Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals. Ground-level ozone, though less concentrated than ozone aloft, is more of a problem because of its health effects.
There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species.
Although ozone was present at ground level before the Industrial Revolution, peak concentrations are now far higher than the pre-industrial levels, and even background concentrations well away from sources of pollution are substantially higher.
The Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standard are required to take steps to reduce their levels. In May 2008, the EPA lowered its ground level ozone standard from 80 ppb to 75 ppb. This proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 ppb, and the World Health Organization recommends 51 ppb. Many public health and environmental groups also supported the 60 ppb standard. On the other hand, the EPA had already designated over 300 mostly urban counties as out of compliance, and lowering the standard to 75 ppb put hundreds more in non-compliance. Lowering it further to 60 ppb would likely have left most of the US in non-compliance. Manufacturers, employers, and others argued that the cost of compliance with the lower standard would be prohibitive. The EPA has also developed an Air Quality Index to help explain air pollution levels to the general public. Eight-hour average ozone concentrations of 85 to 104 ppb are described as "Unhealthy for Sensitive Groups", 105 ppb to 124 ppb as "unhealthy" and 125 ppb to 404 ppb as "very unhealthy".
Satellites can measure tropospheric ozone. Measurements specifically of ground-level ozone require in situ monitoring technology.
Although the ozone levels found on the ground are the same chemical composition as those found in the ozone layer, they are not necessarily a byproduct of a thinning ozone layer in Earth's atmosphere. The upper-level layer protects people from the sun's most damaging ultraviolet rays, and helps to keep the climate of the Earth manageable for humans.
Ground-level ozone, however, is a health hazard. It is formed through a complex chemical reaction involving hydrocarbons, nitrogen oxides and sunlight on calm summer days where the weather may also be warm and humid. High levels of ground ozone affects the breathing process and aggravates asthma in chronic sufferers. The young, elderly, and those with lung diseases are especially suspectible.
Ozone is most likely to exceed safety limits from May through October when seasonal heat and sunlight are at their highest. However, similar conditions can occur at other times of the year in specific urbanized areas; namely the Los Angeles area, which is well-known for smog formation.
A major cause of ground-level ozone is due to pollutants in the air released by heavy industry (manufacturing plants, refineries, coal-fired power plants). Therefore, Ozone Action Days occur most frequently in the Midwestern United States. In recent years, many sites have taken steps to help reduce the amount of pollutants they discharge.
Secondary sources include automotive emissions (leaky auto exhaust systems, excessive engine idling) and liberal use of household chemicals or sprays. It is believed that nearly fifty percent of pollutant ozone molecules are attributed to the presence of these.
Heavy industries make up a high percentage of pollutants causing ground ozone. Without drastically altering or eliminating industrial production in an area altogether, air quality improvements are very slight, though noticeable. Non-industrial pollutants, while not thought of to be a major pollutant group, can be more controlled with more positive change occurring.
Basic steps in limiting ground ozone during Ozone Action Days are:
- Controlling of auto emissions
- Eliminate excessive engine idling
- Ensure automotive exhaust system functions properly
- Avoid unnecessary driving whenever possible
- Don't gas up until after 6:00 pm
- Limit the use of lawn mowers and outdoor grills to after 6:00 pm
- Limit the use of aerosol cans around the home (for example, hair gel instead of hair spray)
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For more information (background, pictures, experiments and references): Ozone