Volatile organic compounds (VOCs) are organic molecules - specifically hydrocarbons - that are classified as a pollutant as they produce undesirable effects in the atmosphere. They are defined as volatile because they evaporate quickly and easily into the air. They react with nitrogen oxides in the air to form ground level ozone, another harmful pollutant, and they also contribute to the formation of secondary particulate matter.
VOCs occur in many forms and are emitted from both human sources, such as power plants, and natural sources, such as trees and plants. Although natural sources account for about 85% of the VOCs in the air, the more reactive and therefore concerning ones are those produced by human activities. Especially in cities where there less plants, the dominant emissions of VOCs come from anthropogenic sources, as shown in Figure 1. These compounds play a key role in the formation of ozone and photochemical smog, which are very harmful for human health.
Visit the UC Davis ChemWiki here to see how VOCs contribute to smog by the production of ozone.
VOCs occur in many forms (some more harmful than others) and different sources emit different forms. Such sources include petroleum use, trash incineration, and evaporation of industrial chemicals. Gasoline alone contains almost 100 different hydrocarbons including iso-octane, heptane and ethane. A major contributor to VOCs is the evaporation of various hydrocarbon-rich liquids such as gasoline from car tanks or refueling stations, industrial solvents such as oil-based paint, barbecue starter fluid, and cleaning products.
As stated before, VOC emissions from vegetation is more than 5 times that of human sources (in Canada, as a whole). However this number greatly decreases towards large cities, until it is human sources accounting for most of the harmful hydrocarbons. In densely populated cities such as Los Angeles or Mexico City the smog produced in part by VOCs has become a major problem, and the health effects associated with it are increasingly worrisome.
The emissions of volatile organic compounds has diminished in recent years due to public awareness and improved regulation. Many gasoline stations use vapor-recovery systems to minimize the VOCs emitted from the pumping of gas. Every new vehicle sold is also equipped with a catalytic converter, which chemically alter harmful VOCs and nitrogen oxides into less harmful pollutants. Many scientists also suggest to pump gasoline at night to reduce the amount of VOCs that can react with sunlight.
The data visualization below shows a recent decrease in VOC levels in Canada. It can also be altered to view various other pollutants, most of which have been declining as well. For a more in-depth look at pollution data, including a graph showing where VOC emissions primarily come from, click here.