Sources
Vegetation fires and technological activities (e.g. fossil fuel and biofuel burning, industrial activities, waste disposal) are sources of CO and of non-methane volatile organic compounds (NMVOC). Soils and oceans are also small sources of carbon monoxide. Vegetation is a large source of NMVOC (isoprene, terpenes, and other compounds). Emissions of methane are not indicated for clarity. Methane is present in the troposphere at an approximately constant mixing ratio (1.7 ppm=1.7 parts per million) owing to its long lifetime of about 10 years.
Atmospheric oxidation
CH4 and the NMVOC are oxidized in the atmosphere mostly by the hydroxyl radical (OH), which is the main cleansing agent of the troposphere. Ozone and the nitrate radical are also important oxidants for a number of unsaturated NMVOC (e.g. terpenes). The oxidation of organic compounds generates a plethora of intermediate compounds like carbonyls, alcohols, peroxides, ketones, and nitrates. Their fate is not always well understood.
Some of these oxygenated intermediates have very low volatilities in atmospheric conditions, and they can condense on existing aerosols, and in some cases they can even nucleate to form new particles. As such they play an important role in the climate system and the formation and fate of the clouds.
The global production of Secondary Organic Aerosols (SOA) is so far very uncertain. Although the ultimate step in the gas-phase oxidation mechanism of any VOC is production of CO2, a significant fraction of the NMVOC by-products (e.g. SOA but also gas-phase organic acids, alcohols etc.) is expected to be deposited on the Earth's surface by wet deposition (washout in clouds, rainout by rain droplets) or dry deposition (direct sticking on the Earth's surface or on vegetation foliage).
