The current global surface distribution of CO (Figure 2.2) is asymmetric, with average surface northern hemisphere mixing ratios being about a factor of 3 higher than those of the southern hemisphere. Highest values occur at northern middle latitudes presumably reflecting the large anthropogenic source here but perhaps also the terpenoid source. It is expected that equatorial regions will also contribute significantly to CO production. However, the relatively constant surface CO mixing ratios in tropical regions may reflect stronger loss processes because of higher OH levels. In general, the surface values reflect the relative differences in the hemispheric source strengths, with most of the biosphere related sources and anthropogenic sources occurring in the northern hemisphere.
The CO distribution versus latitude and pressure obtained by Seiler and Fishman (1981), Figure 2.3, shows explicitly the influence of equatorial source regions where there is rapid influence of equatorial source regions and rapid transport of surface sources to higher altitudes. It also clearly shows that CO mixing ratios are quite variable in the vertical, and adequate resolution will be required to obtain these details. The interhemispheric distribution at higher altitudes appears more uniform than the surface mixing ratios according to the fall data of Reichle et al. (1986) (Figure 2.4), taken with the MAPS instrument on the shuttle STS-2 flight (November, 1981). However, their data also shows substantial latitudinal and longitudinal variations that presumably reflect rapid vertical transport of CO from surface or mixing layer sources. Interestingly, there is no obvious signature for mid-latitude Northern anthropogenic sources in their data indicating the important role of dynamics in determining the distribution.