Continuous CO2 Measurements Research Articles

Measurements of carbon dioxide in an Oregon metropolitan region

Ambient concentrations of atmospheric carbon dioxide (CO2) are reported for the Portland, Oregon (USA) metropolitan region for the late July through December, 2009 period. Three stationary locations were established: a downtown location on the campus of Portland State University; a residential site in southeast Portland; and a rural station on Sauvie Island, located ∼30 km northwest of Portland in the Columbia River Gorge. Continuous measurements of CO2 at each site average 403–408 ppm and show considerable variability at each site (360–610 ppm) due to CO2 sources, sinks and meteorological variability. Within this variability, a marked 20–30 ppm diurnal cycle is observed due to photosynthetic activity and variations in the planetary boundary layer. In-city CO2 concentrations are on average enhanced by 5–6 ppm over the Sauvie Island site during upgorge wind conditions, a difference which is greatest in the afternoon. Measurements of the 13C/12C ratio of CO2 in downtown Portland are significantly depleted in 13C relative to 12C compared with background air and suggest that regional CO2 is dominated by petroleum sources (75–80%). High degrees of relationship between CO2 variability and primary air pollutants CO and NO (r2 = 0.80 and 0.77), measured by the Oregon Department of Environmental Quality at the Southeast Portland location, corroborate this finding and illustrate the importance of traffic emissions on elevated ambient CO2 concentrations.

In situmeasurements confirm the seasonal dominance of benthic algae over phytoplankton in nearshore primary production of a large lake

Summary1. Despite the recognition of its importance, benthic primary production is seldom reported, especially for large lakes. We measuredin situbenthic net primary production by monitoring flux in dissolved inorganic carbon (DIC) concentration in benthic incubation chambers, based on continuous measurements of CO2(aq)flux, alkalinity, and the temperature‐dependent dissociation constants of carbonic acid (K1andK2). This methodology has the advantages of monitoring net primary production directly as change in carbon, maintaining continuous water recirculation, and having sufficient precision to detect change in DIC over short (i.e. 15 min) incubations, even in alkaline waters.2. Benthic primary production onCladophora‐dominated rocky substrata in western Lake Ontario was measured biweekly. Maximum biomass‐specific net photosynthetic rates were highest in the spring (2.39 mgC g Dry Mass−1 h−1), decreased to negative rates by early summer (−0.76 mgC g DM−1 h−1), and exhibited a regrowth in late summer (1.98 mgC g DM−1 h−1).3. ACladophoragrowth model (CGM), previously validated to predictCladophorabiomass accrual in Lake Ontario, successfully simulated the seasonality and magnitude of biomass‐specific primary production during the first cohort ofCladophoragrowth. Averaged over this growing season (May–Aug), mean areal net benthic production at the estimated depth of peak biomass (2 m) was 405 mg C m−2 d−1.4. We measured planktonic primary production in proximity to the benthic study and constructed a depth‐resolved model of planktonic production. Using the CGM, benthic primary production was compared with planktonic primary production for the period May–Aug. Net benthic production from the shoreline to the 12 m contour (1–2 km offshore) equalled planktonic production. Closer to shore, benthic primary production exceeded planktonic primary production. Failure to account for benthic primary production, at least during abundantCladophoragrowth, will lead to large underestimates in carbon and nutrient flows in the nearshore zone of this Great Lake.

Regional-scale geostatistical inverse modeling of North American CO<sub>2</sub> fluxes: a synthetic data study

Abstract. A series of synthetic data experiments is performed to investigate the ability of a regional atmospheric inversion to estimate grid-scale CO2 fluxes during the growing season over North America. The inversions are performed within a geostatistical framework without the use of any prior flux estimates or auxiliary variables, in order to focus on the atmospheric constraint provided by the nine towers collecting continuous, calibrated CO2 measurements in 2004. Using synthetic measurements and their associated concentration footprints, flux and model-data mismatch covariance parameters are first optimized, and then fluxes and their uncertainties are estimated at three different temporal resolutions. These temporal resolutions, which include a four-day average, a four-day-average diurnal cycle with 3-hourly increments, and 3-hourly fluxes, are chosen to help assess the impact of temporal aggregation errors on the estimated fluxes and covariance parameters. Estimating fluxes at a temporal resolution that can adjust the diurnal variability is found to be critical both for recovering covariance parameters directly from the atmospheric data, and for inferring accurate ecoregion-scale fluxes. Accounting for both spatial and temporal a priori covariance in the flux distribution is also found to be necessary for recovering accurate a posteriori uncertainty bounds on the estimated fluxes. Overall, the results suggest that even a fairly sparse network of 9 towers collecting continuous CO2 measurements across the continent, used with no auxiliary information or prior estimates of the flux distribution in time or space, can be used to infer relatively accurate monthly ecoregion scale CO2 surface fluxes over North America within estimated uncertainty bounds. Simulated random transport error is shown to decrease the quality of flux estimates in under-constrained areas at the ecoregion scale, although the uncertainty bounds remain realistic. While these synthetic data inversions do not consider all potential issues associated with using actual measurement data, e.g. systematic transport errors or problems with the boundary conditions, they help to highlight the impact of inversion setup choices, and help to provide a baseline set of CO2 fluxes for comparison with estimates from future real-data inversions.

Permanent shallow subsoil CO 2 flux chambers for monitoring of onshore CO 2 geological storage sites

Public concern over the possibility of migration of stored CO 2 to the surface with resulting damage to vegetation or hazard to humans and animals is a matter which will need to be addressed to be able to satisfy likely regulatory requirements for onshore CO 2 storage in a number of jurisdictions. While soil CO 2 concentration is readily measured continuously and in situ with current technology, the measurement of CO 2 flux at depths below the soil A horizon may be a more sensitive and meaningful technique for early detection of a near surface CO 2 plume. We describe a system for the continuous measurement of soil CO 2 flux at a depth of approximately 1.3 m and present results from three instruments deployed at the Otway Basin Pilot Project in Victoria, Australia and one development system deployed at Sutton, near the Australian Capital Canberra.

Coupling of carbon dioxide and water vapor exchanges of irrigated and rainfed maize–soybean cropping systems and water productivity

Continuous measurements of CO 2 and water vapor exchanges made in three cropping systems (irrigated continuous maize, irrigated maize–soybean rotation, and rainfed maize–soybean rotation) in eastern Nebraska, USA during 6 years are discussed. Close coupling between seasonal distributions of gross primary production (GPP) and evapotranspiration (ET) were observed in each growing season. Mean growing season totals of GPP in irrigated maize and soybean were 1738 ± 114 and 996 ± 69 g C m −2, respectively (±standard deviation). Corresponding mean values of growing season ET totals were 545 ± 27 and 454 ± 23 mm, respectively. Irrigation affected GPP and ET similarly, both growing season totals were about 10% higher than those of corresponding rainfed crops. Maize, under both irrigated and rainfed conditions, fixed 74% more carbon than soybean while using only 12–20% more water. The green leaf area index (LAI) explained substantial portions (91% for maize and 90% for soybean) of the variability in GPP PAR (GPP over a narrow range of incident photosynthetically active radiation) and in ET/ET o (71% for maize and 75% for soybean, ET o is the reference evapotranspiration). Water productivity (WP or water use efficiency) is defined here as the ratio of cumulative GPP or above-ground biomass and ET (photosynthetic water productivity = ∑GPP/∑ET and biomass water productivity = above-ground biomass/∑ET). When normalized by ET o, the photosynthetic water productivity ( W P E T o ) was 18.4 ± 1.5 g C m −2 for maize and 12.0 ± 1.0 g C m −2 for soybean. When normalized by ET o, the biomass water productivity ( W P E T o ) was 27.5 ± 2.3 g DM m −2 for maize and 14.1 ± 3.1 g DM m −2 for soybean. Comparisons of these results, among different years of measurement and management practices (continuous vs rotation cropping, irrigated vs rainfed) in this study and those from other locations, indicated the conservative nature of normalized water productivity, as also pointed out by previous investigators.

Source-receptor relationships for airborne measurements of CO<sub>2</sub>, CO and O<sub>3</sub> above Siberia: a cluster-based approach

Abstract. We analysed results of three intensive aircraft campaigns above Siberia (April and September 2006, August 2007) with a total of ~70 h of continuous CO2, CO and O3 measurements. The flight route consists of consecutive ascents and descents between Novosibirsk (55° N, 82° E) and Yakutsk (62° N, 129° E). We performed retroplume calculations with the Lagrangian particle dispersion model FLEXPART for many short segments along the flight tracks. To reduce the extremely rich information on source regions provided by the model calculation into a small number of distinct cases, we performed a statistical clustering – to our knowledge for the first time – into potential source regions of the footprint emission sensitivities obtained from the model calculations. This technique not only worked well to separate source region influences but also resulted in clearly distinct tracer concentrations for the various clusters obtained. High CO and O3 concentrations were found associated with agricultural fire plumes originating from Kazakhstan in September 2006. A statistical analysis indicates that summer uptake of CO2 is largely explained (~50% of variance) by air mass exposure to uptake by Siberian and sub-arctic ecosystems. This resulted in an average 5 to 10 ppm difference with overlaying air masses. Stratosphere-troposphere exchange is found to strongly influence the observed O3 mixing ratios in spring, but not in summer. European emissions contributed to high O3 concentrations above Siberia in April 2006 and August 2007, while emissions from North Eastern China also contributed to higher O3 mixing ratios in summer, but tend to lower mixing ratios in spring, when the airmass aerosol burden is important. In the lower troposphere, large-scale deposition processes in the boreal and sub-arctic boundary layer is a large O3 sink, resulting in a ~20 ppb difference with overlaying air masses. Lagrangian footprint clustering is very promising and could also be advantageously applied to the interpretation of ground based measurements including calculation of tracers' sources and sinks.

The Association of High-Magnitude Cerebral Passivity and Intraventricular Hemorrhage in Premature Infants

The Association of High-Magnitude Cerebral Passivity and Intraventricular Hemorrhage in Premature Infants

Uncertainty analysis of CO2 flux components in subtropical evergreen coniferous plantation

We present an uncertainty analysis of ecological process parameters and CO 2 flux components (R eco , NEE and gross ecosystem exchange (GEE)) derived from 3 years' continuous eddy covariance measurements of CO 2 fluxes at subtropical evergreen coniferous plantation, Qianyanzhou of ChinaFlux. Daily-differencing approach was used to analyze the random error of CO 2 fluxes measurements and bootstrapping method was used to quantify the uncertainties of three CO 2 flux components. In addition, we evaluated different models and optimization methods in influencing estimation of key parameters and CO 2 flux components. The results show that: (1) Random flux error more closely follows a double-exponential (Laplace), rather than a normal (Gaussian) distribution. (2) Different optimization methods result in different estimates of model parameters. Uncertainties of parameters estimated by the maximum likelihood estimation (MLE) are lower than those derived from ordinary least square method (OLS). (3) The differences between simulated R eco , NEE and GEE derived from MLE and those derived from OLS are 12.18% (176 g C &middot m -2 &middot a -1 ), 34.33% (79 g C &middot m -2 &middot a -1 ) and 5.4% (92 g C &middot m -2 &middot a -1 ). However, for a given parameter optimization method, a temperature-dependent model (T_model) and the models derived from a temperature and water-dependent model (TW_model) are 1.31% (17.8 g C &middot m -2 &middot a -1 ), 2.1% (5.7 g C &middot m -2 &middot a -1 ), and 0.26% (4.3 g C &middot m -2 &middot a -1 ), respectively, which suggested that the optimization methods are more important than the ecological models in influencing uncertainty in estimated carbon fluxes. (4) The relative uncertainty of CO 2 flux derived from OLS is higher than that from MLE, and the uncertainty is related to timescale, that is, the larger the timescale, the smaller the uncertainty. The relative uncertainties of R eco , NEE and GEE are 4%-8%, 7%-22% and 2%-4% respectively at annual timescale. © Science in China Press and Springer-Verlag GmbH 2009.

50 Years of Continuous Measurement of CO2 on Mauna Loa

50 Years of Continuous Measurement of CO₂ on Mauna Loa

Continuous measurement and modeling of CO2 losses from a peatland stream during stormflow events

While streams draining peatland and wetland systems are known to be supersaturated in CO2 with respect to the atmosphere, relatively little is known about short‐term temporal variability in response to extreme hydrological events. Here, we use submerged, nondispersive infrared (NDIR) sensors to make continuous measurements of CO2 concentrations during 18 storm events in a Scottish peatland stream. Individual storms exhibited 3 distinct types of hysteresis loop. We suggest that differences in loop form may be due to differences in the relative contributions of soil water or differences in the contributing catchment source area. We found a negative concentration‐discharge relationship over the full study period, suggesting that CO2‐rich deep peat/groundwater was the major source of aquatic CO2 under low flow conditions. By removing the effect of dilution and estimating additions and losses of CO2, we also show the importance of both surface peat CO2 inputs into the stream and evasion loss during stormflow. The best model of temporal variability in CO2 was achieved by separating the data set into “stormflow” and “dry periods.” Downstream CO2 export during the study period was dominated by stormflow events (71%), highlighting the importance of accurately accounting for high‐flow CO2 sources.

First continuous measurements of CO 2 mixing ratio in central London using a compact diffusion probe

We present one year of data from the first continuous measurements of carbon dioxide mixing ratio in central London. Measurements were made at an 87 m tower site using the new Vaisala CARBOCAP GMP343 instrument, which was found to provide a compact and inexpensive method for mixing ratio monitoring, in an environment where conventional CO 2 sensors could not be accommodated. Measurements were compared with a monitoring site outside London, showing that the city's CO 2 “dome” was an order of magnitude smaller than measured at lower levels in other cities. During the night time in the summer, the mixing ratio in central London was found to be significantly lower than at the rural site. This was thought to be explained by the proximity of biogenic sources to the rural sensor, differences between urban and rural mixing heights and/or the interception of a vertical mixing ratio gradient at two different measurement heights. Estimation of the CO 2 loading of the air entering the city therefore proved problematic during the summer. As a result, we propose that monitoring of CO 2 emission rate using this type of measurement may only be possible in the winter when the influence of these factors is minimal.

Slagging Characteristics during Residential Combustion of Biomass Pellets

Limited availability of sawdust and planer shavings and an increasing demand for biomass pellets in Europe are pushing the market toward other, more problematic raw materials with broader variation in total fuel ash content and composition of the ash forming elements as well as in their slagging tendencies. The main objective in the present work was therefore to determine the influence of fuel−ash composition on residual ash and slag behavior. Twelve different biomass pellets were used: reed canary grass (two different samples), hemp (two different samples), wheat straw, salix, logging residues (two different samples), stem wood (sawdust) as well as spruce, pine, and birch bark. The different pellet qualities were combusted in a commercial under fed pellet burner (20 kW) installed in a reference boiler. Continuous measurements of O2, CO, CO2, HCl, SO2, and total particle matter mass concentrations were determined in the exhaust gas directly after the boiler. The collected slag deposits, the corresponding ...

Atmospheric monitoring and verification technologies for CO 2 geosequestration

The paper describes various techniques for measuring emissions to the atmosphere from geologically stored carbon dioxide, from point, line and area sources at scales of metres to several kilometres. Flux chambers are suitable for measuring small leakage rates from sources at known locations but many samples are required because of large spatial heterogeneity in the fluxes. Micrometeorological eddy covariance, relaxed eddy accumulation and flux-gradient techniques are suitable for measuring leakage from large area sources, while integrated horizontal mass balance, tracer methods and plume dispersion approaches are applicable for line and point sources. Distinguishing between leakage signals and natural fluctuations in CO 2 concentrations due to biogenic sources pose significant challenges and the use of naturally occurring tracers such as CO 2 isotopologues or introduced tracers such as SF 6 added to the sequestered CO 2 will assist with this problem. Forward Lagrangian dispersion calculations showed that CO 2 concentrations 0–80 m downwind of a point source would be readily detectable above all natural variations for point sources >0.3 g CO 2 s −1 (about 10 tonnes of CO 2 per year). The inverse problem involves solving for the unknown emission rate from measured wind fields and down wind concentration perturbations. An optimum monitoring strategy for inverse analysis will require continuous measurements of CO 2 and tracer compounds upwind and downwind of the possible leak location, coupled with transport modelling to determine leakage fluxes, and to differentiate them from other sources. Computations using The air pollution model (TAPM) showed that expected perturbations in CO 2 concentrations at distances of several hundred metres from a leak of 32 g CO 2 s −1 (about 1000 tonnes CO 2 per year, or about 0.01% per year of a typical amount to be stored) will be detectable, but this anomaly will be very small compared to natural variations, thereby complicating the inverse analysis. While the techniques canvassed here have proven successful for measuring fluxes in other applications, none has yet been demonstrated for geosequestration. The next step is to test them in the field.

A simulation of the importance of length of growing season and canopy functional properties on the seasonal gross primary production of temperate alpine meadows.

Along snowmelt gradients, the canopies of temperate alpine meadows differ strongly in their structural and biochemical properties. Here, a study is made of the effects of these canopy dissimilarities combined with the snow-induced changes in length of growing season on seasonal gross primary production (GPP). Leaf area index (LAI) and community-aggregated values of leaf angle and leaf nitrogen content were estimated for seven alpine plant canopies distributed along a marked snowmelt gradient, and these were used as input variables in a sun-shade canopy bulk-photosynthesis model. The model was validated for plant communities of early and late snowmelt sites by measuring the instantaneous CO(2) fluxes with a canopy closed-chamber technique. A sensitivity analysis was conducted to estimate the relative impact of canopy properties and environmental factors on the daily and seasonal GPP. Carbon uptake was primarily related to the LAI and total canopy nitrogen content, but not to the leaf angle. For a given level of photosynthetically active radiation, CO(2) assimilation was higher under overcast conditions. Sensitivity analysis revealed that increase of the length of the growing season had a higher effect on the seasonal GPP than a similar increase of any other factor. It was also found that the observed greater nitrogen content and larger LAI of canopies in late-snowmelt sites largely compensated for the negative impact of the reduced growing season. The results emphasize the primary importance of snow-induced changes in length of growing season on carbon uptake in alpine temperate meadows. It was also demonstrated how using leaf-trait values of the dominants is a useful approach for modelling ecosystem carbon-cycle-related processes, particularly when continuous measurements of CO(2) fluxes are technically difficult. The study thus represents an important step in addressing the challenge of using a plant functional-trait approach for biogeochemical modelling.

Comparison of Continuous, Stat, and Intermittent Bolus Cardiac Output Measurements during Cardiac Surgery

(46.2%) for CCI and 0.09 ± 0.60 L/min/m 2 (44.0%) for SCI, compared with ICI measurements. Conclusions: We found that the agreement of CCI, SCI, and ICI was poor. This disagreement increased during periods of open thorax and open pericardium when compared to the period of closed thorax. Continuous CO measurements through pulmonary artery catheter should be interpreted with caution during OPCAB. (Korean J Anesthesiol 2008; 54: 603∼8)

Long-term measurements of CO, NO, NO 2, benzene, toluene and PM 10 at a motorway location in an Austrian valley

Continuous measurements of CO, NO, NO 2, PM 10, benzene and toluene were conducted over a one-year period at a motorway location in the alpine Inn valley. Diurnal changes in concentrations of detected compounds were observed to vary according to traffic type and frequency, and meteorological conditions of the valley atmosphere, which were both additionally recorded at this location. Seasonal variations were also clearly seen, with lowest levels of compound abundances present in summer and highest levels in winter. Annual mean abundances of compounds were assessed with respect to guideline limits and it was found that NO 2 and PM 10 exceeded their legislation thresholds. This investigation also included the first continuous year-long measurements of benzene at this location, yielding an annual mean volume mixing ratio of 0.6 ppbV, which is well within the EU guideline limit of 1.5 ppbV. Additionally, benzene abundances were found to correlate well with CO concentrations. A significant correlation was also found between toluene and benzene. However, the ratio toluene:benzene shifted from about 2:1 in summer to approximately 1:1 in winter. This may be explained by a strong evaporative and therefore temperature dependent toluene source, while benzene concentrations mainly result from non-ambient temperature dependent exhaust pipe emissions. This seasonal variation must be taken into account whenever this ratio is used to determine the chemical age of an air mass.

Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand

Continuous half-hourly measurements of soil CO 2 efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration ( R s) on soil temperature ( T s) and water content ( θ). Daily mean R s varied from a minimum of 0.1 μmol m −2 s −1 in February to a maximum of 9.2 μmol m −2 s −1 in mid-July. Daily mean T s at the 2-cm depth was the primary variable accounting for the temporal variation of R s and no differences between Arrhenius and Q 10 response functions were found to describe the seasonal relationship. R s at 10 °C ( R s10) and the temperature sensitivity of R s ( Q 10Rs) calculated at the seasonal time scale were 3.8 μmol m −2 s −1 and 3.8, respectively. Temperature normalization of daily mean R s ( R sN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of R s during the growing season. Daily R sN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer ( θ fc, ∼0.30 m 3 m −3): (1) R sN was strongly reduced when θ decreased below θ fc, which reflected a reduction in microbial decomposition, and (2) R sN slightly decreased when θ increased above θ fc, which reflected a restriction of CO 2 or O 2 transport in the soil profile. Diurnal variations of half-hourly R s were usually out of phase with T s at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime R s10 and Q 10Rs parameters calculated from half-hourly nighttime measurements of R s and T s at the 2-cm depth (when there was steady cooling of the soil) varied greatly during the growing season and ranged from 6.8 to 1.6 μmol m −2 s −1 and 5.5 to 1.3, respectively. On average, daily nighttime R s10 (4.5 μmol m −2 s −1) and Q 10Rs (2.8) were higher and lower, respectively, than the values obtained from the seasonal relationship. Seasonal variations of these daily parameters were highly correlated with variations of θ in the 0–15 cm soil layer, with a tendency of low R s10 and Q 10Rs values at low θ. Overall, the use of seasonal R s10 and Q 10Rs parameters led to an overestimation of daily ranges of half-hourly R s (Δ R s) during drought conditions, which supported findings that the short-term temperature sensitivity of R s was lower during periods of low θ. The use of daily nighttime R s10 and Q 10Rs parameters greatly helped at simulating Δ R s during these periods but did not improve the estimation of half-hourly R s throughout the year as it could not account for the diurnal hysteresis effect.

Identification and quantification of plankton bloom events in the Baltic Sea by continuous pCO 2 and chlorophyll a measurements on a cargo ship

Continuous measurements of the surface water CO 2 partial pressure ( pCO 2) and the chlorophyll a fluorescence were performed in the Baltic Sea using a fully automated measurement system deployed on a cargo ship. The ship commuted regularly at two day intervals between the Mecklenburg Bight (Luebeck) and the Gulf of Finland (Helsinki). The pCO 2 data collected during June 2003 and September 2004 were used to identify biological production events such as the spring bloom and the midsummer cyanobacteria bloom in five different sub-regions. To quantify the net biomass production, the decrease of the total CO 2, NC T (normalized to a uniform alkalinity), during the production periods was calculated using the pCO 2, temperature and salinity records and the mean alkalinity. Taking into account the CO 2 air/sea exchange and the formation of dissolved organic carbon, a simple mass balance yielded the net production of particulate organic carbon which represents the total biomass. The chlorophyll a concentrations obtained from the fluorescence data showed peaks that in most cases coincided with the production maxima and thus supported the interpretation of the pCO 2 data. The production during both the spring bloom (2004) and the midsummer nitrogen fixation period (2003) increased by a factor of about three from the southwest to the northeast. For the spring bloom our estimates were significantly higher than those based on the winter nutrient supply and Redfield C / N and C / P ratios. This indicated the existence of additional nutrient sources such as dissolved organic nitrogen, early nitrogen fixation and preferential P mineralization. Midsummer NC T minima were observed only in 2003 and used to quantify the nitrogen fixation activity and to characterize its interannual variability.

Forest floor vegetation plays an important role in photosynthetic production of boreal forests

We estimated gross photosynthetic production (GPP) of the forest floor vegetation in a 40-year-old Scots pine stand in southern Finland with three different methods: measurements of CO 2 exchange of single leaves of field and ground layer species, measurement campaigns of forest floor net CO 2 efflux at different irradiances with a manually operated soil chamber, and continuous measurements of forest floor net CO 2 efflux with an automatic transparent chamber system. We upscaled the measured light response curves from the manual soil chambers using the biomass distribution of the forest floor species, a modelled seasonal pattern of photosynthetic capacity and a model of light extinction down the canopy. Leaf gas exchange measurements as well as measurements of net CO 2 efflux with the manual chamber indicated saturation of photosynthesis at relatively low (50–400 μmol m −2 s −1) light levels. Leaf and patch level measurements gave similar rates of photosynthetic CO 2 fixation per unit leaf biomass suggesting that reduction in photosynthetic production due to within-patch shading was small. Upscaling of photosynthetic production to the stand level and continuous measurements with the automatic soil chambers indicated that momentary photosynthetic production by the forest floor vegetation in the summer was typically about 2 μmol m −2 (ground) s −1. Cumulative upscaled GPP over the period of no snow (from 20 April to 20 November) in year 2003 was 131 g C m −2. Continuous measurements with the automatic soil chamber system were in line with the upscaling, the cumulative GPP being 83 g C m −2 and the seasonal pattern of photosynthetic rate similar to that of the upscaled photosynthesis.

Relationship between soil CO 2 concentrations and forest-floor CO 2 effluxes

To better understand the biotic and abiotic factors that control soil CO 2 efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO 2 effluxes and soil CO 2 concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island. We used small solid-state infrared CO 2 sensors for long-term continuous real-time measurement of CO 2 concentrations at different depths, and measured half-hourly soil CO 2 effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO 2 diffusivity in undisturbed soil cores. The method accounts for the CO 2 production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity by a power law function, which was independent of soil depth. CO 2 concentration at all depths increased with increase in soil temperature, likely due to a rise in CO 2 production, and with increase in soil water content due to decreased diffusivity or increased CO 2 production or both. It also increased with soil depth reaching almost 10 mmol mol −1 at the 50-cm depth. Annually, soil CO 2 efflux was best described by an exponential function of soil temperature at the 5-cm depth, with the reference efflux at 10 °C ( F 10) of 2.6 μmol m −2 s −1 and the Q 10 of 3.7. No evidence of displacement of CO 2-rich soil air with rain was observed. Effluxes calculated from soil CO 2 concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO 2 efflux originated in the top 20 cm soil. Calculated CO 2 production varied with soil temperature, soil water content and season, and when scaled to 10 °C also showed some diurnal variation. Soil CO 2 efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO 2 storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO 2 efflux was proportional to CO 2 concentration at the 50-cm depth with the slope determined by soil water content, which was consistent with a simple steady-state analytical model of diffusive transport of CO 2 in the soil. The latter proved successful in calculating effluxes during 2004.