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Ground-Level
Ozone
Ozone that occurs naturally in the upper
atmosphere surrounding the Earth provides a filter for the damaging ultraviolet
light emitted by the Sun. At ground level, ozone is harmful to living things.
Ground-level ozone is an air pollutant that damages human health, vegetation,
and many common materials. It is a key ingredient of urban smog.
Sources
Ground-level ozone is not emitted
directly into the air, but rather is formed by gases called oxides of nitrogen
(NOx) and volatile organic compounds (VOC), which in the presence of heat and
sunlight, react to form ozone. Ground-level ozone forms readily in the
atmosphere, usually during hot weather. As a result, it is known as a
"summer-time" air pollutant. Emissions of NOx are produced primarily
when fossil fuels are burned in motor vehicle engines, power plants, and
industrial boilers. There are hundreds of thousands of sources of VOC
emissions including automobile emissions, gasoline vapors, chemical solvents,
and consumer products like paints.
Health & Environmental Effects
Repeated exposure to ozone pollution for
several months may cause permanent structural damage to the lungs. Because
ozone pollution usually forms in hot weather, anyone who spends time outdoors
in the summer is at risk, particularly children, moderate exercisers, and
outdoor workers.
Even when inhaled at very low levels,
ground-level ozone triggers a variety of health problems including aggravated
asthma, reduced lung capacity, and increased susceptibility to respiratory
illnesses like pneumonia and bronchitis.
Ground-level ozone is also responsible
for 1 to 2 billion dollars in reduced crop production in the U.S. each year.
Because ground-level ozone interferes with the ability of plants to produce
and store food, they are more susceptible to disease, insects, other
pollutants, and harsh weather. Ozone also damages the foliage of trees and
other plants, ruining the appearance of cities, national parks, and recreation
areas.
Regional Transport
Under the Clean Air Act, EPA has set
acceptable levels, called National Ambient Air Quality Standards, for ozone in
the air we breathe. Some parts of the U.S. are currently unable to meet these
standards. These areas are described as "nonattainment" areas. Tens
of millions of Americans live in ozone "nonattainment" areas,
primarily in parts of the Northeast, Lake Michigan area, Atlanta, southeastern
Texas, and parts of California. Many of these nonattainment areas have focused
a great deal of effort on reducing VOC and, in some cases, NOx emissions from
stationary (factories) and mobile (vehicles) sources within their
jurisdictions. In several cases, emission controls are not producing the
reductions in ground-level concentrations of ozone needed to meet the national
health standard.
VOC+NOx+HEAT+Sunlight=Ozone
According to this
simplified equation, volatile organic compounds and oxides of nitrogen react, in
the presence of heat and sunlight, to form ozone.
Ozone "precursors," such as
NOx emissions, as well as ozone itself, can be carried hundreds of miles from
their origins, causing air pollution over wide regions. Although many urban
areas have made efforts to control ozone by reducing local NOx and VOC
emissions, incoming ozone transported from upwind areas also needs to be
addressed in order to meet the National Ambient Air Quality Standards. High
levels of ozone entering some nonattainment areas can make achieving the
national ozone standard difficult and costly, unless upwind sources are
identified and controlled. If these sources fall within a certain state's
boundaries, it can take measures to control them. If, as is often the case,
these sources fall beyond the political boundaries of that state, it must work
with EPA and other states to reduce air pollution on a regional scale. Often,
it is more cost-effective to reduce emissions from upwind sources than to
control emissions from smaller and smaller businesses in the nonattainment
areas being affected downwind.
Some regional strategies for reducing
ground-level ozone include:
- reducing NOx emissions from power
plants and industrial combustion sources
- introducing low-emission cars and
trucks
- burning gasoline reformulated to
reduce VOC, NOx, and other emissions.
Ozone, VOC, and NOx air emissions
from upwind industrial/urban areas contribute to ozone concentrations hundreds
of miles downwind in rural and other urban areas. When combined with local air
emissions, regionally transported ozone causes some areas to exceed the
National Ambient Air Quality Standards (NAAQS) for ozone.
Ground-level ozone is also responsible
for 1 to 2 billion dollars in reduced crop production in the U.S. each year.
Because ground-level ozone interferes with the ability of plants to produce
and store food, they are more susceptible to disease, insects, other
pollutants, and harsh weather.
U.S. Environmental Protection Agency
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