What Are The Processes Controlling Air Quality?

STRANGE BUT TRUE

Agriculture and industrial activity have grown dramatically along with the human population. Consequently, in parts of the world, increased emissions of pollutants have significantly degraded air quality. Respiratory problems and even premature death due to air pollution occur in urban and some rural areas of both the industrialized and developing countries. Wide-spread burning for agricultural purposes (biomass burning) and forest fires also contribute to poor air quality, particularly in the tropics. The list of culprits in the degradation of air quality includes tropospheric ozone, which is a toxic gas, and the chemicals that form ozone. These ozone precursors are nitrogen oxides, carbon monoxide, methane, and other hydrocarbons. Human activities such as biomass burning, inefficient coal combustion, other industrial activities, and vehicular traffic all produce ozone precursors.

The U.S. Environmental Protection Agency (EPA) has identified six criteria pollutants: carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, lead, and particulates (aerosols). Of these six pollutants, ozone has proved the most difficult to control. Ozone chemistry is complex, making it difficult to quantify the contributions to poor local air quality. Pollutant emission inventories needed for predicting air quality are uncertain by as much as 50 percent. Also uncertain is the amount of ozone that enters the troposphere from the stratosphere.

For local governments struggling to meet national air quality standards, knowing more about the sources and transport of air pollutants has become an important issue. Most pollution sources are local but satellite observations show that winds can carry pollutants for great distances, for example from the western and mid-western states to the east coast of the United States, and sometimes even from one continent to another. Observations and models show that pollutants from Southeast Asia contribute to poor air quality in India. Pollutants crossing from China to Japan reach the west coast of the United States. Pollutants originating in the United States can reduce air quality in Europe. Precursor gases for as much as ten percent of ozone in surface air in the United States may originate outside the country. We have yet to quantify the extent of inter-regional and inter-continental pollution transport.

Tropospheric ozone comes from several sources. Biomass burning and industrial activity produce carbon monoxide (CO) and volatile organic compounds (VOCs) which are oxidized to form ozone. Nitrogen oxides (NOx) from industrial processes, biomass burning, automobile exhaust and lightning also form tropospheric ozone. A small amount of tropospheric ozone also comes from the stratospheric ozone layer. This illustration is also available at a higher resolution. (Image courtesy of Barbara Summey, SSAI.)