The brown haze that sometimes blankets urban areas located in mountain basins is known as smog, but it isn't smoke and fog, as the name suggests. It's a combination of chemicals produced by the interaction of emissions from fossil fuel combustion and sunlight. Unlike classical smog, photochemical smog affects people in hot, sunny climates, and although it hasn't produced any mass calamities comparable to the Great Smog of 1952, it's still dangerous.
Source of Photochemical Smog
The bulk of the fossil fuel emissions that produce photochemical smog come from power plants that burn coal, oil and natural gas and from cars and trucks that burn gasoline. The primary pollutants in these emissions are nitrogen oxides and volatile organic compounds, of which there are several varieties. Nitrogen oxides undergo a variety of reactions in the presence of sunlight that produce ozone, which is a highly corrosive chemical. In the upper stratosphere, ozone performs the beneficial job of filtering ultraviolet sunlight, but near the ground, it's a pollutant. It causes respiratory and eye problems, damages rubber and plastic and retards plant growth.
From Nitrogen Oxide to Ozone
The ozone in photochemical smog is the product of a series of reactions. When VOCs emerge from a smokestack or tailpipe, they combine with hydroxide molecules in the air to form water and complex molecules that in turn combine with the nitrogen oxide that is emitted at the same time. The reaction produces nitrogen dioxide, which gives photochemical smog its yellowish color. Nitrogen dioxide breaks down again into nitrogen oxide in sunlight, but this degradation produces a free oxygen radical. The highly reactive radical combines with molecular oxygen in the air to form ozone.
The hydrocarbons present in the VOCs enhance the formation of ozone; they combine with free oxygen radicals to produce molecules with three oxygen atoms called peroxides. Peroxides combine with oxygen molecules to produce ozone, and they also react with nitrogen oxides to produce nitrogen dioxide. Besides fueling more ozone-producing reactions, nitrogen dioxide molecules also produce another pollutant -- peroxyacetyl nitrates, or PAN -- when they react with the hydrocarbon molecules present in VOCs. This component of photochemical smog is the one most responsible for eye irritation, and it's more damaging to plants than ozone.
The Importance of Inversion Layers
Cities in which photochemical smog is a problem are typically situated in valleys or near mountain ranges which produce temperature inversion layers. In normal atmospheric conditions, air temperature decreases with altitude, which allows warmer air, as well as the pollutants in it, to rise and disperse. A temperature inversion layer is a blanket of warm air that prevents this natural circulation. Inversion layers form for several reasons. For example, in mountainous areas they may form when cool mountain air falls, heating by compression as it does so. Some inversion layers disperse quickly, but others are more stable and create dangerous smog conditions for days.