Interpreting CO<Subscript>2</Subscript> Fluxes Over a Suburban Lawn: The Influence of Traffic Emissions

Turf-grass lawns are ubiquitous in the United States. However direct measurements of land–atmosphere fluxes using the eddy-covariance method above lawn ecosystems are challenging due to the typically small dimensions of lawns and the heterogeneity of land use in an urbanised landscape. Given their typically small patch sizes, there is the potential that CO₂ fluxes measured above turf-grass lawns may be influenced by nearby CO₂ sources such as passing traffic. In this study, we report on two years of eddy-covariance flux measurements above a 1.5 ha turf-grass lawn in which we assess the contribution of nearby traffic emissions to the measured CO₂ flux. We use winter data when the vegetation was dormant to develop an empirical estimate of the traffic effect on the measured CO₂ fluxes, based on a parametrised version of a three-dimensional Lagrangian footprint model and continuous traffic count data. The CO₂ budget of the ecosystem was adjusted by 135gCm⁻² in 2007 and by 134gCm⁻² in 2008 to determine the natural flux, even though the road crossed the footprint only at its far edge. We show that bottom-up flux estimates based on CO₂ emission factors of the passing vehicles, combined with the crosswind-integrated footprint at the distance of the road, agreed very well with the empirical estimate of the traffic contribution that we derived from the eddy-covariance measurements. The approach we developed may be useful for other sites where investigators plan to make eddy-covariance measurements on small patches within heterogeneous landscapes where there are significant contrasts in flux rates. However, we caution that the modelling approach is empirical and will need to be adapted individually to each site.     

Consequences of Uncertainties in CO<Subscript>2</Subscript> Density for Estimating Net Ecosystem CO<Subscript>2</Subscript> Exchange by Open-path Eddy Covariance

Errors in the estimation of CO₂ surface exchange by open-path eddy covariance, introduced during the removal of density terms [Webb et al. Quart J Roy Meteorol Soc 106:85–100, (1980) - WPL], can happen both because of errors in energy fluxes [Liu et al. Boundary-Layer Meteorol 120:65–85, (2006)] but also because of inaccuracies in other terms included in the density corrections, most notably due to measurements of absolute CO₂ density (ρ _c ). Equations are derived to examine the propagation of all errors through the WPL algorithm. For an open-path eddy covariance system operating in the Sierra de Gádor in south-east Spain, examples are presented of the inability of an unattended, open-path infrared gas analyzer (IRGA) to reliably report ρ _c and the need for additional instrumentation to determine calibration corrections. A sensitivity analysis shows that relatively large and systematic errors in net ecosystem exchange (NEE) can result from uncertainties in ρ _c in a semi-arid climate with large sensible heat fluxes (H _s ) and (wet) mineral deposition. When ρ_c is underestimated by 5% due to lens contamination, this implies a 13% overestimation of monthly CO₂ uptake.     

Future Expansion Of Agriculture and Pasture Acts to Amplify Atmospheric CO<Subscript>2</Subscript> Levels in Response to Fossil-Fuel and Land-Use Change Emissions

The expansion of crop and pastures to the detriment of forests results in an increase in atmospheric CO₂. The first obvious cause is the loss of forest biomass and soil carbon during and after conversion. The second, generally ignored cause, is the reduction of the residence time of carbon when, for example, forests or grasslands are converted to cultivated land. This decreases the sink capacity of the global terrestrial biosphere, and thereby may amplify the atmospheric CO₂ rise due to fossil and land-use carbon release. For the IPCC A2 future scenario, characterized by high fossil and high land-use emissions, we show that the land-use amplifier effect adds 61 ppm extra CO₂ in the atmosphere by 2100 as compared to former treatment of land-use processes in carbon models. Investigating the individual contribution of each of the six land-use transitions (forest &#x02194; crop, forest &#x02194; pasture, grassland crop) to the amplifier effect indicates that the clearing of forest and grasslands to arable lands explains most of the CO₂ amplification. The amplification effect is 50% higher than in a previous analysis by the same authors which considered neither the deforestation of pastures nor the ploughing of grasslands. Such an amplification effect is further examined in sensitivity tests where the net primary productivity is considered independent of the atmospheric CO₂. We also show that the land-use changes, which have already occurred in the recent past, have a strong inertia at releasing CO₂, and will contribute to about 1/3 of the amplification effect by 2100. These results suggest that there is an additional atmospheric benefit of preserving pristine ecosystems with high turnover times.     

Choosing a Stabilization Target for CO<Subscript>2</Subscript>

The choice of stabilization target for CO₂ concentration depends on the following: what is considered to be dangerous anthropogenic interference with the climate system; the forcings that might arise from non-CO₂ gases; and the climate sensitivity. These three factors are specified here probabilistically, as probability density functions (pdfs), and combined to produce a pdf for the CO₂ concentration target. There is a probability of 17% that the stabilization target should be less than the present level, and the median target is 536 ppm. The effects of reducing the emissions of non-CO₂ gases and/or implementing adaptation strategies are considered probabilistically and shown to alter these figures significantly.     

The millennial atmospheric lifetime of anthropogenic CO<Subscript>2</Subscript>

The notion is pervasive in the climate science community and in the public at large that the climate impacts of fossil fuel CO₂ release will only persist for a few centuries. This conclusion has no basis in theory or models of the atmosphere/ocean carbon cycle, which we review here. The largest fraction of the CO₂ recovery will take place on time scales of centuries, as CO₂ invades the ocean, but a significant fraction of the fossil fuel CO₂, ranging in published models in the literature from 20–60%, remains airborne for a thousand years or longer. Ultimate recovery takes place on time scales of hundreds of thousands of years, a geologic longevity typically associated in public perceptions with nuclear waste. The glacial/interglacial climate cycles demonstrate that ice sheets and sea level respond dramatically to millennial-timescale changes in climate forcing. There are also potential positive feedbacks in the carbon cycle, including methane hydrates in the ocean, and peat frozen in permafrost, that are most sensitive to the long tail of the fossil fuel CO₂ in the atmosphere.     

Spectra of CO<Subscript>2</Subscript> and Water Vapour in the Marine Atmospheric Surface Layer

Spectra of CO₂ and water vapour fluctuations from measurements made in the marine atmospheric surface layer have been analyzed. A normalization of spectra based on Monin–Obukhov similarity theory, originally developed for wind speed and temperature, has been successfully extended also to CO₂ and humidity spectra. The normalized CO₂ spectra were observed to have somewhat larger contributions from low frequencies compared to humidity spectra during unstable stratification. However, overall, the CO₂ and humidity spectra showed good agreement as did the cospectra of vertical velocity with water vapour and CO₂ respectively. During stable stratification the spectra and cospectra displayed a well-defined spectral gap separating the mesoscale and small-scale turbulent fluctuations. Two-dimensional turbulence was suggested as a possible source for the mesoscale fluctuations, which in combination with wave activity in the vertical wind is likely to explain the increase in the cospectral energy for the corresponding frequency range. Prior to the analysis the turbulence time series of the density measurements were converted to time series of mixing ratios relative to dry air. Some differences were observed when the spectra based on the original density measurements were compared to the spectra based on the mixing ratio time series. It is thus recommended to always convert the density time series to mixing ratio before performing spectral analysis.     

Quick measurement of CH<Subscript>4</Subscript>, CO<Subscript>2</Subscript> and N<Subscript>2</Subscript>O emissions from a short-plant ecosystem

Combining improved injector, gas line and valve-driving models, a gas chromatograph (GC) equipped with Hydrogen Flame Ionization Detector (FID) and Electron Capture Detector (ECD), can measure CH4, CO2, and N2O simultaneously in an air sample in four minutes. Test results show that the system has high sensitivity, resolution, and precision; the linear response range of the system meets the requirement of flux measurements in situ. The system is suitable for monitoring fluxes of the main greenhouse gases in a short-plant field since it is easy to use, efficacious, and constant and reliable in collecting data.     

Tilt corrections over complex terrain and their implication for CO<Subscript>2</Subscript> transport

Observational data from sonic anemometers are commonly rotated from sonic to streamline coordinates, a procedure that is called tilt correction. Tilt corrections are often used to post-process air velocity data collected from sonic anemometers to allow objective interpretation of air flow data relative to the Earth. Since streamline coordinates depend on dynamical characteristics of the flow, the tilt correction depends not only on temporal and spatial variations of the flow, but also on local circulations. We found that ensemble- averaged slope flows are approximately parallel to the terrain slope close to the ground within the canopy layer, but not above, due to the influence of the diurnal variation of local vertical circulations. As a result, the diurnal variation of the observed vertical velocity in streamline coordinates at 21.5 m above the ground over 11-m tall forest canopies can be opposite to that calculated from the continuity equation. To estimate CO₂ transport over sloping terrain, a workable reference coordinate system is needed such that multiple sonic anemometers have a common reference relative to the Earth. Streamline coordinate systems can be the choice of the common reference coordinate system only if flow, at least ensemble-averaged flow, is parallel to terrain slopes. The choice of the reference coordinate system and its implication in investigation of CO₂ transport are discussed. The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Estimates of anthropogenic CO<Subscript>2</Subscript> uptake in a global ocean model

A global ocean general circulation model (L30T63) is employed to study the uptake and distribution of anthropogenic CO₂ in the ocean. A subgrid-scale mixing scheme called GM90 is used in the model. There are two main GM90 parameters including isopycnal diffusivity and skew (thickness) diffusivity. Sensitivities of the ocean circulation and the redistribution of dissolved anthropogenic CO₂ to these two parameters are examined. Two runs estimate the global oceanic anthropogenic CO₂ uptake to be 1.64 and 1.73 Pg C yr^-1 for the 1990s, and that the global ocean contained 86.8 and 92.7 Pg C of anthropogenic CO₂ at the end of 1994, respectively. Both the total inventory and uptake from our model are smaller than the data-based estimates. In this presentation, the vertical distributions of anthropogenic CO₂ at three meridional sections are discussed and compared with the available data-based estimates. The inventory in the individual basins is also calculated. Use of large isopycnal diffusivity can generally improve the simulated results, including the exchange flux, the vertical distribution patterns, inventory, storage, etc. In terms of comparison of the vertical distributions and column inventory, we find that the total inventory in the Pacific Ocean obtained from our model is in good agreement with the data-based estimate, but a large difference exists in the Atlantic Ocean, particularly in the South Atlantic. The main reasons are weak vertical mixing and that our model generates small exchange fluxes of anthropogenic CO₂ in the Southern Ocean. Improvement in the simulation of the vertical transport and sea ice in the Southern Ocean is important in future work.     

Simulation of Non-Homogeneous CO<Subscript>2</Subscript> and Its Impact on Regional Temperature in East Asia

Carbon dioxide (CO₂) is an important greenhouse gas that influences regional climate through disturbing the earth’s energy balance. The CO₂ concentrations are usually prescribed homogenously in most climate models and the spatiotemporal variations of CO₂ are neglected. To address this issue, a regional climate model (RegCM4) is modified to investigate the non-homogeneous distribution of CO₂ and its effects on regional longwave radiation flux and temperature in East Asia. One-year simulation is performed with prescribed surface CO₂ fluxes that include fossil fuel emission, biomass burning, air–sea exchange, and terrestrial biosphere flux. Two numerical experiments (one using constant prescribed CO₂ concentrations in the radiation scheme and the other using the simulated CO₂ concentrations that are spatially non-homogeneous) are conducted to assess the impact of non-homogeneous CO₂ on the regional longwave radiation flux and temperature. Comparison of CO₂ concentrations from the model with the observations from the GLOBALVIEW-CO₂ network suggests that the model can well capture the spatiotemporal patterns of CO₂ concentrations. Generally, high CO₂ mixing ratios appear in the heavily industrialized eastern China in cold seasons, which probably relates to intensive human activities. The accommodation of non-homogeneous CO₂ concentrations in the radiative transfer scheme leads to an annual mean change of–0.12 W m^–2 in total sky surface upward longwave flux in East Asia. The experiment with non-homogeneous CO₂ tends to yield a warmer lower troposphere. Surface temperature exhibits a maximum difference in summertime, ranging from–4.18 K to 3.88 K, when compared to its homogeneous counterpart. Our results indicate that the spatial and temporal distributions of CO₂ have a considerable impact on regional longwave radiation flux and temperature, and should be taken into account in future climate modeling.     

Accounting for CO<Subscript>2</Subscript> variability over East Asia with a regional joint inversion system and its preliminary evaluation

A regional surface carbon dioxide (CO₂) flux inversion system, the Tan-Tracker-Region, was developed by incorporating an assimilation scheme into the Community Multiscale Air Quality (CMAQ) regional chemical transport model to resolve fine-scale CO₂ variability over East Asia. The proper orthogonal decomposition-based ensemble four-dimensional variational data assimilation approach (POD-4DVar) is the core algorithm for the joint assimilation framework, and simultaneous assimilations of CO₂ concentrations and surface CO₂ fluxes are applied to help reduce the uncertainty in initial CO₂ concentrations. A persistence dynamical model was developed to describe the evolution of the surface CO₂ fluxes and help avoid the “signal-to-noise” problem; thus, CO₂ fluxes could be estimated as a whole at the model grid scale, with better use of observation information. The performance of the regional inversion system was evaluated through a group of single-observation-based observing system simulation experiments (OSSEs). The results of the experiments suggest that a reliable performance of Tan-Tracker-Region is dependent on certain assimilation parameter choices, for example, an optimized window length of approximately 3 h, an ensemble size of approximately 100, and a covariance localization radius of approximately 320 km. This is probably due to the strong diurnal variation and spatial heterogeneity in the fine-scale CMAQ simulation, which could affect the performance of the regional inversion system. In addition, because all observations can be artificially obtained in OSSEs, the performance of Tan-Tracker-Region was further evaluated through different densities of the artificial observation network in different CO₂ flux situations. The results indicate that more observation sites would be useful to systematically improve the estimation of CO₂ concentration and flux in large areas over the model domain. The work presented here forms a foundation for future research in which a thorough estimation of CO₂ flux variability over East Asia could be performed with the regional inversion system.     

CO<Subscript>2</Subscript> dynamics on the shelf of the East Siberian Sea

Expedition data obtained in the coastal-shelf zone of the East Siberian Sea in September 2003, 2004, and 2008 are generalized. Studies of carbonate system in water and CO₂ fluxes between ocean and atmosphere in this region confirmed that it was reasonable to divide the water area studied into two biogeochemical provinces and that the ecosystem of its coastal part is mainly of a heterotrophic nature. In different years, the extent of water supersaturation in carbon dioxide in the East Siberian Sea and the area of the CO₂ release significantly changed. Geographic localization of the atmosphere action centers over the Arctic and their intensity were main determining factors; that told both on the formation of a basic character of the atmospheric and hydrological processes and on the dynamics of the CO₂ exchange between water and air.     

Practical applicability of high frequency correction theories to CO<Subscript>2</Subscript> flux measured by a closed-path system

The theoretical correction of CO₂ fluxes for high frequency attenuation in closed-path systems was re-summarized and its applicability examined using both measurements obtained at an Asiaflux forest site and empirical transfer functions used in previous studies. For our measurement system, the theoretical transfer function was applicable to high frequency correction, even when condensation occurred in the sampling line. Further, in respect to some measurement systems described in previous studies, it was found that the theoretical function was potentially applicable along with the empirical functions used. Meanwhile, in some systems significant errors could not be resolved by re-estimation of the theories. In these systems, because of undefined buffering effects, the actual response lag time decided by the maximum covariance method or by measurement of the system response time using tracer gas was significantly different from the lag time calculated from the tube dimensions and the measured flow rate. If the average flow rate calculated by the actual lag time was used to determine the theoretical function, the theoretical function became closer to, and sometimes agreed with, the empirical function. Any remaining deviation from each function might be associated with pressure fluctuations, but this problem was unable to be examined here. The results suggested that an empirical formulation for each site is considered applicable rather than a theoretical approach, although the theories are being developed to practical application.     

半干旱区农田和草地与大气间二氧化碳和水热通量的模拟研究

集成生物圈模型(IBIS)是目前最复杂的基于动态植被模型的陆面生物物理模型之一。通过应用该模型对国际协调强化观测计划(CEOP)半干旱区基准站之一的吉林通榆观测站(44°25′N,122°52′E)草地和农田生态系统2003年全年的CO2和水、热通量变化进行模拟,并将结果与涡度相关法测定的观测值进行了对比分析,以检验IBIS模型在半干旱区的模拟能力。对比结果表明:除CO2通量模拟结果不够理想外,IBIS模型较好地模拟了通榆观测站的感热通量和潜热通量。模拟与观测比较的相关系数均通过了0.05以上显著性水平的信度检验。总体上看,模型对农田生态系统模拟的偏差小于对退化草地的模拟。     

Long-term effects of anthropogenic CO<Subscript>2</Subscript> emissions simulated with a complex earth system model

A new complex earth system model consisting of an atmospheric general circulation model, an ocean general circulation model, a three-dimensional ice sheet model, a marine biogeochemistry model, and a dynamic vegetation model was used to study the long-term response to anthropogenic carbon emissions. The prescribed emissions follow estimates of past emissions for the period 1751–2000 and standard IPCC emission scenarios up to the year 2100. After 2100, an exponential decrease of the emissions was assumed. For each of the scenarios, a small ensemble of simulations was carried out. The North Atlantic overturning collapsed in the high emission scenario (A2) simulations. In the low emission scenario (B1), only a temporary weakening of the deep water formation in the North Atlantic is predicted. The moderate emission scenario (A1B) brings the system close to its bifurcation point, with three out of five runs leading to a collapsed North Atlantic overturning circulation. The atmospheric moisture transport predominantly contributes to the collapse of the deep water formation. In the simulations with collapsed deep water formation in the North Atlantic a substantial cooling over parts of the North Atlantic is simulated. Anthropogenic climate change substantially reduces the ability of land and ocean to sequester anthropogenic carbon. The simulated effect of a collapse of the deep water formation in the North Atlantic on the atmospheric CO₂ concentration turned out to be relatively small. The volume of the Greenland ice sheet is reduced, but its contribution to global mean sea level is almost counterbalanced by the growth of the Antarctic ice sheet due to enhanced snowfall. The modifications of the high latitude freshwater input due to the simulated changes in mass balance of the ice sheet are one order of magnitude smaller than the changes due to atmospheric moisture transport. After the year 3000, the global mean surface temperature is predicted to be almost constant due to the compensating effects of decreasing atmospheric CO₂ concentrations due to oceanic uptake and delayed response to increasing atmospheric CO₂ concentrations before.     

Effect of Data Assimilation Parameters on The Optimized Surface CO<Subscript>2</Subscript> Flux in Asia

In this study, CarbonTracker, an inverse modeling system based on the ensemble Kalman filter, was used to evaluate the effects of data assimilation parameters (assimilation window length and ensemble size) on the estimation of surface CO₂ fluxes in Asia. Several experiments with different parameters were conducted, and the results were verified using CO₂ concentration observations. The assimilation window lengths tested were 3, 5, 7, and 10 weeks, and the ensemble sizes were 100, 150, and 300. Therefore, a total of 12 experiments using combinations of these parameters were conducted. The experimental period was from January 2006 to December 2009. Differences between the optimized surface CO₂ fluxes of the experiments were largest in the Eurasian Boreal (EB) area, followed by Eurasian Temperate (ET) and Tropical Asia (TA), and were larger in boreal summer than in boreal winter. The effect of ensemble size on the optimized biosphere flux is larger than the effect of the assimilation window length in Asia, but the importance of them varies in specific regions in Asia. The optimized biosphere flux was more sensitive to the assimilation window length in EB, whereas it was sensitive to the ensemble size as well as the assimilation window length in ET. The larger the ensemble size and the shorter the assimilation window length, the larger the uncertainty (i.e., spread of ensemble) of optimized surface CO₂ fluxes. The 10-week assimilation window and 300 ensemble size were the optimal configuration for CarbonTracker in the Asian region based on several verifications using CO₂ concentration measurements.     

Influence of dynamic vegetation on climate change arising from increasing CO<Subscript>2</Subscript>

A dynamic global vegetation model (DGVM) is coupled to an atmospheric general circulation model (AGCM) to investigate the influence of vegetation dynamics on climate change under conditions of global warming. The model results are largely in agreement with observations and the results of previous studies in terms of the present climate, present potential vegetation, present net primary productivity (NPP), and pre-industrial carbon budgets. The equilibrium state of climate properties are compared among pre-industrial, doubled, and quadrupled atmospheric CO₂ values using DGVM–AGCM and current AGCM with fixed vegetation to evaluate the influence of dynamic vegetation change. We also separated the contributions of temperature, precipitation and CO₂ fertilization on vegetation change. The results reveal an amplification of global warming climate sensitivity by 10% due to the inclusion of dynamic vegetation. The total effects of elevated CO₂ and climate change also lead to an increase in NPP and vegetation coverage globally. The reduction of albedo associated with this greening results in enhanced global warming. Our separation analysis indicates that temperature alters vegetation at high latitudes such as Siberia or Alaska, where there is a switch from tundra to forest. On the other hand, CO₂ fertilization provides the largest contribution to greening in arid/semi-arid region. Precipitation change did not cause any drastic vegetation shift.     

Atmospheric Environmental Capacity of SO2 in Winter over Lanzhou in China： A Case Study

The total emission control method based on atmospheric environmental capacity is the most effective in air pollution mitigation. The atmospheric environmental capacities of SO2 on representative days over Lanzhou are estimated using the numerical models RAMS, HYPACT and a linear programming model, according to the national ambient air quality standard of China （NAAQSCHN）. The results show that the fields of meteorological elements and SO2 simulated by the models agree reasonably well with observations. The atmospheric environmental capacity of SO2 over Lanzhou is around 111.7 × 10^3 kg d^-1, and in order to meet the air quality level Ⅱ of the NAAQSCHN, SO2 emissions need to be reduced by 20%.     

Seasonal and diurnal dynamics of CO<Subscript>2</Subscript> emission from oligotrophic peat soil surface

The results of research of diurnal and seasonal dynamics of CO₂ emission from the oligotrophic swamp surface in the southern taiga subzone of Western Siberia in 2005–2007 are under consideration. During the summertime, the intensity of CO₂ emission increases from spring to the midsummer and then decreases by the fall. A mean CO₂ emission value was 118 mg CO₂/(m² hour). The analysis of diurnal dynamics of CO₂ emission showed that the maximum CO₂ flux is observed at 16:00, while the minimum, at 07:00. Mean amplitude of diurnal variations of the CO₂ emission is 74 mg CO₂/(m² hour). The relations established between air temperature and CO₂ flux allowed calculating carbon dioxide emission for the periods between measurements. It was found that in the summertime, the period between 10:00 and 13:00 was optimal for measuring CO₂ emission with a chamber method.