Global Air Pollution and Intercontinental Transport

Ozone due to manmade pollution is the main irritant in "smog" but it is also a natural component of the atmosphere.  Understanding the effects of global pollution on local ozone exposure requires tracking ozone from both natural and manmade sources.

High in the atmosphere, above 10km (30,000 ft.) altitude, ozone is formed naturally by ultraviolet (UV) light driving the breakup and recombination of oxygen molecules.  The high altitude interaction of oxygen and UV creates the "Stratospheric Ozone Layer" and protects plants and animals at the surface from the harmful effects of excess UV exposure (Figure 1.).

In unpopulated areas and over the oceans, a small amount of natural low altitude ozone is formed from sunlight driving reactions of natural volatile organic compounds (VOCs) from vegetation and natural nitrogen oxides (NOx), primarily from lightning and forest fires.

Figure 1.  The Stratospheric Ozone Layer screens out most harmful solar UV radiation. (courtesy USEPA Sunwise Program)

In populated areas, sunlight drives reactions involving VOCs (volatile organic carbon compounds - primarily unburned or partially burned gasoline vapor, solvents, etc., and secondarily from vegetation), and NOx (nitrogen oxides, primarily from motor vehicles, power plants, etc.) to form the ozone we associate with "smog" (Figure 2).

The high concentrations of ozone in high pollution episodes in urban centers such as Shanghai, Mexico City, or Los Angeles are due to meteorological conditions that trap locally produced pollution near the ground.

Figure 2.  Ground level ozone is formed when sunlight drives reactions of VOCs and NOx (NJ DEP).

Pollution impacts on global ozone occur when, instead of trapping pollutants, winds drive wide dispersion of VOC and NOx.  Under these conditions, both local and global ozone production combine to drive up ozone levels from regional to hemispheric scales.  The effects of human activity on global surface ozone from 1890 to 2025 is shown in Figure 3.