Near the planet's surface, CO varies with season, proximity to source regions, and latitude. Above the boundary layer, and in the middle and upper troposphere, CO also shows seasonal cycles and spatial distributions. At altitudes higher than a few kilometers, mixing ratios are largely determined by surface source distributions and by vertical and horizontal transport.29'30 Mixing ratios determined in the free troposphere at mountain observatories in the Northern Hemisphere are typically lower than measurements made at sea level at similar times and latitudes.18 CO mixing ratios in the free troposphere, studied from aircraft, show interhemispheric differences similar to those at the surface (higher levels in the north compared to the south).30'31 In the Northern Hemisphere, CO often decreases with height,31 reflecting the abundance of surface sources, but this may be seasonally dependent.32 In the Southern Hemisphere, CO may increase with altitude or remain relatively constant,31 and strong convective transport in the tropics can deliver CO to the middle and upper troposphere. Across the tropopause, CO mixing ratios fall below 50 ppb.33 Transport from the stratosphere brings air with low CO into the troposphere.
3 GLOBAL CO BUDGET 83
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