Since around 1942, in Los Angeles of the United States, air pollution slightly different from conventional smog began to be produced. This smog caused gray-brown haze, poor visibility, eye irritation and specific damage to plants. This smog was produced only on hot sunny days. In the early 1950’s, Professor A.H.Haagen-Smit of California Institute of Technology clarified that this pollution is mainly composed of ozone produced by irradiating vehicle exhaust gas combined with sunlight.
Nitrogen dioxide emitted from vehicles and factories into the air is brownish when it is highly concentrated. Nitrogen dioxide absorbs ultraviolet light energy from the visible rays of the sun and is photodegraded into nitric oxide (NO) and atomic oxygen (O). The reaction properties of the atomic oxygen generated here are high and it forms ozone by reacting with oxygen (O2) immediately. However, this reaction alone does not result in a high concentration of ozone. Various hydrocarbons such as HC and organic compounds emitted from cars are present in the air, and these substances play an important role in producing photochemical smog. The presence of hydrocarbons leads to an accumulation and high concentration of ozone in the atmosphere and forms harmful organic compounds such as aldehyde, acrolein and PAN(peroxyacetyl nitrate). The major component of photochemical smog is produced as a secondary chemical reaction in the air, not by a direct source like cars. Since ozone possesses strong oxidizing properties, the generic name used is photochemical oxidant, and it is the causative substance of photochemical smog.
Ozone has strong oxidizing properties. High ozone concentrations cause eye and throat irritation and respiratory problems. Additionally, ozone has an adverse affect on agricultural crops. PAN has much more harmful effect. The environmental standard for hourly value on photochemical oxidant in Japan is less than 0.06ppm. Because health hazards for local residents mainly appear as acute effects, a limit of 0.06ppm is regulated specifically for the prevention short-term exposure problems, including eye irritation and respiratory problem. On the other hand, a different assessment method was employed concerning the effect on plants. AOT 40 is often used as the index. This assessment indicates how long the ozone concentration remains over 40ppb. It is based on exposure accumulation of a concentration which has an adverse affect on plants.
Since the reaction properties of ozone are high, high ozone concentrations cannot remain. It reacts with various compounds in the air or matter on the earth’s surface, especially plants, and disappears. The production of ozone stops as the sunlight becomes weaker before the sunset. This phenomenon needs a constant time to react because it is based on chemical reaction in the air. In addition, high ozone concentrations are observed in suburbs some kilometers away from cities where the primary pollutant like nitrogen oxide is exhausted, because the air mass and pollutants can travel in the atmosphere. In Japan, the achievement rate of the environmental standard on photochemical oxidant is quite low. The rate at less than 1% has lasts. When the ozone concentration for hourly value is over 0.12ppm, a photochemical smog warning is issued, and when over 0.24ppm, a photochemical smog alarm is issued. The system is used to supply hourly, real time information to residents to encourage them to be aware and reduce their pollution emissions. For the measurement of the ozone concentration, the ultraviolet absorption method is used, and in Japan it is used in combination with the neutral potassium iodide method.
Since the latter method can measure various acidic agents, the results are shown as photochemical oxidant.
As noted above, the three major factors which cause photochemical smog are nitrogen oxide, hydrocarbon and ultraviolet rays from the sun. Because it is impossible to control sunlight on a large scale, many countries attempt to control the emission of nitrogen oxide and hydrocarbons in order to prevent photochemical smog. The methods of control of nitrogen oxide are almost exactly the same as that of acid rain. The control of the causes of hydrocarbon started in April 2006 in Japan, and it is expected that this activity will reduce the concentration of suspended particulate matter in the air.
Because the causative substance of photochemical smog is mainly generated in urban areas, this used to be considered a form of typical, urban-type regional air pollution. However, two other factors must be considered. One is the phenomenon that ozone in the stratosphere descends onto the earth’s surface. It may be the cause of the high ozone concentrations in early spring. The other factor is the wide-area ozone transport by tropospheric ozone production. In Japan, the transfer from the ocean side, but also from the continent side is considered important. This indicates that photochemical smog has both sides of a local pollution problem and global-scale environmental issue. Therefore, it is not easy to achieve an effective solution.