[1] The ability to reliably estimate CO 2 fluxes from current in situ atmospheric CO 2 measurements and future satellite CO 2 measurements is dependent on transport model performance at synoptic and shorter timescales. The TransCom continuous experiment was designed to evaluate the performance of forward transport model simulations at hourly, daily, and synoptic timescales, and we focus on the latter two in this paper. Twenty-five transport models or model variants submitted hourly time series of nine predetermined tracers (seven for CO 2 ) at 280 locations. We extracted synoptic-scale variability from daily averaged CO 2 time series using a digital filter and analyzed the results by comparing them to atmospheric measurements at 35 locations. The correlations between modeled and observed synoptic CO 2 variabilities were almost always largest with zero time lag and statistically significant for most models and most locations. Generally, the model results using diurnally varying land fluxes were closer to the observations compared to those obtained using monthly mean or daily average fluxes, and winter was often better simulated than summer. Model results at higher spatial resolution compared better with observations, mostly because these models were able to sample closer to the measurement site location. The amplitude and correlation of model-data variability is strongly model and season dependent. Overall similarity in modeled synoptic CO 2 variability suggests that the first-order transport mechanisms are fairly well parameterized in the models, and no clear distinction was found between the meteorological analyses in capturing the synoptic-scale dynamics.

[1] A forward atmospheric transport modeling experiment has been coordinated by the TransCom group to investigate synoptic and diurnal variations in CO 2 . Model simulations were run for biospheric, fossil, and air-sea exchange of CO 2 and for SF 6 and radon for [2000][2001][2002][2003]. Twenty-five models or model variants participated in the comparison. Hourly concentration time series were submitted for 280 sites along with vertical profiles, fluxes, and meteorological variables at 100 sites. The submitted results have been analyzed for diurnal variations and are compared with observed CO 2 in 2002. Mean summer diurnal cycles vary widely in amplitude across models. The choice of sampling location and model level account for part of the spread suggesting that representation errors in these types of models are potentially large. Despite the model spread, most models simulate the relative variation in diurnal amplitude between sites reasonably well. The modeled diurnal amplitude only shows a weak relationship with vertical resolution across models; differences in near-surface transport simulation appear to play a major role. Examples are also presented where there is evidence that the models show useful skill in simulating seasonal and synoptic changes in diurnal amplitude.

Inverse modeling techniques used to quantify surface carbon fluxes commonly assume that the uncertainty of fossil fuel CO(2) (FFCO(2)) emissions is negligible and that intra-annual variations can be neglected. To investigate these assumptions, we analyzed the differences between four fossil fuel emission inventories with spatial and temporal differences over Europe and their impact on the model simulated CO(2) concentration. Large temporal flux variations characterize the hourly fields (similar to 40% and similar to 80% for the seasonal and diurnal cycles, peak-to-peak) and annual country totals differ by 10% on average and up to 40% for some countries (i.e., the Netherlands). These emissions have been prescribed to seven different transport models, resulting in 28 different FFCO(2) concentrations fields. The modeled FFCO(2) concentration time series at surface sites using time-varying emissions show larger seasonal cycles (+2 ppm at the Hungarian tall tower (HUN)) and smaller diurnal cycles in summer (-1 ppm at HUN) than when using constant emissions. The concentration range spanned by all simulations varies between stations, and is generally larger in winter (up to similar to 10 ppm peak-to-peak at HUN) than in summer (similar to 5 ppm). The contribution of transport model differences to the simulated concentration std-dev is 2-3 times larger than the contribution of emission differences only, at typical European sites used in global inversions. These contributions to the hourly (monthly) std-dev's amount to similar to 1.2 (0.8) ppm and similar to 0.4 (0.3) ppm for transport and emissions, respectively. First comparisons of the modeled concentrations with (14)C-based fossil fuel CO(2) observations show that the large transport differences still hamper a quantitative evaluation/validation of the emission inventories. Changes in the estimated monthly biosphere flux (Fbio) over Europe, using two inverse modeling approaches, are relatively small (less that 5 %) while changes in annual Fbio (up to similar to 0.15% GtC yr(-1)) are only slightly smaller than the differences in annual emission totals and around 30% of the mean European ecosystem carbon sink. These results point to an urgent need to improve not only the transport models but also the assumed spatial and temporal distribution of fossil fuel emission inventories

The isotopic composition of molecular hydrogen (H₂) contains independent information for constraining the global H₂ budget. To explore this, we have implemented hydrogen sources and sinks, including their stable isotopic composition and isotope fractionation constants, into the global chemistry transport model TM5. For the first time, a global model now includes a simplified but explicit isotope reaction scheme for the photochemical production of H₂. We present a comparison of modelled results for the H₂ mixing ratio and isotope composition with available measurements on seasonal to inter annual time scales for the years 2001–2007. The base model results agree well with observations for H₂ mixing ratios. For δD[H₂], modelled values are slightly lower than measurements. A detailed sensitivity study is performed to identify the most important parameters for modelling the isotopic composition of H₂. The results show that on the global scale, the discrepancy between model and measurements can be closed by adjusting the default values of the isotope effects in deposition, photochemistry and the stratosphere-troposphere exchange within the known range of uncertainty. However, the available isotope data do not provide sufficient information to uniquely constrain the global isotope budget. Therefore, additional studies focussing on the isotopic composition near the tropopause and on the isotope effects in the photochemistry and deposition are recommended

Abstract. The isotopic composition of molecular hydrogen (H 2 ) produced by photochemical oxidation of methane (CH 4 ) and Volatile Organic Compounds (VOCs) is a key quantity in the global isotope budget of (H 2 ). The many individual reaction steps involved complicate its investigation. Here we present a simplified structure-activity approach to assign isotope effects to the individual elementary reaction steps in the oxidation sequence of CH 4 and some other VOCs. The approach builds on and extends the work by Gerst and Quay (2001) and Feilberg et al. (2007b). The description is generalized and allows the application, in principle, also to other compounds. The idea is that the C-H and C-D bonds -seen as reactive sites -have similar relative reaction probabilities in isotopically substituted, but otherwise identical molecules. The limitations of this approach are discussed for the reaction CH 4 +Cl. The same approach is applied to VOCs, which are important precursors of H 2 that need to be included into models. Unfortunately, quantitative information on VOC isotope effects and source isotope signatures is very limited and the isotope scheme at this time is limited to a strongly parameterized statistical approach, which neglects kinetic isotope effects. Using these concepts we implement a full hydrogen isotope scheme in a chemical box model and carry out a sensitivity study to identify those reaction steps and conditions that are most critical for the isotope composition of the final H 2 product. The reaction scheme is directly applicable in global chemistry models, which can thus include the isotope pathway of H 2 produced from CH 4 and VOCs in a consistent way.