MOPITT Mission Description Document (30 October 1993)

B CO and CH4 Chemistry

B.1 Detailed Equations of OH Chemistry

One of the major cleaning agents in the atmosphere is OH whose primary source in the daytime background troposphere is

(Levy, 1971). OH is transferred to HO2 by reaction with O3

and CO

The HOx (OH + HO2) thus formed are lost via reaction with HO2,

or HNO3

(followed by washout of HNO3), and by formation

and rainout of H2O2. Depending on the magnitude of the NO mixing ratios, the reaction

may also be important in the HOx cycle. The above cycle is intimately related to the production and destruction of O3 in the troposphere. If NOx mixing ratios are elevated then the oxidation of CH4 or CO will result in production of O3, otherwise O3 is destroyed (Logan et al., 1981). Clearly CO plays an importance in the chemistry and climate of the troposphere and long term monitoring of CO proves a means of assessing and isolating the various elements associated with global change.

B.2 Detailed Chemistry of Methane

The distribution of Ch4 and its oxidation paths are much better defined than those of the terpenoids (McConnell et al., 1971, Wofsy et al., 1972). The life time of CH4 (approx. 7 years) is long enough that CH4 has a relatively uniform distribution around the globe. CH4 reacts with OH to produce CH3 which associated rapidly with O2 to produce CH3O2

which may then react with NO or HO2 to form CH3O

(A)

(B)

Formaldehyde (HCHO) is then formed by the reaction with O2

Photolysis of HCHO results in CO formation

The uncertainty in the production rate relates to the abundance of NO in the atmosphere. If NO has a high mixing ratio the oxidation scheme will proceed via (A) above with no loss of CO from the scheme. However, if NO densities are relatively low, scheme (B) will apply and CH3OOH will be produced and this may be washed out.