Surface CO2 Concentrations Research Articles

Systematic monthly climate changes in the northern and southern hemispheres

A new comprehensive analysis of the monthly surface temperatures published data in the northern and southern hemispheres is presented. Linear regressions between surface temperatures and CO2 concentrations at consecutive latitude geographical regions indicated that there is a systematic opposite global warming effect between mirror regions on the northern and southern hemispheres during local winters and summers. The validity of the analysis is first tested against well known estimation of the global warming. This completely independent regression procedure produced estimated global temperature rise result during the last century which is in very good agreement with that estimated in IPCC 2007 report.

Dual-chamber measurements of δ13C of soil-respired CO2 partitioned using a field-based three end-member model

The contribution of old soil C (SOM) to total soil respiration (RS) in forest has been a crucial topic in global change research, but remains uncertain. Isotopic methods, such as natural variations in carbon isotope composition (δ13C) of soil respiration, are more frequently being applied, and show promise in separating heterotrophic and autotrophic contributions to RS. However, natural and artificial modification of δ13CRs can cause isotopic disequilibria in the soil-atmosphere system generating a mismatch between what is usually measured and what process-based models will predict. Here we report the partitioning of the soil surface CO2 flux in a warm Mediterranean forest into components derived from root, litter/humus, and SOM sources using a new, three end-member mixing model, and compare this with the conventional partitioning into autotrophic and heterotrophic components. The three end-member mixing model takes into account both the discrimination during CO2 respiration/decomposition of the three components, as well as the fractions of their CO2 fluxes integrated over the total soil profile mass. In addition, we used a novel dual-chamber technique to ensure that δ13CRs was subjected to minimal artefacts during measurement.We observed that by using measured soil surface CO2 concentrations as a baseline level for the dual-chamber operation, it was possible to achieve and monitor the necessary conservation of the soil CO2 steady-state diffusion conditions during the measurements, without using permanent collars inserted deeply into the soil. When RS (8.64 g CO2 m2 d−1) was partitioned into two components, the mean autotrophic and heterotrophic respiration was 56 and 44%, respectively. When RS was partitioned using the three-way model, however, roots, litter/humus, and SOM contributed 30, 33, and 37% of the total flux. Our results confirm that to improve the estimates of the partitioning method, it is important to distinguish the fractional contribution of the long-term SOM-derived flux from younger and more labile sources.

Understanding the kinetics and Re promotion of carbon nanotube supported cobalt catalysts by SSITKA

CO hydrogenation is carried out on Co/CNT and CoRe/CNT catalysts at 483K, 1.85bar, H2/CO=10. The study addresses a fundamental understanding of the effects of reaction conditions (syngas pressure, temperature and GHSV) and Re promotion on the Co activity and selectivity by SSITKA. A systematic SSITKA study was performed to gain a better understanding of the surface reactions at different conditions by measuring the surface concentrations of CO and CHx intermediates on Co. The TOFs of CO conversion and methane formation are found to be sensitive to H site coverage, which is affected by the competitive adsorption between H2 and CO. Higher temperature mainly increases the reaction rate constant, which changes more significantly as compared with the site coverage of the H and CO. CO pressure and temperature have more significant effect on the methane selectivity than H2 pressure. Re has little effect on the TOF and selectivity of the Co/CNT catalyst. SSITKA experiment reveals that both NCO and NCHx increase slightly with Re promotion. The surface coverage of CO is about ten percent higher on the Re promoted sample, suggesting that Re might be able to enhance CO adsorption a little bit. However, Re promotion does not change the intrinsic site activity and apparent activation energy. The apparent activation energy for methane formation is found to be about 118kJ/mol for both promoted and unpromoted catalysts.

Atmospheric studies of habitability in the Gliese 581 system

The M-type star Gliese 581 is orbited by at least one terrestrial planet candidate in the habitable zone, i.e. GL 581 d. Orbital simulations have shown that additional planets inside the habitable zone of GL 581 would be dynamically stable. Recently, two further planet candidates have been claimed, one of them in the habitable zone. In view of the ongoing search for planets around M stars which is expected to result in numerous detections of potentially habitable Super-Earths, we take the GL 581 system as an example to investigate such planets. In contrast to previous studies of habitability in the GL 581 system, we use a consistent atmospheric model to assess surface conditions and habitability. Furthermore, we perform detailed atmospheric simulations for a much larger subset of potential planetary and atmospheric scenarios than previously considered. A 1D radiative-convective atmosphere model is used to calculate temperature and pressure profiles of model atmospheres, which we assumed to be composed of molecular nitrogen, water, and carbon dioxide. In these calculations, key parameters such as surface pressure and CO2 concentration as well as orbital distance and planetary mass are varied. Results imply that surface temperatures above freezing could be obtained, independent of the here considered atmospheric scenarios, at an orbital distance of 0.117 AU. For an orbital distance of 0.146 AU, CO2 concentrations as low as 10 times the present Earth's value are sufficient to warm the surface above the freezing point of water. At 0.175 AU, only scenarios with CO2 concentrations of 5% and 95% were found to be habitable. Hence, an additional Super-Earth planet in the GL 581 system in the previously determined dynamical stability range would be considered a potentially habitable planet.

Oxygen and Carbon Dioxide Distribution And Movement in Passively Aerated Compost Piles

Distributions of O2 and CO2 concentrations across a cross section of a full-scale passively aerated, mechanically turned, compost piles were measured as a function of time over an 11 day long period covering two pile turnings. The compost pile had a triangular cross section, was 1.8 m high, 4.4 m wide, 80 m long and consisted of sewage sludge, yard/park waste and screening residue from previously composted materials. The measurements were conducted in one cross section of the pile. The O2 and CO2 concentration measurements were used in combination with earlier published measurements of air permeability and air pressure inside the compost pile to calculate O2 and CO2 fluxes across the pile surface as functions of time and location as well as estimation of total specific oxygen consumption rates in the compost. Distributions of O2 and CO2 concentrations inside the pile were relatively constant with time and exhibited high O2 concentrations near the surface and high CO2 concentrations near the center of the pile. Maximum O2 fluxes in the compost occurred along the lower edges of the pile and equalled up to 15 kg/m−2 h−1 while maximum CO2 fluxes occurred at the center top of the pile and equalled up to 700 g m−2 h−1. Average daily CO2 emissions from the compost were up to 3.4 kg m3 d−1 while the corresponding O2 flux into the compost pile was up to 53 kg m3 d−1. Average O2 consumption was 1.4 kg m−3 d−1 while average CO2 production was 1.5 kg m−3 d−1 at the measurement location over the 11 day experimental period.

Impact of model resolution on chemical ozone formation in Mexico City: application of the WRF-Chem model

Abstract. The resolution of regional chemical/dynamical models has important effects on the calculation of the distributions of air pollutants in urban areas. In this study, the sensitivity of air pollutants and photochemical ozone production to different model resolutions is assessed by applying a regional chemical/dynamical model (version 3 of Weather Research and Forecasting Chemical model – WRF-Chemv3) to the case of Mexico City. The model results with 3, 6, 12, and 24 km resolutions are compared to local surface measurements of CO, NOx, and O3. The study shows that the model resolutions of 3 and 6 km provide reasonable simulations of surface CO, NOx, and O3 concentrations and of diurnal variations. The model tends to underestimate the measurements when the resolution is reduced to 12 km or less. The calculated surface CO, NOx, and O3 concentrations at 24 km resolution are significantly lower than measured values. This study suggests that the ratio of the city size to the threshold resolution is 6 to 1, and that this ratio can be considered as a test value in other urban areas for model resolution setting. There are three major factors related to the effects of model resolution on the calculations of O3 and O3 precursors, including; (1) the calculated meteorological conditions, (2) the spatial distribution for the emissions of ozone precursors, and (3) the non-linearity in the photochemical ozone production. Model studies suggest that, for the calculations of O3 and O3 precursors, spatial resolutions (resulting from different meteorological condition and transport processes) have larger impacts than the effect of the resolution associated with emission inventories. The model shows that, with coarse resolution of emission inventory (24 km) and high resolution for meteorological conditions (6 km), the calculated CO and O3 are considerably improved compared to the results obtained with coarse resolution for both emission inventory and meteorological conditions (24 km). The resolution of the surface emissions has important effects on the calculated concentration fields, but the effects are smaller than those resulting from the model resolution. This study also suggests that the effect of model resolution on O3 precursors leads to important impacts on the photochemical formation of ozone. This results directly from the non-linear relationship between O3 formation and O3 precursor concentrations. Finally, this study suggests that, considering the balance between model performance and required computation time on current computers, the 6 km resolution is an optimal resolution for the calculation of ozone and its precursors in urban areas like Mexico City.

Neural networks for CO2 profile retrieval from the data of GOSAT/TANSO-FTS

The feasibility of the retrieval of CO2 vertical profiles and its column-averaged concentration by reflected solar radiation measured by the TANSO-FTS sensor onboard the GOSAT satellite is demonstrated. Model spectra in the 0.76-µm O2 band and CO2 bands near 1.6 and 2.06 µm were used to train the neural network for CO2 retrieval. Separate neural networks were developed for each of the four scanning angles; the solar zenith angle was considered as a continuous variable. An accuracy of better than 1 ppm for column-averaged values and better than 4 ppm for the surface CO2 concentration were achieved. A 1: 300 noise level in the spectra was set for all spectral ranges.

New technique to analyse global distributions of CO2 concentrations and fluxes from non-processed observational data

We have developed a new observational screening technique for inverse model. This technique was applied to our transport models with re-analysed meteorological data and the inverse model to estimate the global distribution of CO2 concentrations and fluxes. During the 1990s, we estimated a total CO2 uptake by the biosphere of 1.4–1.5 PgC yr-1 and a total CO2 uptake by the oceans of 1.7–1.8 PgC yr-1. The uncertainty of global CO2 flux estimation is about 0.3 PgC yr-1. We also obtained monthly surface CO2 concentrations in the marine boundary layer to precisions of 0.5–1.0 ppm. To utilize non-processed (statistical monthly mean) observational data in our analysis, we developed a quality control procedure for such observational data including a repetition of inversion. This technique is suitable for other inversion setups. Observational data by ships were placed into grids and used in our analysis to add to the available data from fixed stations. The estimated global distributions are updated and extended every year.

Assessing the regional impacts of Mexico City emissions on air quality and chemistry

Abstract. The impact of Mexico City (MCMA) emissions is examined by studying its effects on air quality, photochemistry, and on ozone production regimes by combining model products and aircraft observations from the MILAGRO experiment during March 2006. The modeled influence of MCMA emissions to enhancements in surface level NOx, CO, and O3 concentrations (10–30% increase) are confined to distances <200 km, near surface. However, the extent of the influence is significantly larger at higher altitudes. Broader MCMA impacts (some 900 km Northeast of the city) are shown for specific outflow conditions in which enhanced ozone, NOy, and MTBE mixing ratios over the Gulf of Mexico are linked to MCMA by source tagged tracers and sensitivity runs. This study shows that the "footprint" of MCMA on average is fairly local, with exception to reactive nitrogen, which can be transported long range in the form of PAN, acting as a reservoir and source of NOx with important regional ozone formation implications. The simulated effect of MCMA emissions of anthropogenic aerosol on photochemistry showed a maximum regional decrease of 40% in J[NO2→NO+O], and resulting in the reduction of ozone production by 5–10%. Observed ozone production efficiencies are evaluated as a function of distance from MCMA, and by modeled influence from MCMA. These tend to be much lower closer to MCMA, or in those points where modeled contribution from MCMA is large. This research shows that MCMA emissions do effect on regional air quality and photochemistry, both contributing large amounts of ozone and its precursors, but with caveat that aerosol concentrations hinder formation of ozone to its potential due to its reduction in photolysis rates.

Characterization of Au-Rh/TiO<SUB>2</SUB> Bimetallic Nanocatalysts by CO and CH<SUB>3</SUB>CN Adsorption: XPS, TEM and FTIR Measurements

On X-ray photoelectron spectra of the Au-Rh/TiO2 catalysts the position of Au4f peak was practically unaffected by the presence of rhodium, the peak position of Rh3d, however, shifted to lower binding energy with the increase of gold content of the catalysts. Rh enrichment in the outer layers of the bimetallic crystallites was experienced. The bands due to Au0-CO, Rh0-CO and (Rh0)2-CO were observed on the IR spectra of bimetallic samples, no signs for Rh+-(CO)2 were detected on these catalysts. The band due to CH3CN on Lewis acid centers shifted to lower wavenumbers with the increase of Rh content, which shows that the strength of Lewis acid sites weakens with the increase of Rh content of the catalysts. CH3CN, on the other hand, dissociates producing CN(a) species even at this temperature. From the shift to higher wavenumbers of the band due to CN(a) the strengthening of the C-N bond with increasing Rh content has been established. The results were interpreted by electron donation from titania through gold to rhodium and by the higher particle size of bimetallic crystallites. The effects of catalyst composition, reaction temperature and composition of the reacting gas mixtures have been studied on the oxidation of CO in the presence of hydrogen (PROX process). The presence of both O2 and H2 reduced the surface concentration of CO adsorbed on metallic sites. Mass spectroscopic analysis of the gas phase showed that gaseous CO2 formed in the highest amount in CO+O2 mixture, the presence of H2 suppressed the amount of CO2 produced. This negative effect of hydrogen was the lowest on 1% Rh/TiO2, the highest inhibition was observed on Au/TiO2 systems.

Determination of composition in stoichiometric Co–N ultrathin films by nitrogen plasma sputtering

This work utilizes low-energy sputtering to incorporate the generated nitrogen plasma into an epitaxial 1.4nm Co film on the surface of a ZnO(002) substrate. In this method, ultrathin Co–N amorphous films were formed. Interestingly, Co is key to the formation of Co–N films. Without the deposition of Co on the ZnO(002), nitride films cannot be formed. Observations of the surface composition of the Co–N films after the firing of a N+ ion beam onto it demonstrated that the surface concentration of Co reduced at the same rate as the reduction in the concentration of N upon successive sputtering. Theoretical calculations based on the Auger peak-to-peak amplitudes established that the composition of the amorphous Co–N thin films may be Co3N2.

Seasonal CO2 rectifier effect and large‐scale extratropical atmospheric transport

In atmospheric transport models, the covariation of the atmospheric transport and annually neutral biospheric CO2 flux is usually evident as the annual zonal mean surface CO2 concentration gradient. Using the NIES transport model and CO2 flux from the Biome‐BGC model, the covariations of different transport mechanisms and CO2 flux were examined and quantified. Including the covariation of the total transport (processes included in the NIES model) and CO2 flux, the annual average pole to pole CO2 concentration gradient is 3.5 ppm and interhemispheric difference of the average extratropical surface concentration is 2.5 ppm. The conventional covariation mechanism of the seasonal variation of planetary boundary layer mixing height and CO2 flux accounts for approximately 45% of the CO2 concentration gradient. Another important contribution to the CO2 concentration gradient in this model is the covariation of the extratropical anomaly transport (mainly by cyclones and anticyclones) and the biospheric flux, which accounts for about 55%. This alternate physical mechanism is the association of stronger meridional (north–south) anomaly transport (under strong baroclinic instability condition) with higher CO2 concentration from soil respiration in the winter and weaker anomaly transport (weak baroclinic instability condition) with lower CO2 concentration from photosynthetic uptake in the summer. The net result of the meridional transport and flux covariation is a north–south annual zonal mean CO2 concentration gradient.

Surface species and gas phase products in the preferential oxidation of CO on TiO2-supported Au-Rh bimetallic catalysts

The oxidation of CO in the presence of hydrogen (PROX process) was investigated on bimetallic Au-Rh catalysts at 300–373 K by Fourier transform infrared spectroscopy and mass spectroscopy. The effects of catalyst composition, reaction temperature and composition of the reacting gas mixtures have been studied. The IR studies revealed the formation of bi- and monodentate carbonates, bicarbonates and hydrocarbonates on the catalysts surfaces; these surface species proved to be not involved in the surface reactions. The formation of adsorbed formaldehyde was observed on all surfaces, except 1% (0.25Au+0.75Rh)/TiO2. Adsorbed CO2 (as the surface product of CO oxidation) was not detected on any surface. The presence of both O2 and H2 reduced the surface concentration of CO adsorbed on the metallic sites. Mass spectroscopic analysis of the gas phase showed that gaseous CO2 was formed in the highest amount in the CO+O2 mixture, the presence of H2 suppressed the amount of CO2 produced. This negative effect of H2 was the lowest on the 1% Rh/TiO2 and 1% (0.25Au+0.75Rh)/TiO2 catalysts.

Kinetic instabilities and intra-thread diffusion limitations in CO oxidation reaction at Pt/fiber-glass catalysts

Abstract Experimental and mathematical simulation studies of CO oxidation reaction at Pt-based glass-fiber catalyst showed that reaction phenomena in fabric and threads of woven glass-fiber catalysts is rather complicated by transfer limitations, which may impose especially valuable influence on reactions with strongly non-linear kinetics. Such limitations are related to extra-thread and intra-thread heat and mass transfer. Additionally, in thin beds, consisting of one or few layers of woven catalyst, the significant role may be played by experimentally confirmed transient oscillations between CSTR and plug-flow regimes, therefore, the axial heat and mass dispersion is an important transfer factor for such beds as well. Mathematical modelling of phenomena inside the catalyst thread on the basis of elaborated kinetic model showed possibility of existence of steady-state multiplicity as a self-sufficient result of intra-thread transfer limitations. In general, observed reaction rates at the catalyst thread and location of multiplicity region may depend upon many factors (thread diameter and porosity, surface CO concentration and temperature and even choice of reaction media balance gas), which should be taken into account for correct formulation of lab experimental procedure, interpretation of experimental data and process scale-up.

Direct observation of the kinetically relevant site of CO hydrogenation on supported Ru catalyst at 700 K by time-resolved FT-IR spectroscopy

Time-resolved FT-IR spectra of carbon monoxide hydrogenation over alumina-supported ruthenium particles were recorded on the millisecond time scale at 700 K using pulsed release of CO and a continuous flow of H(2)-N(2) (ratio 0.067 or 0.15, 1 atm total pressure). Adsorbed carbon monoxide was detected along with gas phase products methane (3016 and 1306 cm(-1)), water (1900-1300 cm(-1)), and carbon dioxide (2348 cm(-1)). Aside from adsorbed CO, no other surface species were observed. The rate of formation of methane is 2.5 +/- 0.4 s(-1) and coincides with the rate of carbon dioxide growth (3.4 +/- 0.6 s(-1)), thus indicating that CH(4) and CO(2) originate from a common intermediate. The broad band of adsorbed carbon monoxide has a maximum at 2010 cm(-1) at early times (36 ms) that shifts gradually to 1960 cm(-1) over a period of 3 s as a result of the decreasing surface concentration of CO. Kinetic analysis of the adsorbed carbon monoxide reveals that surface sites absorbing at the high frequency end of the infrared band are temporally linked to gas phase product growth. Specifically, a (linear) CO site at 2026 cm(-1) decays with a rate constant of 2.9 +/- 0.1 s(-1), which coincides with the rise constant of CH(4). This demonstrates that the linear CO site at 2026 cm(-1) is the kinetically most relevant one for the rate-determining CO dissociation step under reaction conditions at 700 K.

Evaluation of Carbon Uptake and Emissions by Forests in Korea During the Last Thirty Years (1973–2002)

The contribution of Korean forests to carbon sequestration for anthropogenic carbon emissions was evaluated. In addition, monitoring of carbon species released from forest fires was conducted. Despite a high carbon uptake by Korean forests, a tremendous increase in fossil fuel burning resulted in a small contribution by forests to carbon removal. The removal efficiency had a 5-31% range with an average of 12% during the period 1973-2002. In 2000, the amount of carbon released from burned trees corresponded to 1.6% of carbon uptake by forests. The distribution of surface CO concentration (ppb) derived from MOPITT (Measurement of Pollution in the Troposphere) showed high CO levels over the East/Japan Sea on April 10, 2000 when the largest forest fires occurred along the east coast of Korea. Trajectory analysis and ground CO measurements also indicated that CO levels over the East/Japan Sea were influenced by forest fires. This study suggests that continuous monitoring of carbon emissions from forest fires is needed for a more reliable estimate of carbon flux in the environment.

Voltammetric study of interaction of Co(phen) 33+ with DNA at gold nanoparticle self-assembly electrode

Modifying electrode surfaces on the molecule scale allow developing new electrochemical biosensors. A new strategy for the immobilization of calf thymus DNA on the surface of gold nanoparticles which are co-immobilized at a gold electrode through 4,4 ′-bis(methanethiol) biphenyl (MTP) molecule by assembly process is demonstrated. The DNA modified electrode was incubated in Co(phen) 3 3+ solution of an aqueous buffer or an acetonitrile (AN) solution, then it was rinsed and placed in a Co(phen) 3 3+ free buffer solution or AN solution, followed by cyclic voltammetric experiments. Clear redox peaks of Co(phen) 3 3+ were observed both in an aqueous and AN solutions. The concentration of supporting electrolyte on electrochemical behavior was discussed. It was found that the surface coverage value of DNA molecules on modified gold nanoparticle and the redox current of adsorbed Co(phen) 3 3+ were decrease with increasing the size of gold nanoparticles (6, 25, 42, 73, and 93 nm). In aqueous solution, the electron transfer rate constant of Co(phen) 3 3+/2+ redox couple became slow with increasing the diameter of gold nanoparticle, and the speed almost had nothing to do with the diameter in nonaqueous solution. The surface concentration of Co(phen) 3 3+ adsorption on DNA modified electrode decreased and rate constant of adsorption kinetics increased with increasing the interactive temperature. In AN solution, the electrostatic interaction between DNA and Co(phen) 3 3+/2+ was greatly reduced, however, compare with in aqueous solution the interaction between DNA and reduced form of Co(phen) 3 2+ was more strongly than oxidized form Co(phen) 3 3+. The surface concentration of Co(phen) 3 3+ adsorption on DNA modified electrode reach maximum value when the interactive temperature about 20 °C, and rate constant of adsorption kinetics nearly independent of the interactive temperature. The results show that the DNA can adsorb on the modified electrode firmly and the Co(phen) 3 3+/2+ adsorbed on DNA give good electrochemical response both in aqueous and nonaqueous solutions. It was confirmed that the DNA modified electrode can be applied in a nonaqueous system and the modified electrode can be used to investigate the interaction between DNA and electroactive species both in aqueous and nonaqueous systems.

Investigating the sources of synoptic variability in atmospheric CO2 measurements over the Northern Hemisphere continents: a regional model study

Continuous measurements of atmospheric CO2 over the continents are potentially powerful tools for understanding regional carbon budgets, but our limited understanding of the processes driving the high-frequency variability in these measurements makes interpretation difficult. In this paper we examine the synoptic variability (∼days) of surface CO2 concentrations in four continental records from Europe and North America. Three source functions corresponding to the ocean, land biosphere and anthropogenic sources and sinks for CO2 have been implemented in a regional atmospheric transport model. In previous carbon studies, monthly biospheric fluxes have typically been used, but here high spatiotemporal (daily, 1 ◦ × 1 ◦ ) resolution biospheric fluxes are obtained from the NCAR Land Surface Model (LSM). A high-pass filter is used to remove atmospheric variability on time scales longer than 2 months, and the resulting simulated concentration fields replicates reasonably well the magnitude and seasonality of the synoptic variability across the four observation sites. The phasing of many of the individual events are also captured, indicating that the physical and biogeochemical dynamics driving the model variability likely resemble those in nature. The observations and model results show pronounced summer maxima in the synoptic CO2 concentration variability at the two stations located in North America, while a slightly different seasonality with high variability throughout fall and winter is observed at the European sites. The mechanisms driving these patterns are studied and discussed based on correlations between the concentration anomalies and the driving atmospheric physical variables and surfaces fluxes in the simulations. During the summer, the synoptic variability over the continents has a significant contribution from variations in regional net primary production, which in turn is modulated by regional, synoptic temperature variability. In winter the synoptic variability is partitioned about equally between biospheric and anthropogenic CO2 and is mainly driven by local vertical mixing and synoptic variations in atmospheric circulation working on the large-scale atmospheric gradient. This study highlights the importance for future modeling work of improved high temporal resolution (at least daily) surface biosphere, oceanic and anthropogenic flux estimates as well as high vertical and horizontal spatiotemporal resolution of the driving meteorology.

Investigating the sources of synoptic variability in atmospheric CO2 measurements over the Northern Hemisphere continents: a regional model study

Continuous measurements of atmospheric CO2 over the continents are potentially powerful tools for understanding regional carbon budgets, but our limited understanding of the processes driving the high-frequency variability in these measurements makes interpretation difficult. In this paper we examine the synoptic variability (⊼days) of surface CO2 concentrations in four continental records from Europe and North America. Three source functions corresponding to the ocean, land biosphere and anthropogenic sources and sinks for CO2 have been implemented in a regional atmospheric transport model. In previous carbon studies, monthly biospheric fluxes have typically been used, but here high spatiotemporal (daily, 1¼×1¼) resolution biospheric fluxes are obtained from the NCAR Land Surface Model (lsm). A high-pass filter is used to remove atmospheric variability on time scales longer than 2 months, and the resulting simulated concentration fields replicates reasonably well the magnitude and seasonality of the synoptic variability across the four observation sites. The phasing of many of the individual events are also captured, indicating that the physical and biogeochemical dynamics driving the model variability likely resemble those in nature.The observations and model results show pronounced summer maxima in the synoptic CO2 concentration variability at the two stations located in North America, while a slightly different seasonality with high variability throughout fall and winter is observed at the European sites. The mechanisms driving these patterns are studied and discussed based on correlations between the concentration anomalies and the driving atmospheric physical variables and surfaces fluxes in the simulations. During the summer, the synoptic variability over the continents has a significant contribution from variations in regional net primary production, which in turn is modulated by regional, synoptic temperature variability. In winter the synoptic variability is partitioned about equally between biospheric and anthropogenic CO2 and is mainly driven by local vertical mixing and synoptic variations in atmospheric circulation working on the large-scale atmospheric gradient. This study highlights the importance for future modeling work of improved high temporal resolution (at least daily) surface biosphere, oceanic and anthropogenic flux estimates as well as high vertical and horizontal spatiotemporal resolution of the driving meteorology.

Effect of clouds on photolysis and oxidants in the troposphere

Cloud layers in the troposphere influence photolysis rates (J values) and hence concentrations of chemical species. In order to study the impact of clouds on photolysis rates and oxidants, we have developed a simplified version of the National Center for Atmospheric Research (NCAR) Tropospheric Ultraviolet‐Visible (TUV) model and have coupled the simplified TUV (otherwise known as the fast TUV (FTUV)) into the NCAR/Atmospheric Chemistry Division global transport chemical model (Model for Ozone and Related Chemical Tracers (MOZART‐2)). The FTUV model has the same physical processes as the TUV model, except that the wavelength bins between 121 and 750 nm are reduced from 140 to 17. As a result, FTUV is about 8 times faster than the original TUV. Differences in the calculated photolysis rates between TUV and FTUV are generally less than 5% in the troposphere. Subgrid vertical distributions of clouds are also considered in the calculation of photolysis rates in MOZART‐2. The method used in this study is a mixed maximum and random overlap scheme. The subgrid method increases the computation time for photolysis rates by a factor of 3 compared to a simple method in which clouds are uniformly distributed over the MOZART‐2 grids. Our calculation shows that the uniform cloud distribution method tends to significantly overestimate back scattering on the top of clouds and overestimates the impact on photochemistry in the troposphere. The results suggest that clouds have important impacts on tropospheric chemistry. Global mean OH concentration increases by about 20% due to the impact of clouds. As a result, the calculated CH4 lifetime changes to 11 years for clear sky and 9 years for cloudy sky. The latter value is closer to the methane lifetime estimated from previous studies. Calculated CO surface concentrations are compared with observed values, showing an improvement when the impact of clouds on the photolysis rates is taken into account. Clouds also have important impacts on tropospheric ozone budget. Our calculation suggest that because of clouds, the globally averaged photolysis rates of J[O3], J[CH2O], and J[NO2] are enhanced in the troposphere by about 12, 13, and 13%, respectively, leading to an 8% increase in the tropospheric O3 concentrations. Our study suggests that clouds strongly influence photolysis rates and hence play an important role in controlling the concentrations of the tropospheric oxidants. Such effects should be carefully considered and included in regional and global chemical transport models.