Smog is a visible form of air pollution that commonly affects urban and industrial areas. It is a complex atmospheric phenomenon resulting from reactions involving various pollutants. This hazy condition reduces visibility.
Defining Smog
The term “smog” originated in the early 20th century as a combination of “smoke” and “fog” to describe a smoky haze. While “classic” or “sulfurous” smog was linked to coal burning, modern concerns center on “photochemical smog,” which does not require smoke or fog to form. Photochemical smog is characterized by a brownish tint and is observed over cities during warmer, sunnier periods.
Primary Pollutants and Their Sources
The formation of photochemical smog begins with the release of specific precursor chemicals: nitrogen oxides (NOx) and volatile organic compounds (VOCs). Nitrogen oxides, including nitric oxide (NO) and nitrogen dioxide (NO2), are generated from high-temperature combustion. Major sources of NOx are vehicle exhaust, industrial combustion, and domestic burning.
Volatile organic compounds are carbon-containing chemicals that readily evaporate into the air. These compounds come from vehicle emissions, industrial solvents, gasoline vapors, and consumer products like paints and cleaning supplies. Natural sources, such as plant emissions and forest fires, also contribute VOCs.
The Photochemical Process
Sunlight is the primary catalyst for photochemical smog formation. The process begins when nitrogen dioxide (NO2) absorbs ultraviolet (UV) radiation from sunlight. This causes NO2 to break apart into nitric oxide (NO) and a single oxygen atom (O). This free oxygen atom is highly reactive and quickly combines with molecular oxygen (O2) to form ground-level ozone (O3), a significant component of photochemical smog that is detrimental at ground level.
Without VOCs, nitric oxide (NO) would react with ozone (O3), converting it back into oxygen and nitrogen dioxide, limiting ozone accumulation. However, VOCs interfere with this natural ozone-scavenging process by reacting with NO, which prevents it from breaking down ozone. This allows ground-level ozone concentrations to increase significantly. Reactions involving VOCs and NOx can also lead to the formation of other harmful secondary pollutants, such as peroxyacyl nitrates (PANs) and various aldehydes. This complex, sunlight-driven chemical chain reaction is continuous.
Atmospheric Conditions Affecting Smog
Certain atmospheric conditions and geographical features influence smog formation and concentration. A temperature inversion, where warm air sits above cooler air near the ground, acts like a lid. This traps pollutants, including smog precursors, close to the Earth’s surface and prevents their vertical dispersion. Longer inversions lead to more pollutant accumulation and higher smog levels.
Geographical features also trap air masses and pollutants. Cities in valleys or basins, surrounded by mountains, are susceptible to smog formation. These elevated landforms act as natural barriers, hindering air movement and allowing pollutants to build up. Light winds or stagnant air conditions further contribute to pollutant accumulation. When winds are weak, pollutants are not effectively dispersed, remaining concentrated and reacting more readily to form smog.