Air Pollution: Photochemical Smog

Sunlight Plus Cars Equals Photochemical Smog

Photochemical reaction is any chemical reaction activated by light. Photochemical smog is a mixture of primary and secondary pollutants formed under the influence of UV radiation from the sun.

In greatly simplified terms,

                                                                                  Ground level ozone (O3)                                        

                                                                                + Other photochemical

                                                                                VOCs + NOx + heat + sunlight  >>> oxidants

                                                                               +aldehydes

                                                                               + Other secondary air pollutants

The formation of photochemical smog begins when exhaust from morning commuter traffic releases large amounts of NO and VOCs into the air over a city. The NO is converted to reddish-brown NO2, explaining why photochemical smog is sometimes called brown-air smog. When exposed to ultraviolet radiation from the sun, some of the NO2 reacts in complex ways with other pollutants. The resulting photochemical smog is a mixture of pollutants, including ground-level ozone.

Some pollutants, known as photochemical oxidants, can damage lung tissue. Hotter days lead to higher levels of ozone and other components of smog. As traffic increases on a sunny day, photochemical smog (dominated by O3) usually builds up to peak levels by late morning, irritating people’s eyes, and respiratory tracts.

All modern cities have some photochemical smog, but it is much more common in cities with sunny, warm, and dry climates and lots of motor vehicles. Examples are Los Angeles, Denver, and Salt Lake City in the United States; Sydney, Australia; São Paulo, Brazil; Buenos Aires, Argentina; Bangkok, Thailand; Jakarta, Indonesia; and Mexico City, Mexico. According to a 1999 study, if 400 million people in China drive conventional gasoline-powered cars by 2050 as projected, the resulting photochemical smog could regularly cover the entire western Pacific, extending to the United States.