The role of methane in tropospheric chemistry

Abstract

While methane is chemically quite inert to reactions with atmospheric molecular species, it does react with atomic species and molecular radicals. Because of its relatively large abundance in the global troposphere and because of the apparent annual increase of its concentration, it is worthwhile exploring the role of methane in tropospheric chemistry, especialy the role in air pollution.

From a detailed analysis of the reactions of methane with atmospheric species we conclude that while methane does play a role in generating the general background of ozone that permeates the global troposphere, it plays only a minor role in urban and rural ozone formation. In urban areas, most of which fail to meet the ozone standards established by the Clean Air Act, the prime facilitators of ozone formation are various higher hydrocarbon emissions. In rural and forested areas, biogenic emissions of terpenes, pinenes and isoprenes take a dominant role in creating ozone via their peroxy radical. Thus, the omission of methane in urban and rural ozone modeling is justified. The models should use a general ozone background level which in most areas of the U.S. amounts to 10-30 ppb. This background ozone may be due in part to global methane concentrations. Further, for ozone control purposes in exceedance areas, little would be gained by monitoring methane at the emission sources or in the ambient.

Annually between 400-800 million tons (MTy-1) of CH4 are emitted, of which 100-200 MTy come from enteric fermentation, 280-500 MTy-l from wetlands and rice paddies, 16-50 MTy-1 from fossil fuel leakage and combustion and the rest from other minor sources. These emissions produce in the northern hemisphere an annual average concentration of methane in air of about 1600-1700 parts per billion by volume, abbreviated henceforth ppb. The concentration appears to increase by about 1% y-l. For comparison, the tropospheric abundance of other hydrocarbons ranges from 0.1-10 ppb. In urban, polluted areas both methane and non-methane-hydrocarbon (NMHC) concentrations could be larger. Unfortunately, few measurements are available that include the whole spectrum of hydrocarbons.

The most important tropospheric reaction of methane is with the hydroxyl radical OH. This reaction initiates a chain that leads to the formation of the methyl peroxy radical CH302, and to a lesser extent to formaldehyde HCHO and methyl nitrate CH3 ONO 2 . The CH302 radical may be an important oxidant that converts atmospheric nitric oxide NO into nitrogen dioxide NO2 . Nitrogen dioxide is the major precursor of ozone 03. However, peroxy radical formation is much more rapid from higher alkanes, unsaturated hydrocarbons and aromatic hydrocarbons. The NMHC's react 25-830 times faster with hydroxyl than methane. The extent to which CH4 contributes to 03 concentrations is very much dependent on the ratio of ambient concentrations of NMHC/CH4. In our estimation, methane is a contributor to the general tropospheric ozone concentration or "background" ozone, which amounts to 10-30 ppb in the US. In rural and forested areas, where biogenic emissions of terpenes, pinenes and isoprenes are large, these hydrocarbons form rapidly peroxy radicals contributing to excess ozone formation over that of the general background. In urban areas, there are large emissions of higher hydrocarbons due to evaporation of fuels, solvents and paints, industrial (e.g. refinery) emissions, and incomplete combustion of fuels. In urban areas, we expect that these emissions are the prime facilitators of ozone formation, as well as the precursors of other "smog" ingredients, e.g. aldehydes, ketones, acids, organic nitrates and peroxyacetyl nitrate (PAN).