Tropospheric budgets for methane, carbon monoxide, and related species

A tropospheric photochemical model is presented that provides a homogeneous gas phase radical chain mechanism for the removal of tropospheric methane, carbon monoxide, molecular hydrogen, and formaldehyde. The chain reaction that destroys methane serves as the tropospheric source for formaldehyde, which in turn serves as the major source of tropospheric carbon monoxide and molecular hydrogen. Simple relationships between methane and its destruction products are derived that give a surface mixing ratio for carbon monoxide of 0.1 ppm, independent of the radical number densities, and an upper limit for formaldehyde mixing ratio of 1 ppb. Detailed calculations of total column production and loss rates based on earlier calculated altitude profiles for the number densities of the hydroxyl radical and formaldehyde yield the following: For methane, a total column loss rate of 4.5 × 1011 molecules/cm2 sec and a tropospheric residence time of 2 years; for carbon monoxide, a total column production of 4.5 × 1011 molecules/cm2 sec, a tropospheric residence time of 0.1 year (assuming the measured value of 0.12 ppm for the carbon monoxide mixing ratio), and a uniform mixing ratio of 0.07 ppm based solely on atmospheric destruction and production; for molecular hydrogen, a column production rate of 1.6 × 1011 molecules/cm2 sec, a uniform mixing ratio of 0.7 ppm, and a tropospheric residence time of 2 years.