FIGURE 5.9 The seasonally changing global distribution of CO pollution observed by Terra MOPITT at an altitude of 700 hPa (about 3 km). Averages are shown separately for March 2006 (top) and September 2006 (bottom). High CO pollution levels are shown in red. In addition to chemical production, northern hemisphere pollution sources are predominantly urban and industrial, while high CO in the tropics and southern hemisphere often results from biomass burning. NOTE: ppbv = parts per billion by volume. SOURCE: Modified from Edwards et al. (2006). Reprinted with permission by American Geophysical Union, copyright 2006.