What Causes a Brown Cloud?
Brown clouds are caused by human activity. These clouds are composed of aerosol particles that may contain combinations of soot, nitrogen dioxide and sulphur. The large brown cloud frequently seen over Hong Kong is composed of these particles mixing with "several small, local clouds," according to Deena Gudzer.
But brown clouds are not just a problem for Hong Kong. In fact, brown clouds occur in many areas. Their composition may vary slightly by region but human activity is always the cause.
Phoenix, Arizona offers an interesting study. At one time Arizona was known for its clean air and often people moved there seeking relief for respiratory problems. But the tremendous growth that the area has experienced has led to a dramatic decrease in air quality. By 2005 Maricopa County received a low score for air quality, based on ozone and particulates, according to the American Lung Association (About.com).
Rapid population growth brings increased development. Activities such as construction, industrial pollution and increased burning of fossil fuels add significantly to atmospheric pollution. When the explosive growth occurs in a region with particular geographic or climate characteristics, the ominous brown cloud occurs most frequently.
In other parts of the world the source of pollution may vary. In less economically developed countries, cooking fires can be a major source of these particles. Where it is common to burn crop debris or where meals are prepared using dung as fuel, particles are released into the atmosphere.
Burning fossils fuels is another big contributor to the formation of brown clouds. When fossil fuels are burned they release sulphur and tiny soot particles. As more fossil fuels are burned for transportation, the more aerosol particles are released.
Forest fires, too, add to the brown clouds. Some of these fires are natural, of course. But many of these fires are deliberate acts of human activity as forests are burned to clear land for agriculture. In many parts of the world, slash and burn is the most prevalent method of clearing land.
The largest brown cloud forms over India, where ocean currents and ocean surface temperatures amplify human pollution. The region has seen tremendous population growth and industrialization. Often industries lack modern capabilities that moderate the effects of pollution. The region also experiences great swings in atmospheric conditions due to geography.
Many scientists feel that the quickest, simplest solution is to provide more efficient methods of cooking and home heating, especially in the third world. The good news is that the particles making up brown clouds dissipate in only a few weeks (Guzder). Cutting them off at their source would prove beneficial very quickly.
Eastwood, Steve. Brown Air in the Valley. About.com. Jan 2006.
Guzder, Deena. Study Gets Inside the World's "Brown Cloud." Time. Jan 23 2009.