Every product we use may have some impact on the environment. The air that we breathe, the water we drink and the food we eat all contain naturally occurring chemicals. For example, the smell of the forest is due to a mixture of certain organic chemicals. Many of these chemicals are classified as volatile organic compounds or VOCs. These chemicals are not harmful in their own right, but if they mix with nitrogen oxides (NOx) under the right conditions (presence of sunlight and warm temperatures) they can react to produce ground-level ozone that can have harmful effects on human health.
VOCs may contribute to the formation of ozone in a complex series of reactions that requires the presence of heat, sunlight, and nitrogen oxides or NOx, whose primary source is automobile exhaust.
To reduce ozone levels in the atmosphere, both VOC and NOx emissions can be controlled; but, depending on their relative amounts, one component or the other will be the most effective target. For instance, in a NOx rich urban environment, the rate of ozone formation is directly related to the amount of VOC available. The opposite may be true in rural environments where VOCs are in excess and the removal of NOx translates into an ozone decline.
The VOCs emitted into an urban environment come from the following sources:
- Vegetation - biogenic sources are generally important in rural environments, but some cities in the Southeast have major emissions from trees and plants.
- Mobile - mobile emissions come from the transportation sector and are the result of evaporative and exhaust emissions from the use of on-road and off-road vehicles; including trains, planes, watercraft, and construction equipment.
- Stationary - these emissions are from fixed locations and include the fugitive releases of VOCs from industrial and nonindustrial locations that use or manufacture volatile chemicals
- Area - area sources of VOCs emissions include the widely dispersive releases associated with the use of consumer and commercial products.
Although each of these sources contributes to the atmospheric burden of VOCs, some sources are more amenable to control than others, especially in an urban environment. Vegetative sources, for example, cannot be reasonably controlled; and cities like Atlanta where biogenic emissions dominate the landscape, need to extract a larger percentage of their VOC reductions from the remaining source categories.
Although the relative contribution of the three major VOC sources can vary according to location, the pie chart for the Sacramento nonattainment region is reasonably representative of the distribution observed in many urban environments. As shown, mobile emissions are the dominant source of VOCs in most situations. Solvent VOC emissions from the three area sources categories are by comparison far lower, comprising about 19% of the overall total. The solvent VOCs emitted from individual area sources each contribute to less than 10% of the total emission inventory. Though the solvent VOCs from these product areas are not major contributors, they are potentially controllable sources that are often addressed in any ozone attainment plan.