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Is Your Bus Exhausting? Stevens Institute of Technology


LEARN MORE: The Ozone Between Us

Ozone (O3) is a gas that occurs in two layers of the atmosphere. The stratospheric or "good" ozone layer, which extends upward from about 10 to 30 miles from the earth's surface, protects life on earth from the sun's harmful ultraviolet rays (UV-b). However, ozone found in the troposphere, the layer of the atmosphere that extends from the earth's surface to about 10 miles up, is deemed ground level or "bad" ozone. At ground level, ozone is an air pollutant that damages human health, vegetation, many common materials, and is a key ingredient of smog.

Ozone has the same chemical structure whether it occurs miles above the earth or at ground level. At ground level, ozone is formed when certain compounds react in the presence of direct sunlight.

VOCs + NOx + Sunlight = Ozone

VOCs (volatile organic compounds) sources include: household products such as paints, paint strippers, and other solvents, wood preservatives, aerosol sprays, cleansers and disinfectants, moth repellents and air fresheners, stored fuels and automotive products, hobby supplies and dry-cleaned clothing. Other sources outside the home include gasoline stations, autobody paint shops, and print shops. In addition to all the man made sources of VOCs, natural sources of VOCs exist. For example, trees naturally release small amounts of VOCs.

NOx (nitrogen oxide gases) sources include: automobiles, trucks and buses, and off-road engines such as aircraft, locomotives, construction equipment, gasoline-powered lawn and garden equipment. Other sources include chemical manufacturers, and combustion sources such as power plants burning fossil fuels.

When VOCs and NOx are released into the air, they begin to react. Sunlight and hot weather cause the reaction between VOCs and NOx in the air to speed up. One of the products of the reaction is ground level ozone. If the sunlight is very strong and the weather is very hot, sometimes harmful concentrations of ground level ozone are produced. As a result, ground level ozone is known as a summertime air pollutant.

Many urban areas tend to have high levels of ground level ozone. But even rural areas are subject to increased ozone levels because winds can carry ozone, and the pollutants that form it, hundreds of miles away from the original sources.

Southeastern and Mid-Atlantic cities are on the list of the highest ground level ozone cities in America, along with other cities, such as Los Angeles and Houston. Some of these cities suffer from high levels of ozone air pollution because of local traffic and industry. Other areas without major industry or large populations, suffer due to pollution transported by prevailing winds from other communities. Regardless of how the ground level ozone gets to the cities, it can pose health threats to all the inhabitants; people, animals and plants.

Often industry is blamed entirely for ground level ozone air pollution, but actually private citizens are responsible for a significant percentage of the air pollutants that lead to ozone smog. Motor vehicle emissions are the single greatest contributor to ground level ozone pollution.  In particular, diesel engines, like those found in school buses, emit high levels of NOx and particulate matter (PM), and in addition, a complex mixture of gases, many of which are known or suspected cancer causing agents.

Ground level ozone is believed to be the most common air pollutant and the cause of injury to the environment and human health. Although we have some control over sources of some of the ozone producing air pollutants, there is no control over the heat and sunlight that turns those pollutants into ground level ozone.

The United States Environmental Protection Agency (USEPA) coordinates ground level ozone information through the AIRNow web site. The web site coordinates real-time ground level ozone monitor readings from across the country and compiles the information into animations and forecast maps.  The location of monitoring stations reflects the quantity and accuracy of the data collected. Although ozone monitors represent ozone measurements over a large area, there are areas of the country that may not appear to have an ozone problem simply because there may not be any monitoring stations to report ground level ozone that may be present. The monitors have been placed in areas that have a higher likelihood to have ground level ozone present. It is not practical to place monitors everywhere due to the high costs (hundreds of thousands of dollars) of each monitor and the maintenance of the monitors once in place.


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