Atmospheric CH4, CO, and CO2

The chemistry of atmospheric CH4, CO, and CO2 is treated with a one-dimensional model incorporating the effects of eddy diffusion in the altitude region of 0–120 km. Methane is well mixed up to about 20 km, and its mixing ratio declines steadily at higher altitudes. Carbon monoxide is well mixed in the troposphere. Its mixing ratio decreases initially in the lower stratosphere, reaches a minimum value of the order of 10−8 near 30 km, and increases steadily at higher elevations. Carbon dioxide is well mixed in the troposphere. Its mixing ratio declines somewhat in the stratosphere, and this decline reflects the importance of anthropogenic production of CO2 and the relatively long time constants for stratospheric diffusion. Mixing ratios of CO and CO2 are comparable near 120 km. Combustion is a minor source of CO in comparison with oxidation of CH4. Oxidation of CH4 also provides a significant source of stratospheric H2O and H2. The principal sinks for CO and CH4 involve reactions with OH. Carbon dioxide is removed at the surface and to a lesser extent in the atmosphere by photolysis. Mean atmospheric residence times for CO and CH4 are 0.3 and 7 years, respectively.