Transformation, There are two sources of air pollutants:
Natural
Trees and bushes emit millions of tons of volatile organic compounds in the air. Volcanoes spew out ash, acid mists, hydrogen sulfides and other toxic gases. Pollen, spores, viruses, bacteria and other small bits of organic compounds are natural sources of air pollution. Bacterial metabolism of decaying vegetation in swamps and of cellulose in the guts of termites and ruminant animals is responsible for as much as two-thirds of the methane (natural gas) in the air.
Anthropogenic (Human caused air pollution)
There are two different types of human caused pollution:
*Primary pollutants or point source pollutants are those released directly from the source into the air in a harmful form.
*Secondary pollutants are modified to a hazardous form after they enter the air or are formed by chemical reactions as components of the air mix and interact. Solar radiation often provides the energy for these reactions. Photochemical oxidants and atmospheric acids are both secondary pollutants produced by this mechanism.
There are six Primary "Criteria" Pollutants (Regulated by EPA under the Clean Air Act of 1970 as amended)
These are particulate materials, lead, nitrogen oxides, carbon monoxides, volatile organic compounds (VOC) and sulfur dioxides. However, the two primary "criteria" pollutants of most concern to this area are the precursors of ozone, nitrogen oxides and VOCs.
Nitrogen Oxides (NOx)
Nitrogen Oxides are highly reactive gases formed when nitrogen in fuel or combustion air is heated to temperatures above 650 degrees celsius. NO oxidizes further in the atmosphere to nitrogen dioxide (NO2), a reddish brown gas that gives photochemical smog its distinctive color. Because of their diverse forms, the general term NOx is used to describe these gases. The major sources of man-made NOx emissions are high-temperature combustion processes, such as those occurring in automobiles and power plants. Home heaters and gas stoves also produce substantial amounts of NO2 in indoor settings. Natural sources of NOx are lightning and soil microbes. Nitrogen oxides combine with water to make nitric acid (HNO3), which is also a major component of atmospheric acidification. They contribute to a wide range of environmental effects, including potential changes in the composition and competition of some species of vegetation in wetland and terrestrial systems, visibility impairment, acidification of freshwater bodies, eutrophication(i.e., explosive algae growth leading to a depletion of oxygen in the water) of estuarine and coastal waters (e.g., Chesapeake Bay), and increases in levels of toxins harmful to fish and other aquatic life.
Transportation and power plants are the two major producers of NOx.
Health effects of NOx
Short-term exposures (e.g., less than 3 hours) can lead to:
Changes in airway responsiveness
Increases in repiratory illnesses in children (5-12 years old)
Long-term exposures can lead to:
Increased susceptibility to repiratory infection
Lung alterations
Volatile Organic Compounds (VOCs)
Volatile Organic Compounds are organic chemicals that exist in gases in the air. Plants are the largest source of VOCs. About 400 million tons of methane (CH4) are produced by natural wetlands and rice paddies and by bacteria in the guts of termites and ruminant animals. These volatile hydrocarbons are generally oxidized to CO and CO2 in the atmosphere. In addition to these natural sources of VOCs, a large number of synthetic organic chemicals, such as benzene, toluene, formaldehyde, etc are released into the air by human activities. These chemicals play an important role in the formation of photochemical oxidants. Anthropogenic sources of VOCs are mainly unburned or partially burned hydrocarbons from transportation, power plants, chemical plants, and petroleum factories. Incomplete combustion implies than the products include carbon monoxide and volatile organic compounds:
Organic fuel + O2 =CO2 + CO + H2O + Volatile organic compounds + HEAT
Organic fuel is almost anything that burns.
Complete combustion can only be achieved under very controlled systems, but it results in: organic fuel+O2=CO2+H2O+Heat.
By achieving more complete combustion less VOC's are created, but more CO2 is created. If you burn anything in air, NOx is an additional product.
Source:http://epa.gov/oar/oaqps/gooduphigh
Not only are NOx produced from combustion from vehicles and industry but VOCs are largely produced by these sectors.
VOC Emissions, 1988-97
1988-97: 20% decrease
1996-97: no change
Source: epa.gov
Trends in NO2 and VOC Levels
Traditionally, the EPA has focused on reducing VOCs emissions in nonattainment areas. However, the EPA and the states have recognized a need for more aggressive measures in the reduction of NOx. Still, NOx and VOCs underscore the importance of this new approach. In fact, volatile organic compound emissions decreased 20 percent between 1988 and 1997, while NOx emissions decreased only 1 percent. Additionally, VOCs emissions from highway vehicles have declined 38 percent since 1988, while highway vehicle NOx emissions have declined 8 percent since their peak level in 1994. Further, between 1970 and 1997 emissions of VOCs have decreased 38 percent whereas emissions of NOx have increased 11 percent and NOx emissions from coal-fired power plants have increased 44 percent.
epa.gov
To determine whether our area needs to decrease our NOx or VOCs concentrations in order to become an attainment area, we must find our "position" on ozone isopleths.
The Ozone Isopleth
The concentration of ozone can be represented as a topographic plot, called an ozone isopleth; in this type of plot, ozone concentration is a function of NOx and VOC concentrations. The colored regions represent areas of equal O3 concentration.
Since emissions of NOx and VOC vary depending on geographic region, different places are “located” at various points on this isopleth.
Based on a ChemTest program used to determine concentration values, here is an example of an ozone isopleth:
Ozone isopleths are influential in showing that changes in ozone concentration (increases, decreases, or essential no change) depend on the original NOx and VOCs concentrations of a region.
The region with lower VOC concentration and higher NOx concentration (such as point A) is the VOC-limited region. In this region, altering the VOC concentration will alter the ozone concentration; however, altering the NOx concentration causes little change.
In the case of point A, increasing the VOC concentration results in a higher ozone concentration and decreasing the VOC concentration results in a lower ozone concentration; however, increasing or decreasing the NOx concentration results in little alteration of the ozone concentration.
The region with higher VOC concentration and lower NOx concentration (such as point B) is the NOx-limited region. In this region, changing the VOC concentration has little impact on the ozone concentration, while controlling NOx concentration does impact concentration of ozone.
Should we control NOx or VOC emissions
(which is the most effective policy)?
Since the triangle area is located in the “B” region (NOx-limited region), we must focus on the control of NO2 emissions to effectively reduce ozone concentration in our area. Additionally, we have more control over reducing NOx emissions since we are the ones responsible for the majority of the NO2 production. By comparison, making a major reduction in VOCs is a more difficult task (since lots of VOCs occur naturally) and may not even effectively reduce ozone
unc.edu ... original with diagrams, and charts |
