The Science Behind Smog: Unraveling the Gray Haze

On a day in 1943, Los Angeles was engulfed in a dense gas, causing widespread panic as residents feared a chemical attack. However, this was no act of war, but rather an encounter with smog. This article delves into the science of smog, exploring its composition, formation, and the reasons it plagues certain cities.

The Origins of Smog

The term "smog," a blend of "smoke" and "fog," emerged in the early 20th century to describe the thick, gray haze prevalent in industrial cities like London, Glasgow, and Edinburgh. This industrial smog resulted from the combination of smoke from coal-burning stoves and factories with moisture in the air.

However, the smog that struck Los Angeles was different. It had a yellowish tint and a chemical odor. This type of smog remained a mystery until chemist Arie Haagen-Smit identified the key components:

• Volatile Organic Compounds (VOCs): These compounds readily vaporize and contain elements like carbon, oxygen, hydrogen, chlorine, and sulfur. While some VOCs are naturally produced by plants and animals, others originate from man-made sources such as solvents, paints, glues, and petroleum.
• Nitrogen Oxides: Released during the incomplete combustion of gas in motor vehicles, nitrogen oxides contribute to the yellowish color of this type of smog.

Photochemical Smog Formation

VOCs and nitrogen oxides react with sunlight to produce secondary pollutants known as PANs (peroxyacyl nitrates) and tropospheric, or ground-level, ozone. These pollutants cause eye irritation and damage lung tissue, making them key components of photochemical smog, the type that affected Los Angeles.

Why Some Cities Suffer More

Both industrial and photochemical smog result from a combination of man-made pollution, local weather, and geography. London's high humidity made it susceptible to industrial smog, while photochemical smog thrives in urban areas with calm winds and dry, warm, sunny weather. The ultraviolet radiation from sunlight provides the energy needed to break down molecules that contribute to smog formation.

Cities surrounded by mountains, like Los Angeles, or located in basins, like Beijing, are particularly vulnerable because the smog has nowhere to dissipate. This is also due to a phenomenon called temperature inversion, where a layer of pollution-filled air becomes trapped near the Earth's surface by a slightly warmer layer above, preventing the pollutants from dispersing.

The Harmful Effects of Smog

Smog is more than just an aesthetic problem. Both forms of smog can:

• Irritate the eyes, nose, and throat.
• Exacerbate conditions like asthma and emphysema.
• Increase the risk of respiratory infections like bronchitis.

Smog is especially harmful to young children and older adults, and exposure during pregnancy has been linked to low birth weight and potential birth defects. Additionally, secondary pollutants in photochemical smog can damage and weaken crops, making them more susceptible to insects and decreasing yield.

Combating Smog: A History of Change

For decades, smog was considered an unavoidable consequence of civilization. Londoners grew accustomed to the infamous "pea soup fog" until the Great Smog of London in 1952, which shut down transportation and caused over 4,000 respiratory deaths. This tragedy led to the Clean Air Act of 1956, which banned burning coal in certain areas and significantly reduced smog levels.

Similarly, regulations on vehicle emissions and gas content in the US have reduced volatile compounds in the air and, consequently, smog levels.

Smog Today: A Global Challenge

Despite progress, smog remains a major problem worldwide. Countries like China and Poland, which rely heavily on coal for energy, experience high levels of industrial smog. Photochemical smog and airborne particles from vehicle emissions affect many rapidly developing cities, from Mexico City and Santiago to New Delhi and Tehran.

Governments have implemented various measures to combat smog, such as banning cars from driving on certain days. As more than half of the world's population lives in cities, shifting to mass transit and reducing our reliance on fossil fuels may be essential for cleaner air and healthier lives.