The various sources of atmospheric CO are shown in Table 2.1. Oxidation of carbon bearing compounds in the atmosphere represents at least half of the sources of CO, with oxidation of terpenoids, (C5H8)n, potentially representing the largest single chemical source. However, at the present time, because of uncertainty in the global strength of the terpenoids and the details of their oxidation scheme, this is perhaps the most uncertain of the chemical sources. Isoprene (C5H8) is one of the most abundant of the terpenoids and it is produced mainly from various types of trees (Duce et al., 1983).
Oxidation of the terpenoids produces many products, among which formaldehyde (HCHO) and acetaldehyde (CH3CHO) are perhaps the most important. Further oxidation of the aldehydes leads to production of CO. In general the terpenoids have very short lifetimes, about 0.1 to 1 day, to destruction by OH and O3 (eg. Hov et al., 1984). These sources, then, will tend to be very localized. However, the products and the CO itself have long enough lifetimes that the net effect will be much more distributed (e.g. Henderson et al., 1989).
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) (see Appendix B for details). The lifetime of CH4 (about 7 years) is long enough that CH4 has a relatively uniform distribution around the globe. CH4 reacts with OH to ultimately produce HCHO (Levy, 1971) which then reacts with OH or is photolyzed within a day to produce CO. The source of the uncertainty indicated in Table 2.1 is associated with uncertainty in NOx mixing ratios since if NOx abundances are low, peroxides may be formed and subsequently rained out with no production of CO.
The oxidation process may also result in the net production of HOx radicals. In addition, if NOx levels are sufficiently high (approx. > 50 ppt) then O3 may be produced; at lower NOx levels CO and CH4 oxidation will result in the destruction of O3 (Logan et al., 1981).
As shown in Table 2.1, other non-methane-hydrocarbons (NMHC) may also contribute significantly to the global budget of CO. HCHO and CH3CHO may be produced subsequently producing CO, in a similar manner to the terpenoids. However, other products such as formic and acetic acids may also be formed and they are either rapidly rained out or result in CO2 production directly (Henderson et al., 1989).
CO is also produced in large quantities by burning of fossil fuels and by biomass burning. The contribution of the latter, although quite uncertain, could be very important.
Direct production by biological activity also contributes substantially to the CO atmospheric budget.