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Mobile Sources

Sources of Toxic Air Pollution

Emissions Inventory: As part of the Cumulative Exposure Project (CEP), the U.S. EPA prepared a comprehensive list of air toxics emissions for the entire country in 1990. This is the first time that such a list (known as an emissions inventory) had ever been prepared on a national scale. Although there are bound to be some errors in the details of a massive undertaking such as this, a summary of the emissions inventory can give us some indication of what may be the most important sources of air toxic emissions in Vermont.



Mobile Sources: On and off road vehicles and aircraft.

Area Sources: Burn barrels, gasoline filling stations, woodstoves, paint stripper, surface coatings, drycleaners, industrial boilers, etc. (small stationary sources)

Point Sources: Manufacturing operations (large stationary sources).

In the late 1990s, the U.S. EPA undertook the National-scale Air Toxics Assessment (NATA). This assessment, based on the 1996 national emissions inventory, yielded results useful in understanding the amounts and possible health effects of 33 toxic air pollutants plus diesel particulate matter. (For more information regarding this list of federal hazardous air pollutants (HAPs), please click here). The analysis consisted of four primary steps. These include: (1) an emissions inventory which estimates the release of the 33 HAPs nationwide from mobile, area, and point (or major) sources (2) modeling estimates of the concentrations of these pollutants in the air (3) estimates of the exposure of populations to the HAPs and (4) the resultant risk of both cancer and noncancer health effects due to exposure to these toxic pollutants.

Similar to the 1990 CEP inventory results, the 1996 NATA inventory indicates that approximately 53% of air toxics still come from mobile sources while 47% of toxic air emissions come from area sources. The CEP categorized sources as point, area and mobile. Due to the 1990 Clean Air Act Amendments (CAAA), the definition of the source categories was revised to include major, area and mobile sources. Vermont has very few large stationary sources that are classified as "major" under the CAAA. Therefore, our small industrial sources are classified as "area" sources and hence the difference between the 1990 CEP pie chart above and the 1996 NATA pie chart below.



Eight of the top 10 air toxic pollutants in Vermont, excluding methyl chloride (chloromethane) and styrene, are considered in the National Air Toxics Assessment. NATA divides emission sources into four categories. They are major sources (large point sources), area and other (small point sources), onroad mobile sources and nonroad mobile sources. For simplicity's sake, onroad mobile and nonroad mobile were combined in the following pie charts. These pie charts depict the estimated statewide Vermont emissions in 1996 by source category.


Pie Charts of 1996 Air Toxic Pollutant Emissions by Source Category
(as estimated by NATA)


1,3-Butadiene Mercury
Carbon Tetrachloride Methylene Chloride
Chloroform Tetrachloroethylene

Unlike many larger states, a great majority of the toxic air pollutants emitted in Vermont come from mobile sources and smaller area sources, as the pie charts indicate. In fact, according to the federal definition of major source, Vermont  has very few major sources of HAPs. Based on the 1996 NATA results, the operation of mobile sources is responsible for a majority of the benzene (85%) and 1,3-Butadiene (67%) emissions in the state as well as approximately 49% of the primary formaldehyde emissions. The remainder of these three pollutants is emitted by area and other sources. In 1996, sources within the area and other category emitted 100% of the carbon tetrachloride, chloroform, methylene chloride and tetrachloroethylene and nearly 90% of the mercury compounds.

Specific sources for the top 10 air toxic pollutants in Vermont are listed below:

1. Benzene Benzene in the atmosphere comes from numerous sources. Motor vehicles are considered a significant source of benzene. Benzene is present in both evaporative emissions and in exhaust emissions from motor vehicles. Refueling of motor vehicles is another source of benzene. Benzene is also released by industries in the state and is a component of fossil fuel emissions, including wood stoves. Benzene in Vermont appears to be locally generated as the highest concentrations are observed in urban areas and the concentrations decrease as the sites become more rural. Benzene has an atmospheric half life of 10-12 days.
2. 1,3-Butadiene The primary source of 1,3-butadiene is motor vehicles. 1,3-Butadiene is formed in vehicle exhaust due to incomplete combustion of fuel. Other sources of 1,3-butadiene are waste incinerators and wood fires. The 1,3-butadiene concentrations observed appear to be locally generated as the concentrations are highest in the urban areas of Burlington and Rutland with levels decreasing as the sites become more rural. 1,3-Butadiene has a short atmospheric half-life of 4-6 hours, which also indicates that observed levels are locally generated, as opposed to transported from outside the state.
3. Carbon Tetrachloride Carbon tetrachloride in the atmosphere in Vermont is probably due to transported pollution. The compound was used extensively in degreasers, cleaning fluids and fire extinguishers until it was withdrawn from the market in the 1960s. Carbon tetrachloride is still used as a refrigerant and an aerosol propellant but, these uses are being phased out. Carbon tetrachloride appears to be a regional or transported pollutant as the concentrations do not vary significantly from urban sites to rural sites. The atmospheric half life of carbon tetrachloride is 50-100 years, indicating that it will take a long time for current concentrations to decrease.
4. Chloroform The amount of chloroform normally expected to be present in air ranges from 0.02 to 0.05 ppb (0.1 to 0.2 g/m 3 ). Chloroform has been found in the air from all areas of the United States. Sources of chloroform in the atmosphere are pulp and paper mills and water and wastewater plants that use chlorine as a disinfectant. Chloroform appears to be a transported pollutant. The concentrations do not vary significantly from urban to rural sites and the compound has an atmospheric half life of 2-3 months.
5. Formaldehyde Formaldehyde has numerous atmospheric sources. The compound is a byproduct of combustion, which covers a wide range of sources from internal combustion engines to wood stoves. Formaldehyde is also generated by wood processing plants and glues. Formaldehyde is produced by the atmospheric reactions of other pollutants, including 1,3-butadiene. Formaldehyde in Vermont follows the locally generated pattern. Concentrations are highest in urban sites and decrease at rural locations (except for Winooski 1995). The atmospheric half life of formaldehyde is short, 4-10 hours.
6. Methyl Chloride Outside air contains less than 0.001 ppm (2 g/m 3 ) and city air contains up to 0.001 ppm methyl chloride. These levels are much lower than the levels shown to have toxic effects . Methyl chloride is naturally released into the atmosphere from oceans and biomass, producing low ambient concentrations. Other sources of methyl chloride are wood burning and chlorinated swimming pools. The methyl chloride in the outdoor environment, however, is almost totally from natural sources. Methyl chloride appears to be a transported pollutant. The variation in concentrations between urban and rural sites is not significant and the atmospheric half life is fairly long: 1-2 years.
7. Methylene Chloride Methylene chloride comes from both transported and locally generated sources. Local sources are regulated industrial sources and area sources. Area sources include businesses where methylene chloride is used as a general solvent such as garages and the compound is also found in spray cans and furniture strippers. Methylene chloride has a fairly long atmospheric half life, 3-4 months, indicating the fairly long persistence typical of transported pollutants.
8. Tetrachloroethylene Sources of tetrachloroethylene are local area and point sources. Tetrachloroethylene is used in many industries as a general solvent. The compound is used extensively in dry cleaning and is also produced by waste incinerators. Tetrachloroethylene concentrations are highest at urban sites and decrease at rural sites, indicating that the compound is locally generated. The atmospheric half life is 70-100 days.
9. Mercury Sources of mercury include both transported and local emissions. Waste incineration is a local source of atmospheric mercury.
10. Styrene Styrene sources include local industrial point sources and area sources such as vehicle exhaust and auto body shops. Styrene has a fairly short atmospheric half life, 6-7 hours, indicating that it is probably a locally generated, rather than a transported pollutant.


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Last Updated: 1/22/03


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