Photochemical smog represents a modern type of air pollution that forms through chemical reactions in the atmosphere, resulting in a brown, hazy cloud over urban areas. It is distinctly different from industrial smog, which resulted primarily from burning coal and producing sulfur dioxide and soot. Photochemical smog is a secondary pollutant, meaning it is not directly emitted from a source but forms when primary pollutants undergo transformation in the presence of sunlight. Its formation is driven by atmospheric chemistry powered by solar energy.
The Necessary Raw Materials
The creation of photochemical smog requires two primary precursor pollutants: Nitrogen Oxides (\(\text{NO}_{\text{x}}\)) and Volatile Organic Compounds (VOCs). Nitrogen oxides, including nitric oxide (NO) and nitrogen dioxide (\(\text{NO}_2\)), are generated primarily from high-temperature combustion processes. This occurs in sources like vehicle engines, coal-fired power plants, and industrial boilers, where atmospheric nitrogen and oxygen react under intense heat.
Volatile Organic Compounds are chemicals that easily vaporize. Anthropogenic sources of VOCs include the evaporation of gasoline, industrial solvents, paints, and chemical manufacturing. Natural sources also contribute, such as the emission of isoprene and terpenes from vegetation.
The Essential Role of Sunlight
Sunlight provides the energetic catalyst required to initiate the transformation of primary pollutants into photochemical smog. The specific component responsible is ultraviolet (UV) light, which carries enough energy to break stable chemical bonds. This process is known as photolysis, a light-driven chemical decomposition.
The reaction is triggered when nitrogen dioxide (\(\text{NO}_2\)), the brown-colored component of \(\text{NO}_{\text{x}}\), absorbs UV radiation. This energy causes the \(\text{NO}_2\) molecule to break apart, yielding nitric oxide (NO) and a free oxygen atom (O). Smog formation is most pronounced during summer months and peaks around midday when solar radiation is strongest. High ambient temperatures also accelerate the reaction rates.
The Chemical Chain Reaction
The free oxygen atom (O) produced by the photolysis of nitrogen dioxide quickly combines with molecular oxygen (\(\text{O}_2\)) to form ground-level ozone (\(\text{O}_3\)). Ozone is a major, harmful component of photochemical smog. In the absence of VOCs, ozone would react with nitric oxide (NO), converting it back to nitrogen dioxide (\(\text{NO}_2\)) and limiting ozone buildup through a natural cycle.
The presence of VOCs interrupts this balance, leading to rapid ozone accumulation. VOCs are attacked by hydroxyl radicals (OH), initiating a complex radical chain reaction. This reaction generates peroxy radicals, which then react with NO. This converts NO back to \(\text{NO}_2\) without consuming ozone, effectively short-circuiting the natural ozone destruction mechanism.
The continuous regeneration of \(\text{NO}_2\) allows the photolysis process to repeat, driving up ozone concentration. This chain reaction also forms other secondary pollutants that contribute to the visible haze and irritating qualities of smog. These include aldehydes, which contribute to the odor, and Peroxyacetyl Nitrates (PANs), which cause severe eye irritation and damage vegetation.
Meteorological and Geographic Influences
Atmospheric and geographic conditions prevent the dispersal of smog, leading to severe episodes. A common factor is a temperature inversion, where a layer of warmer air settles above cooler air near the ground. This stable stratification acts like a lid, trapping pollutants and their precursors beneath the warm layer, preventing them from rising and mixing into the upper atmosphere.
Low wind speeds are another significant meteorological factor, as stagnant air allows the concentrations of primary and secondary smog components to build up. Without strong winds, pollutants cannot be diluted or transported away. Topographical features, such as valleys or mountain ranges, physically exacerbate this trapping effect. Cities located in basins, like Los Angeles or Mexico City, are particularly susceptible because the surrounding terrain acts as a barrier to air flow, keeping the polluted air mass concentrated.