Question of the Month: Why is Los Angeles so smoggy?
Although the Los Angeles area is subject to the most stringent air quality regulations in the nation, and though the amount of air pollution we emit per person is the lowest in the country, putting us some 35 years ahead of other American cities, we still have some of the worst air in the country.
One reason for this is the vast population. There are more than 10 million people in the greater Los Angeles area, a far greater number than in the San Francisco/San Jose or San Diego areas, and that many people simply create a lot of air pollution. But other large cities, such as New York, don't suffer from pollutant concentrations in the outdoor air that are as high as ours.
One reason for this is the low atmospheric mixing depth typical of the Los Angeles region. Mixing depth is the elevation to which air from the ground mixes, and in Los Angeles it's only a few thousand feet. For example the top of Mount Wilson, at more than 5,000 feet elevation, is usually in clean air because the pollution doesn't mix to that height. But on the east coast the mixing depth is greater, so pollution from equally large cities, such as New York, is spread through a larger volume of air and isn't as concentrated close to the ground.
Another factor that contributes to the region's smog is our bright sunshine. Sunlight causes photochemical reactions to occur in the atmosphere, in which gaseous pollutants are converted into ozone and into tiny particles that more severely affect visibility.
Tiny particles, those smaller than 10 micrometers (about four ten-thousandths of an inch) in diameter, are a hot topic in the field of air quality these days. Because they are associated with adverse health effects and because they can drastically reduce visibility, the U.S. Environmental Protection Agency is in the process of revising their standards for airborne particles.
Presently, particles smaller than 10 micrometers are regulated as a group. Soon, there will likely be new standards that distinguish between the tiniest particles, those smaller than 2.5 micrometers, and medium-sized particles, those between 2.5 and 10 micrometers. These two size ranges contribute about equally to the particle mass concentration in the region's air, but the smaller particles contribute much more to the visible haze that hangs over the city for most of the summer.
Most of the tiniest particles are produced in two ways. Many of them come right from the pollution source, in the form of exhaust from diesel engines, or smoke from wood fires. The other source of the tiniest particles is the photochemical reactions in the atmosphere I mentioned earlier.
The composition of these particles is very complicated, and illustrates the complex interactions between different types of air pollution. Typically, these particles have a core of either fine dirt or soot. On top of this are condensed some combination of nitrates and sulfates, which are the end products of atmospheric chemical reactions involving nitrogen oxides and sulfur dioxide, and organic vapors, such as are given off by motor vehicle exhaust and paint solvent evaporation.
The greater Los Angeles area contains more than 10 million people, millions of cars, and tens of thousands of businesses large and small. Of course automobile and truck engines contribute significantly to the region's smog, but in such a huge region, thousands of seemingly small polluters, such as dry cleaners, and lawn mowers have a major cumulative effect. It will be interesting to see how the South Coast Air Quality Management District tries to control these small but important sources of pollution.
