Photochemical Smog

Photochemical smog is a noxious mixture of pollutants formed in the presence of sunlight. It is very harmful to humans, animals, and plants. The ingredients required for the production of photochemical smog are oxides of nitrogen (NOx), volatile organic compounds (VOCs), and sunlight. NOx and VOCs are themselves harmful, but in the presence of sunlight, they are involved in a complex series of reactions that create other harmful substances. Industry and transportation sources contribute most of the precursor gases. Photochemical smog is, therefore, most prevalent in urban areas. Ground-level ozone (O3), a secondary pollutant created by reactions between the precursor gases, is the main component of photochemical smog.

Nitric oxide (NO) is emitted when hydrocarbons such as gasoline, diesel fuels, or coal are burned. It is a by-product of incomplete combustion. Once emitted, NO quickly oxidizes to form nitrogen dioxide (NO2), a toxic, foul-smelling gas that gives polluted air its brownish/yellow hue. Collectively, NO and NO2 are called NOx. Volatile organic compounds are hydrocarbons that evaporate readily at normal temperature and pressure. Thousands of compounds fall into this category, and sources include evaporated fumes from fuel in vehicles, fuel combustion, petroleum refining, and industrial and residential solvents such as paints and cleaning fluids.

In the presence of sunlight, NO2 will dissociate, and further reactions with oxygen gas (O2) lead to the formation of ozone (Reactions 1 and 2). However, ozone will also dissociate in the presence of sunlight and react with water vapor to form highly reactive hydroxyl radicals (HO) (Reaction 3). If VOCs are present, the hydroxyl radicals react with VOC to produce peroxy radicals (RO2) (Reaction 4). The peroxy radicals further react with NO to produce NO2 (Reaction 5), and the cycle continues to create O3. These are simplified representations of the reactions:

Early-morning smog over Quito, Ecuador, June 2005

Source: Courtesy of Mark Welford.

Compounds such as peroxyacetyl nitrates, or PAN (CH3-CO-OO-NO2), are also formed in the process. PANs are important because they are harmful, have relatively long lifetimes in cold temperatures, and therefore can be transported to other locations where they release NO2. Carbon monoxide (CO), which is another by-product of fossil fuel combustion, may also be involved in reactions to produce ground-level ozone.

NO2 concentrations are highest during morning peak hour traffic, while ozone peaks near midday. VOCs are most prevalent in summer because of increased evaporation in warm temperatures. The creation of ozone is also more profuse in summer because of the increased intensity of the sun. Ozone is not created at night, due to the lack of sunlight. Photochemical smog, therefore, leads to poorest air quality episodes on summer days, though mixing with other pollutants such as particulate matter can cause poor air quality at any time.