Non-methane Organics in the Remote Troposphere

Abstract

The atmosphere is now understood to contain many gaseous species at concentrations varying by many orders of magnitude. At one end of the concentration scale, gases such as oxygen are vital to sustaining animal respiration, whilst at the other end, trace species such as nitrogen oxides control fundamental geochemical phenomena such as the earth’s ozone shield and the level of hydroxyl radicals present in the troposphere. The most numerous trace gases present are organic molecules which have a variety of sources including combustion processes, biogenic decay on land and in the ocean, solvent usage, natural gas leakage, etc. The measurement of these many compounds at concentration levels down to parts in 10¹² and below has presented atmospheric scientists with severe analytical problems. These are now being solved by using a variety of modern techniques based upon spectroscopy and gas chromatography. Data on the distribution of many compounds is still exceedingly scarce. This is urgently needed to quantify major sources and to test current theories of atmospheric oxidation leading to the creation of soluble molecules capable of being removed by rain. Some of the limited amount of concentration data available is discussed by compound type: namely, halocarbons, hydrocarbons, oxygenated compounds, and sulfur compounds. The main object of this discussion is an attempt to find some consistent indicators for the degree of chemical reaction which can be attributed to attack by hydroxyl radicals in the troposphere. It appears that the level of oxidation indicated by the latitudinal distribution of molecules such as perchloroethylene and acetylene, and possibly benzene, toluene, and some other compounds, is considerably less than that predicted from a study of the carbon monoxide cycle. However, much more data on the distribution of these molecules throughout the troposphere are needed before final conclusions can be drawn. A low oxidation rate for many molecules tends to lower the amounts that can be removed each year by natural processes occurring in the atmosphere. This makes pollution more of a problem, of course, and at the same time it affects the current assessments of the efficiency of the biosphere for producing trace gases in the atmosphere, including those involved in the creation of condensation nuclei by direct gas to particle conversion processes. Finally, there are indications from a comparison of the concentration distribution of acetylene and sulfur dioxide, both of which are largely pollution derived, that oxidation of soluble species in atmospheric droplets may be a very efficient process, possibly by reaction with hydrogen peroxide.