO2 concentrations in the free troposphere show that large-scale spatial gradients induce synoptic-scale temporal variations in ⟨CO2⟩ in the Northern Hemisphere midlatitudes through horizontal advection. Rather than obscure the signature of surface fluxes on atmospheric CO2, these synoptic-scale variations provide useful information that can be used to reveal the meridional flux distribution. We estimate the meridional gradient in ⟨CO2⟩ from covariations in ⟨CO2⟩ and potential temperature, θ, a dynamical tracer, on synoptic timescales to evaluate surface flux estimates commonly used in carbon cycle models. We find that simulations using Carnegie Ames Stanford Approach (CASA) biospheric fluxes underestimate both the ⟨CO2⟩ seasonal cycle amplitude throughout the Northern Hemisphere midlatitudes and the meridional gradient during the growing season. Simulations using CASA net ecosystem exchange (NEE) with increased and phase-shifted boreal fluxes better fit the observations. Our simulations suggest that climatological mean CASA fluxes underestimate boreal growing season NEE (between 45–65° N) by ~40%. We describe the implications for this large seasonal exchange on inference of the net Northern Hemisphere terrestrial carbon sink.

The Total Carbon Column Observing Network (TCCON) produces precise measurements of the column average dry-air mole fractions of CO2, CO, CH4, N2O and H2O at a variety of sites worldwide. These observations rely on spectroscopic parameters that are not known with sufficient accuracy to compute total columns that can be used in combination with in situ measurements. The TCCON must therefore be calibrated to World Meteorological Organization (WMO) in situ trace gas measurement scales. We present a calibration of TCCON data using WMO-scale instrumentation aboard aircraft that measured profiles over four TCCON stations during 2008 and 2009. The aircraft campaigns are the Stratosphere-Troposphere Analyses of Regional Transport 2008 (START-08), which included a profile over the Park Falls site, the HIAPER Pole-to-Pole Observations (HIPPO-1) campaign, which included profiles over the Lamont and Lauder sites, a series of Learjet profiles over the Lamont site, and a Beechcraft King Air profile over the Tsukuba site. These calibrations are compared with similar observations made during the INTEX-NA (2004), COBRA-ME (2004) and TWP-ICE (2006) campaigns. A single, global calibration factor for each gas accurately captures the TCCON total column data within error.

Methane and nitrous oxide are potent greenhouse gases whose atmospheric abundances have increased significantly in the past 200 years, together accounting for approximately half of the radiative forcing associated with increasing concentrations of carbon dioxide. In order to understand the factors causing increase of these gases globally, we need to determine their emission rates at regional to continental scales. We directly link atmospheric observations with surface emissions using a Lagrangian Particle Dispersion Model, and then determine emission rates by optimizing prior emissions estimates. We use measurements from NOAA's tall tower and aircraft program in 2004, The Stochastic Time-Inverted Lagrangian Transport model (STILT) driven by meteorological fields from a customized version of the Weather Research and Forecasting (WRF) model, and EDGAR32FT2000 and Global Emissions Inventory Activity (GEIA) as prior emission estimates. In the US and Canada, methane emission rates are found to be consistent wi...

We present top-down emission constraints for two non-CO2 greenhouse gases in large areas of the U. S. and southern Canada during early summer. Collocated airborne measurements of methane and nitrous oxide acquired during the COBRA-NA campaign in May-June 2003, analyzed using a receptor-oriented Lagrangian particle dispersion model, provide robust validation of independent bottom-up emission estimates from the EDGAR and GEIA inventories. We find that the EDGAR CH4 emission rates are slightly low by a factor of 1.08 +/- 0.15 (2 sigma), while both EDGAR and GEIA N2O emissions are significantly too low, by factors of 2.62 +/- 0.50 and 3.05 +/- 0.61, respectively, for this region. Potential footprint bias may expand the statistically retrieved uncertainties. Seasonality of agricultural N2O emissions may help explain the discrepancy. Total anthropogenic U.S. and Canadian emissions would be 49 Tg CH4 and 4.3 Tg N2O annually, if these inventory scaling factors applied to all of North America. [References: 21]

Measurements of midday vertical atmospheric CO 2 distributions reveal annual-mean vertical CO 2 gradients that are inconsistent with atmospheric models that estimate a large transfer of terrestrial carbon from tropical to northern latitudes. The three models that most closely reproduce the observed annual-mean vertical CO 2 gradients estimate weaker northern uptake of –1.5 petagrams of carbon per year (Pg C year –1 ) and weaker tropical emission of +0.1 Pg C year –1 compared with previous consensus estimates of –2.4 and +1.8 Pg C year –1 , respectively. This suggests that northern terrestrial uptake of industrial CO 2 emissions plays a smaller role than previously thought and that, after subtracting land-use emissions, tropical ecosystems may currently be strong sinks for CO 2 .

We present a general framework for designing and analyzing Lagrangian-type aircraft observations in order to measure surface fluxes of trace gases on regional scales. Lagrangian experiments minimize uncertainties due to advection by measuring tracer concentrations upstream and downstream of the study region, assuring that observed concentration changes represent fluxes within the region. The framework includes ( 1) a receptor-oriented model of atmospheric transport, including turbulent dispersion, ( 2) an upstream tracer boundary condition, ( 3) a surface flux model that predicts the distribution of tracer fluxes in time and space, and ( 4) a Bayesian inverse analysis that combines a priori information with observations to yield optimal estimates of tracer fluxes by the flux model. We use a receptor-oriented transport model, the Stochastic Time-Inverted Lagrangian Transport ( STILT) model, to simulate ensembles of particles representing air parcels transported backward in time from an observation point (receptor), linking receptor concentrations with upstream locations and surface inputs. STILT provides the capability to forecast flight tracks for Lagrangian experiments in the presence of atmospheric shear and dispersion. STILT may be used to forecast flight tracks that sample the upstream tracer boundary condition, or to analyze the data and provide optimized parameters in the surface flux model. We present a case study of regional scale surface CO2 fluxes using data over the United States obtained in August 2000 in the CO2 Budget and Rectification Airborne (COBRA-2000) study. STILT forecasts were obtained using the National Centers for Environmental Prediction Eta model to plan the flight tracks. Results from the Bayesian inversion showed large reductions in a priori errors for estimates of daytime ecosystem uptake of CO2, but constraints on nighttime respiration fluxes were weaker, due to few observations of CO2 in the nocturnal boundary layer. Derived CO2 fluxes from the influence-following analysis differed notably from estimates using a conventional one-dimensional budget ("Boundary Layer Budget'') on a typical day, due to time-variable contributions from forests and croplands. A critical examination of uncertainties in the Lagrangian analyses revealed that the largest uncertainties were associated with errors in forecasting the upstream sampling locations and with aggregation of heterogeneous fluxes at the surface. Suggestions for improvements in future experiments are presented.

[1] We present an analysis framework and illustrate its potential to constrain terrestrial carbon fluxes at the regional scale using observations of CO2 and CO over North America acquired during the CO2 Budget and Rectification Airborne (COBRA) study in 2000. The COBRA data set, presented in detail in a companion paper [Gerbig et al., 2003] provides dense spatial coverage and extensive profiling in the lower atmosphere, revealing strong CO2 signatures of land surface fluxes in the active and relic mixed layers of the atmosphere. We introduce a “receptor-oriented” analysis framework designed to quantitatively interpret the atmospheric signatures of surface processes by linking concentrations at measurement locations (receptors) to surface fluxes in upwind regions. The framework incorporates three main components: (1) the Stochastic Time-Inverted Lagrangian Transport (STILT) model, driven with assimilated winds and running backward in time to map out the source-receptor relationship (footprint) at high temporal and spatial resolution; (2) an observation-based lateral boundary condition for CO2, resolving vertical and meridional gradients; and (3) a simple parameterization for biosphere-atmosphere fluxes that uses eddy covariance observations from the AmeriFlux network as prior estimates for fluxes. This framework allows quantitative comparison between the top-down constraint on fluxes from airborne observations of CO2 with the bottom-up constraint of eddy flux measurements in a Bayesian synthesis inversion. The model is used to investigate the observed representation error (mismatch between point measurements and grid-cell-averaged values in models), evaluated in the companion paper, showing that unresolved spatial variability of surface fluxes gives rise to most of the representation error over the continent. Thus the representation error reflects the effect of aggregation errors. Discrepancies between simulated and observed CO2 distributions are assessed to indicate where improvements are needed, including improved empirical representation of biosphere-atmosphere exchange process and better simulation of convective processes in atmospheric transport models.

[1] We analyze the spatial variability of CO2 measurements from aircraft platforms, including extensive observations acquired over North America during the CO2 Budget and Rectification Airborne (COBRA) study in 2000. The COBRA data set is unique in its dense spatial coverage and extensive profiling in the lower atmosphere. Strong signatures of CO2 fluxes at the land surface were observed in the active and relic mixed layers of the atmosphere (up to ∼20 ppm gradients). Free tropospheric CO2 exhibited significantly less variability except in areas affected by convective transport. Statistical analyses of the COBRA data indicate that CO2 mixed-layer averages can be determined from vertical profiles with an accuracy of approximately ±0.2 ppm, limited by atmospheric variance. Analysis of the associated representation error suggests that models require horizontal resolution smaller than ∼30 km to fully resolve spatial variations of atmospheric CO2 in the boundary layer over the continent. To provide a global context for these data, we analyzed the GLOBALVIEW marine boundary layer (MBL) reference CO2. Comparison of the MBL reference with extensive aircraft data extending over 20 years, covering the whole troposphere over the northern Pacific, shows significant seasonal biases of up to 2 ppm in the free troposphere, indicating that the MBL reference is a suitable boundary condition only for some applications. The spatial variability of CO2 revealed by the COBRA-2000 calls for a suitable analysis framework to derive regional and continental fluxes, presented in a companion paper. The problem requires boundary conditions constrained by both surface and upper tropospheric observations and constraints on terrestrial fluxes that exploit the information content of the highly variable CO2 distribution over land.

Ice-core measurements indicate that atmospheric CO2 concentrations during glacial periods were consistently about 80 parts per million lower than during interglacial periods. Previous explanations for this observation have typically had difficulty accounting for either the estimated glacial O2 concentrations in the deep sea, 13C/12C ratios in Antarctic surface waters, or the depth of calcite saturation; also lacking is an explanation for the strong link between atmospheric CO2 and Antarctic air temperature. There is growing evidence that the amount of deep water upwelling at low latitudes is significantly overestimated in most ocean general circulation models and simpler box models previously used to investigate this problem. Here we use a box model with deep-water upwelling confined to south of 55 degrees S to investigate the glacial-interglacial linkages between Antarctic air temperature and atmospheric CO2 variations. We suggest that low glacial atmospheric CO2 levels might result from reduced deep-water ventilation associated with either year-round Antarctic sea-ice coverage, or wintertime coverage combined with ice-induced stratification during the summer. The model presented here reproduces 67 parts per million of the observed glacial-interglacial CO2 difference, as a result of reduced air-sea gas exchange in the Antarctic region, and is generally consistent with the additional observational constraints.

We present a method for testing the performance of global ocean carbon cycle models using measurements of atmospheric O-2 and CO2 concentration. We combine these measurements to define a tracer, atmospheric potential oxygen (APO approximate to O-2 + CO2), which is conservative with respect to terrestrial photosynthesis and respiration. We then compare observations of APO to the simulations of an atmospheric transport model which uses ocean-model air-sea fluxes and fossil fuel combustion estimates as lower boundary conditions. We present observations of the annual-average concentrations of CO2, O-2, and APO at 10 stations in a north-south transect. The observations of APO show a significant interhemispheric gradient decreasing towards the north. We use air-sea CO2, O-2, and N-2 fluxes from the Princeton ocean biogeochemistry model, the Hamburg model of the ocean carbon cycle, and the Lawrence Livermore ocean biogeochemistry model to drive the TM2 atmospheric transport model. The latitudinal variations in annual-average APO predicted by the combined models are distinctly different from the observations. All three models significantly underestimate the interhemispheric difference in APO, suggesting that they underestimate the net southward transport of the sum of O-2 and CO2 in the oceans. Uncertainties in the model-observation comparisons include uncertainties associated with the atmospheric measurements, the atmospheric transport model, and the physical and biological components of the ocean models. Potential deficiencies in the physical components of the ocean models, which have previously been suggested as causes for anomalously large heat fluxes out of the Southern Ocean, may contribute to the discrepancies with the APO observations. These deficiencies include the inadequate parameterization of subgrid-scale isopycnal eddy mixing, a lack of subgrid-scale vertical convection, too much Antarctic sea-ice formation, and an overestimation of vertical diffusivities in the main thermocline. [References: 55]