Smog is a form of air pollution that often appears as a hazy, brownish or grayish layer, significantly reducing visibility. This atmospheric phenomenon is a complex mixture of various pollutants, rather than a single substance. Its presence is largely a modern concern, particularly in urban environments.
The Ingredients of Smog
Smog formation relies on specific precursor pollutants, primarily nitrogen oxides (NOx) and volatile organic compounds (VOCs). Nitrogen oxides, which include nitric oxide (NO) and nitrogen dioxide (NO2), are largely byproducts of high-temperature combustion processes. Major sources include emissions from vehicles such as cars and trucks, as well as electrical power generation plants and other industrial activities.
Volatile organic compounds are carbon-containing chemicals that readily vaporize into the atmosphere at typical room temperatures. These compounds come from a wide array of sources, including vehicle exhaust, industrial fumes, chemical solvents, paints, and even natural emissions from plants. While NOx and VOCs are the primary chemical reactants, particulate matter also contributes to the visible haze and can be directly emitted or formed through atmospheric reactions.
The Formation of Photochemical Smog
Photochemical smog forms through a series of chemical reactions driven by sunlight. The process often begins when nitrogen dioxide (NO2) absorbs ultraviolet (UV) radiation from the sun. This absorption causes NO2 to break apart, yielding nitric oxide (NO) and a free oxygen atom (O).
The highly reactive free oxygen atom then quickly combines with molecular oxygen (O2) present in the atmosphere to form ground-level ozone (O3). Ground-level ozone is a major component of photochemical smog and is considered a pollutant, unlike the beneficial ozone layer found in the upper atmosphere. Volatile organic compounds play a significant role in enhancing ozone formation. They react with hydroxyl radicals, which are highly reactive species formed through the photolysis of ozone, to produce peroxy radicals. These peroxy radicals then react with nitric oxide, converting it back to nitrogen dioxide, which can then undergo photolysis again to produce more ozone.
The complex interplay between VOCs and NOx in the presence of sunlight also leads to the formation of other secondary pollutants. These include peroxyacetyl nitrates (PANs) and aldehydes.
Environmental Factors Driving Smog
Several atmospheric and environmental conditions significantly influence the formation and concentration of smog. Strong sunlight is a primary driver, as it provides the energy necessary to initiate and sustain the photochemical reactions between pollutants. This is why photochemical smog is often referred to as “summer smog” and is more prevalent during warmer months.
High temperatures further accelerate these chemical reactions, leading to increased rates of ozone production. Calm or stagnant air conditions also contribute to smog accumulation by preventing pollutants from dispersing. Without sufficient wind, the chemical precursors and the resulting smog remain concentrated near the ground.
Temperature inversions are a significant meteorological factor that can worsen smog events. Normally, air temperature decreases with increasing altitude, allowing pollutants to rise and disperse. However, during an inversion, a layer of warmer air traps cooler, pollutant-laden air closer to the Earth’s surface. This atmospheric “lid” prevents the vertical mixing of air, causing pollutants to accumulate and intensify the visible haze.