Jean-François Lamarque, Peter Hess, Louisa Emmons, and John Gille Figure 2 Days since June CO Mixing ratio (ppbv) See description above. AsiaNorth America Europe Figure 1 CO Mixing ratio (ppbv) The top left panel represents the contribution from the source over Southeast Asia, the top right panel the source over North America, the middle left panel the source of Europe, the middle right panel the rest of the surface sources (blue), the contribution from the initial conditions (black), and the contribution from the hydrocarbon oxidation (red), and the bottom left panel the total from all CO colors. In the first three panels, the accumulated contribution from each month is separately shown, as well as the total (black). The horizontal axis is days, from June and the vertical axis is CO concentration in ppbv. Spatial and temporal origin of CO over North America during the TOPSE campaign Abstract. We present the results of a model study of the origin of the carbon monoxide distribution over North America during spring of The origin relates to the geographical distribution (North America, Europe, Southeast Asia, and rest of the world) and temporal (October 1999 to May 2000) of the surface CO sources. For that purpose, the model keeps track of each source separately, with each CO reacting with OH. The analysis focuses of the relative distribution of each CO “color”. It is clear from this model study that the surface CO in the high latitudes of the Northern hemisphere comes from Europe. Model description. For this study we use the Model for Ozone And Related Tracers (MOZART) developed at NCAR [Brasseur et al., 1998; Hauglustaine et al., 1998; Horowitz et al., 2001; see Horowitz’s poster, Session A32B, titled: A Global Simulation of Tropospheric Ozone and Related Tracers: Description and Evaluation of MOZART, Version 2]. In its standard version, MOZART provides the distribution of 56 chemical constituents (including non-methane hydrocarbons) between the surface and the upper stratosphere. In this study, in order to reduce computational cost, we use the model at a horizontal resolution of ~ 5.6º (T21) in both latitude and longitude. The continuity equations for these species are solved and account for advection, convection, and diffusive transport, as well as surface emissions, photochemical conversions, and wet and dry deposition. The evolution of species due to all physical and chemical processes is calculated with a timestep of 20 min. For this study, we use analyzed winds provided by the National Center for Environmental Predictions (NCEP). The vertical discretization consists of 28 sigma levels from the ground to ~ 2 hPa. The simulation described in this study covers the period June 1999 through May Tagging of different CO. In order to follow the different contributions to the total CO, we split the different sources into their geographical origin (Southeast Asia, North America, and Europe) and their time of release (from October 1999 to May 2000). This amounts to a total of 24 different CO “colors”. In addition the initial conditions, the rest of the surface emissions, and the contribution from hydrocarbon oxidation are also tagged. Each color of CO reacts with OH so that the total chemistry is unaffected by the split of CO [Lamarque et al., 1996]. Also, each color is deposited at the surface. Comparison with a simulation in which only a single CO (containing all surface sources and hydrocarbon oxidation) indicates minor differences with the results presented in this study (not shown). These differences are related to the impact of transport on the steeper gradients found in the case of CO “colors”. Results. 1.Analysis over the TOPSE region (40 o N < latitude < 60 o N) The distributions of the different CO colors, averaged between the surface and 400mb and between 110 o W and 90 o W are shown in Figure 1. It must be mentioned that the total CO distribution (lower left panel) presented in this study is consistent with the results presented by P. Hess, which in turn are very close to the TOPSE observations of CO. The maximum total contribution is obviously from North America due to its proximity to large sources. By the end of the simulation, the total contribution from Europe and Asia are comparable. These last two tend to contribute a constant amount over the last 3 months of the simulations, while the North American contribution exhibits more variability. Only the contributions from March and later have significant values, as expected from the lifetime of CO. Because of its recent release, the May source over North America is the largest contributor over this area at the end of the simulation (May ). The March and April sources contribute some very high peaks, followed by a rapid decay, due to the transport around the globe before contributing over this region. Although still a significant fraction of the total, the contribution from the rest (middle right panel) of the surface sources (including all the surface emissions prior to November 1999) decreases steadily from November on, indicating that the three considered regions of surface emissions are the main contributors. The contribution from the initial conditions (plus the stratospheric CO) is relatively small within a few months after the simulation start. There does not seem to be a particular strong seasonal cycle in the contribution from hydrocarbon oxidation. This is probably due to the fact that most of the production is in the tropical regions. 2.Analysis over the TOPSE region (latitude > 60 o N) Results for this area are presented in Figure 2, using the same format as in Figure 1 (see above). In contrast to the previous results, Europe (middle left) is now the main contributor to the CO distribution and rapidly (by January 2000) reaches its maximum contribution. Similarly, the American (upper right) contribution is mostly flat from January on. On the other hand, the contribution from the Asian source (upper left) keeps increasing over the considered, even when the total CO (bottom left panel) is decreasing. Because of the longer lifetime of CO in this region, the contribution from January and February is still a significant fraction of the total, for the European emissions in particular. At all times, the CO from the European source exhibits more variability than from any of the other two regions. This indicates the very episodic nature of the transport. It is interesting to note that while the CO from October and November are rapidly almost equal for the Asian and American sources, the convergence takes much longer for the European source. This is due to a larger source amplitude variation over that particular region. 3. Isentropic view of transport To put the results presented before in a larger perspective, we show in Figure 3 the monthly-averaged total accumulated CO distribution from each sector (Southeast Asia, North America, and Europe), zonally-averaged, in a latitude-isentropic coordinate system. Results are shown for January (Figure 3a) and May 2000 (Figure 3b). Contours are from 0 (dark blue) to 80 ppbv by 5 pbbv. In both months, the largest contributor at the surface in the highest northern latitudes is Europe. Europe and North America are similar in contribution in the mid-troposphere. An indication of quasi- isentropic transport is the fact that isocontours are horizontal for the most part, particularly in winter. In the upper troposphere, the contribution from Asia is relatively larger in January than May due to the existence in May of a barrier around 40 o N which makes the isentropic transport on the 310K much less possible than in winter. There is also a similar albeit smaller barrier at 65 o N in January. References Brasseur, G. P., D. A. Hauglustaine, S. Walters, P. J. Rasch, J.-F. Müller, C. Granier, and X. X. Tie, MOZART: A global chemical transport model for ozone and related chemical tracers, 1, Model description. J. Geophys. Res., 103, 28,265-28,290, Hauglustaine, D. A., G. P. Brasseur, S. Walters, P. J. Rasch, J.-F. Müller, L. K. Emmons, and M. A. Carroll, MOZART: A global chemical transport model for ozone and related chemical tracers, 2, Model results and evaluation, J. Geophys. Res., 103, 28, ,336, Horowitz, L., et al., A global simulation of tropospheric ozone and related tracers: Description and evaluation of MOZART, version 2. To be submitted to JGR, 2001 Lamarque, J.-F., G. Brasseur, P. Hess, and J.-F. Müller, 1996: Three-dimensional study of the relative contributions of the different nitrogen sources in the troposphere. J. Geophys. Res., 22,955-22,968. Figure 3a (January 2000) Figure 3b (May 2000) Latitude Potential temperature (K) Conclusions By use of a chemistry-transport model, we have identified some transport patterns for carbon monoxide in the Northern hemisphere. During the TOPSE period, the largest fraction of the surface CO at high latitudes originates in Europe. In mid- troposphere, the contribution from North America is of the same order as the European one. Finally, in winter, Asian emissions are blocked by a barrier around 40 o N. Europe AsiaNorth America