Smog

Smog is a type of intense air pollution that creates a visible haze in the atmosphere. The term is a portmanteau, which means it combines the words "smoke" and "fog."

This mixture includes various substances like nitrogen oxides, sulfur oxide, ozone, and tiny particles. Smog is a serious environmental issue because it can be toxic to humans. Exposure can lead to severe sickness, shortened life spans, or premature death.

To understand smog, we must distinguish between primary and secondary pollutants. Primary pollutants are emitted directly from a specific source. For example, burning coal releases sulfur dioxide directly into the air. Secondary pollutants are not emitted directly from a source. Instead, they form when primary pollutants undergo chemical reactions in the atmosphere. Ozone is a major secondary pollutant that forms through these complex interactions.

Scientists categorize smog into two main types: summer smog and winter smog. Summer smog is often called photochemical smog. This type is driven by sunlight and warmer temperatures. It is common in cities with sunny, dry climates and many motor vehicles.

Winter smog occurs during colder months. During winter, people use more coal and fossil fuels to heat buildings. This type of smog is often worsened by atmospheric inversions. An inversion happens when warm air traps cold air near the ground. This prevents pollutants from dispersing, keeping them trapped where people breathe.

Photochemical smog formation is a specific, sunlight-driven chemical process. It begins when nitrogen dioxide (NO2) absorbs solar radiation. This causes the molecule to split into nitric oxide (NO) and a free oxygen atom. That oxygen atom then combines with molecular oxygen (O2) to create ozone (O3).

Volatile organic compounds (VOCs), which come from cars and industry, also play a role. These compounds react with nitric oxide, which prevents the ozone from being destroyed. This allows ozone concentrations to build up throughout the day. Other harmful substances like peroxyacetyl nitrates (PANs) and aldehydes also form during these reactions.

Many human activities contribute to the creation of these pollutants. Transportation is a major source, as cars, trucks, and buses emit carbon monoxide, nitrogen oxides, and hydrocarbons. Industrial fumes and coal combustion also release significant amounts of gases. In some regions, such as Delhi, agricultural practices like stubble burning add to the severity.

Even natural events can cause smog-like conditions. Erupting volcanoes release sulfur dioxide and particulate matter. This natural version is often called "vog" to distinguish it from man-made smog. Additionally, plants release hydrocarbons like isoprene that can react in the atmosphere to increase ozone.

Our understanding of smog has evolved through scientific discovery. The word "smog" appeared in print as early as 1880. However, Henry Antoine Des Voeux is often credited with coining the term in a 1905 paper. In the 1940s, a flavor chemist named Arie Haagen-Smit began studying the air in Los Angeles. He adapted his equipment to collect chemicals and identified ozone as a key component.

He discovered that sunlight reacting with automotive exhaust and hydrocarbons was the cause. He worked with Arnold Beckman, who developed tools like air quality monitoring vans to detect these pollutants.

Smog affects many different parts of the world in various ways. Large cities like Los Angeles, Beijing, Delhi, and Mexico City often face high pollution levels.

In some cases, the smog is so thick that it forces cities to close roads, schools, or airports. For example, in 2013, the Chinese city of Harbin faced such closures due to smoke. Because smog travels with the wind, it can move from busy cities into sparsely populated areas. It is a global challenge that connects industrial activity, weather patterns, and human health.