Sources
VOCs comprise volatile hydrocarbons and other organic molecules released into the atmosphere. They may have biogenic or anthropogenic sources. In the UK it is estimated that less than 5% of the VOCs (2.3 million tonnes per year, expressed in terms of carbon) emitted into the atmosphere are emitted from vegetation. The rest comes from transport, including distribution and extraction losses (50%), solvent use (30%) and other industrial processes (15%). Road transport alone accounts for 30% of VOC emissions. Not included in the above are emissions of methane, which although a volatile hydrocarbon, is present in the atmosphere from natural sources at much larger concentrations than other VOCs. The largest emissions of individual VOCs are of butane, toluene, pentane, propane, ethanol and 'white spirit'.
Chemistry and transport
VOCs range from very reactive to almost inert in the atmosphere, with some of the biogenic compounds (terpenes and isoprene) being oxidised within a few hours, and others with lifetimes of years. The main removal process is oxidation by reaction with the hydroxyl radical (OH), which is produced by the action of UV in sunlight on ozone. The oxidation of complex organic molecules leads to fragmentation, production of a range of reactive free radicals, and more stable smaller molecules such as aldehydes. These in turn are oxidised or photolysed to the eventual products of carbon dioxide and water. At night, when OH radicals are not present in significant quantities, other radical reactions (with the nitrate radical, for example) play a part. Molecules containing alkene bonds may also react directly with ozone, but more slowly than with the OH radical.
The wide range of reaction rates means that the range of transport distances is also large, extending from around 100 km to the whole troposphere. Unlike many inorganic pollutants, it appears that uptake and removal of VOCs by vegetation is rather slow, and has little impact on overall removal processes.
Issues
There are human health concerns about carcinogenic VOCs, the most important of which are 1,3-butadiene and benzene. Both come from industrial sources and from vehicle emissions. Target exposure limits for human health have been established in England & Wales to 5µg/m3, and in Scotland &Northern Ireland to 3.25µg/m3, both to be achieved by 2010 (concentrations based on an annual running mean).
Other concerns, applying to all VOCs, arise because of the role of VOCs in producing ozone in air polluted by nitrogen oxides. When VOCs react with the OH radical, they in turn are converted into reactive free radicals that react with nitric oxide. This reaction eventually leads to the production of ozone, and a recycling of free radicals to start the process over again. Both VOCs and nitrogen oxides are necessary for the production of ozone in sunlight. Controls have been introduced to reduce the emissions of both precursors, but the efficacy of such controls depends on a subtle balance of the relative ratios of nitrogen oxides and VOCs, and varies with time and place. Some VOCs are more reactive than others, so that even relatively small emissions of more reactive VOCs may contribute significantly to rates of ozone production.