Photochemical smog how does it form

This particular resource used the following sources:. Skip to main content. Chemistry and the Real World. Search for:. Photochemical Smog. Learning Objective Recall what causes photochemical smog and why it is a problem for humans. Key Points Photochemical smog is composed of primary and secondary pollutants. This lists the logos of programs or partners of NG Education which have provided or contributed the content on this page.

Powered by. Smog is air pollution that reduces visibility. The term "smog" was first used in the early s to describe a mix of smoke and fog. The smoke usually came from burning coal.

Smog was common in industrial areas, and remains a familiar sight in cities today. Today, most of the smog we see is photochemical smog. Photochemical smog is produced when sunlight reacts with nitrogen oxide s and at least one volatile organic compound VOC in the atmosphere. Nitrogen oxides come from car exhaust , coal power plants, and factory emissions. VOCs are released from gasoline , paints, and many cleaning solvents.

When sunlight hits these chemicals, they form airborne particles and ground-level ozone —or smog. Ozone can be helpful or harmful. But when ozone is close to the ground, it is bad for human health. Ozone can damage lung tissue , and it is especially dangerous to people with respiratory illness es like asthma.

Ozone can also cause itchy, burning eyes. Smog is unhealthy to humans and animals, and it can kill plants. Smog is also ugly. It makes the sky brown or gray. Smog is common in big cities with a lot of industry and traffic. Cities located in basin s surrounded by mountains may have smog problems because the smog is trapped in the valley and cannot be carried away by wind. Los Angeles, California, and Mexico City , Mexico, both have high smog levels partly because of this kind of landscape.

Many countries, including the United States, have created laws to reduce smog. Nitric oxide NO and nitrogen dioxide NO 2 are emitted from the combustion of fossil fuels , along with being naturally emitted from things such as volcanos and forest fires it is the immense concentration of these pollutants within cities that is of the most concern however, as natural emissions tend to spread out over larger areas.

When exposed to ultraviolet radiation, NO 2 goes through a complex series of reactions with hydrocarbons to produce the components of photochemical smog—a mixture of ozone , nitric acid , aldehydes , peroxyacyl nitrates PANs and other secondary pollutants. NO 2 , ozone and PANs are called photochemical oxidants because they can react and oxidize certain compounds in the atmosphere or within a person's lungs that are not normally oxidized. Even small traces of these chemicals can affect the respiratory tract of humans and animals, and damage crops and trees.

Photochemical smog has many adverse effects. When combined with hydrocarbons, the chemicals contained within it form molecules that cause eye irritation. Radicals in the air interfere with the nitrogen cycle by preventing the destruction of ground level ozone. Other effects include reduced visibility and respiratory ailments. Ground level ozone is also produced which has various effects on the human body.

That type of smog definition, which comes from coal-burning plants and is common in rainy industrial centers, is known as industrial smog. The modern world is plagued with a new type of smog. Beginning in the s, people in Los Angeles began to notice a persistent brown haze in the air on hot days that caused watery eyes and respiratory problems. They began referring to the haze as smog, but it had a different composition than industrial smog and formed in a different way.

It is officially known as photochemical smog, but even though it affects many cities worldwide, people sometimes call it Los Angeles smog. The unofficial nickname for industrial smog, by the way, is London smog. The formation of photochemical smog involves three primary ingredients: nitrogen oxides, hydrocarbons and sunlight.

The nitrogen oxides and hydrocarbons are by-products of fossil fuel-burning energy plants, and they can even come from natural processes, but the main source is the internal combustion engines in gasoline-powered automobiles.

Nitrous oxide and nitrogen dioxide dissociate in sunlight and combine with trace hydrocarbons to ultimately produce a large number of pollutants. The complex process proceeds in stages:. Many major cities maintain a smog index, and one of the main chemical pollutants they monitor is ozone.