What Causes Smog?

What is ozone?

Ozone is a greenhouse gas made of three atoms of oxygen. Ozone can be helpful or harmful depending on where it is located. Stratospheric ozone, found in the upper atmosphere (which begins about 5 miles above Earth's surface), forms a protective layer that helps block ultraviolet (UV) radiation from the sun. Because too much UV exposure can lead to vision loss and skin cancers, the global community began to phase out substances that deplete the stratospheric ozone layer when a hole appeared in 1980. Due to these efforts, the United Nations reports that the layer will likely recover by around 2066.

Ground-level ozone, on the other hand, is a pollutant and a primary ingredient of smog. It forms when volatile organic compounds, or VOCs, and nitrogen oxides participate in chemical reactions in the presence of sunlight.

• VOCs: Carbon-containing compounds that easily evaporate at room temperature and enter the surrounding air. Many VOCs are emitted naturally by plants. Others come from human-made sources, including products such as paints, pesticides, solvents, and processes such as fuel production and combustion. Gas-powered lawn equipment and older-model cars emit particularly high levels of VOCs. Until relatively recently, car emissions were the largest source of VOCs in most cities. However, the catalytic converter almost completely eliminates these emissions in modern vehicles.
• Nitrogen oxides: A family of gases that cause pollution and are harmful to human health. Trucks and automobiles are responsible for about half of all nitrogen oxide emissions, with electric power plants and other industrial sources contributing most of the rest. In addition to smog, nitrogen oxides are converted in the atmosphere to nitric acid, a component of acid rain.

What is particulate matter?

Airborne particulate matter is made up of tiny liquid droplets and solid particles. While some particulate matter is large enough to be visible to the naked eye, much is microscopic. Particulate matter is directly emitted during combustion, for example from fires or diesel engines. But it also forms in the atmosphere as a result of chemical reactions involving VOCs. Airborne particulate matter reduces visibility and is the reason smog appears brown.

How do ozone and particulate matter affect human health?

Ground-level ozone and particulate matter are particularly harmful to respiratory health. Breathing ozone-polluted air can cause coughing and shortness of breath, damage and inflame airways, and aggravate asthma. Airborne particles, meanwhile, are easily inhalable and can become embedded in the lungs or transferred into the bloodstream, causing serious health problems.

How do ozone and particulate matter affect the environment?

Ozone exposure can leave plants more susceptible to disease and drought and reduce agricultural yields.

Atmospheric particulate matter scatters sunlight back to space, which has a cooling effect on Earth. Particles also provide the seeds around which nearly all clouds form. However, heavy pollution can suppress cloud formation.

How can we reduce smog?

In addition to recommending individual actions, such as choosing fuel-efficient vehicles, driving less, using electric lawn equipment, and selecting low-VOC paints and cleaners, state and federal regulatory agencies set air quality standards for ozone and other harmful air pollutants. Agencies also regulate vehicle and factory emissions and fuel production.

For more than 70 years, Caltech scientists and engineers have informed air-pollution policy with insights on smog formation; innovations in pollution measurement and monitoring; and discoveries about the origin, chemistry, and evolution of pollutants in the atmosphere.

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