[1] In association with Tropospheric Ozone Production about the Spring Equinox (TOPSE) measurement campaign a regional episodic chemical transport model is used to study the seasonal mechanisms of transport of pollutants from their Northern Hemisphere emission regions into the remainder of the troposphere. The model simulates the strong seasonal cycle for CO and hydrocarbons that agrees well with TOPSE measurements. In this study we use the isentropic perspective to analyze transport during different seasons and from different emission regions. Simulations with diagnostic tracers are conducted to quantify (1) the contribution of different emission regions to tracer distribution and (2) the importance of cross-isentropic transport. In the high latitudes during winter, in agreement with previous studies, the European and Siberian emission sources are the largest contributors to the diagnostic tracer distributions in the lower troposphere due to largescale circulation patterns, low temperatures at the source and presence of a cold stable boundary layer that facilitates diabatic cooling. North American emissions are located further south, often south of the polar front, and are on average emitted at higher potential temperatures. Owing to their predominant transport over the relatively warm Atlantic, they experience strong diabatic heating due to the general instability in the air column and heavy precipitation in the storm track. During the summer months the pollution from all emission regions is more likely to be diabatically transported to higher potential temperatures and diluted. In addition, direct transport of pollutants into the Arctic is less frequent during summer due to the differences in the large-scale circulation patterns.

Abstract. In the context of rising greenhouse gas concentrations, and the potential feedbacks between climate and the carbon cycle, there is an urgent need to monitor the exchanges of carbon between the atmosphere and both the ocean and the land surfaces. In the so-called top-down approach, the surface fluxes of CO 2 are inverted from the observed spatial and temporal concentration gradients. The concentrations of CO 2 are measured in-situ at a number of surface stations unevenly distributed over the Earth while several satellite missions may be used to provide a dense and better-distributed set of observations to complement this network. In this paper, we compare the ability of different CO 2 concentration observing systems to constrain surface fluxes. The various systems are based on realistic scenarios of sampling and precision for satellite and in-situ measurements.It is shown that satellite measurements based on the differential absorption technique (such as those of SCIAMACHY, GOSAT or OCO) provide more information than the thermal infrared observations (such as those of AIRS or IASI). The OCO observations will provide significantly better information than those of GOSAT. A CO 2 monitoring mission based on an active (lidar) technique could potentially proCorrespondence to: K. Hungershoefer (katja.hungershoefer@dwd.de) vide an even better constraint. This constraint can also be realized with the very dense surface network that could be built with the same funding as that of the active satellite mission. Despite the large uncertainty reductions on the surface fluxes that may be expected from these various observing systems, these reductions are still insufficient to reach the highly demanding requirements for the monitoring of anthropogenic emissions of CO 2 or the oceanic fluxes at a spatial scale smaller than that of oceanic basins. The scientific objective of these observing system should therefore focus on the fluxes linked to vegetation and land ecosystem dynamics.

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[1] The distributions of NO x and O 3 are analyzed during TOPSE (Tropospheric Ozone Production about the Spring Equinox). In this study these data are compared with the calculations of a global chemical/transport model (Model for OZone And Related chemical Tracers (MOZART)). Specifically, the effect that hydrolysis of N 2 O 5 on sulfate aerosols has on tropospheric NO x and O 3 budgets is studied. The results show that without this heterogeneous reaction, the model significantly overestimates NO x concentrations at high latitudes of the Northern Hemisphere (NH) in winter and spring in comparison to the observations during TOPSE; with this reaction, modeled NO x concentrations are close to the measured values. This comparison provides evidence that the hydrolysis of N 2 O 5 on sulfate aerosol plays an important role in controlling the tropospheric NO x and O 3 budgets. The calculated reduction of NO x attributed to this reaction is 80 to 90% in winter at high latitudes over North America. Because of the reduction of NO x , O 3 concentrations are also decreased. The maximum O 3 reduction occurs in spring although the maximum NO x reduction occurs in winter when photochemical O 3 production is relatively low. The uncertainties related to uptake coefficient and aerosol loading in the model is analyzed. The analysis indicates that the changes in NO x due to these uncertainties are much smaller than the impact of hydrolysis of N 2 O 5 on sulfate aerosol. The effect that hydrolysis of N 2 O 5 on global NO x and O 3 budgets are also assessed by the model. The results suggest that in the Northern Hemisphere, the average NO x budget decreases 50% due to this reaction in winter and 5% in summer. The average O 3 budget is reduced by 8% in winter and 6% in summer. In the Southern Hemisphere (SH), the sulfate aerosol loading is significantly smaller than in the Northern Hemisphere. As a result, sulfate aerosol has little impact on NO x and O 3 budgets of the Southern Hemisphere.

Abstract. A photochemical box model with CO-CH4-NOy-H20 chemistry is used to calculate the diurnally averaged net photochemical rate of change of ozone (hereinafter called the chemical ozone tendency) in the troposphere for different values of parameters: NO x and ozone concentration, temperature, humidity, CO concentration, and surface albedo. To understand the dependency of the chemical ozone tendency on the input parameters, a detailed sensitivity study is performed. Subsequently, the expected variations of the ozone tendencies with altitude, latitude, and season are analyzed. The magnitude of the tendency decreases rapidly with height mostly as a result of lower absolute humidity and temperature. In the upper troposphere (at 190 mbar) the maximum tendencies are below 2 parts per billion by volume/day. Lower temperature and specific humidity cause a shift of the value of NO x at which the ozone production balances the destruction of ozone (balance point) to lower NO x values; these two parameters are also, to a large extent, responsible for lower magnitudes of the tendency at higher latitudes and in winter. In the upper troposphere we find that the net tendency is at least as sensitive to variations in H20 concentration as to NO x. This suggests a possible synergism between direct NO x pollution by aircraft and the indirect modification of H20 by climate change. In the second part of the paper the box model calculated rates are used as ozone's chemical tendency terms during a simulation conducted with the three-dimensional global chemistry transport model (GCTM). The box model is used to calculate the tendencies as a function of NO x and ozone at all tropospheric levels of the GCTM, at nine latitudes and for four seasons using zonally and monthly averaged data: water vapor and temperature from observations and model CO. These tables together with the NO x fields obtained in an earlier GCTM simulation are used in the GCTM simulation of 0 3 if nonmethane hydrocarbon levels are low. The global monthly averaged chemical ozone tendency fields saved during the simulation are presented and analyzed for the present-day and preindustrial conditions. The chemical tendency fields show a strong correlation with the NO x fields. In contrast with the lower and middle troposphere where the tendencies are negative in remote regions over the oceans, in the upper troposphere, where NO x is generally greater than 50 parts per trillion by volume and the balance point is low, the tendencies are generally small but positive. The GCTM simulations of the preindustrial ozone show that in the upper troposphere the presentday ozone tendencies are greater than the simulated preindustrial tendencies. In the boundary layer and in the midtroposphere the present-day tendencies are greater near anthropogenic NO x sources and smaller (generally more negative), due to higher ozone levels, in regions not affected by these sources.