<P> Air pollution has two primary sources, biogenic and anthropogenic . Biogenic sources are natural sources, such as volcanoes that spew particulate matter, lightning strikes that cause forest fires, and trees and other vegetation that release pollen and spores into the atmosphere . </P> <P> Californian greenhouse gas emissions come mostly from transportation, utilities, and industries including refineries, cement, manufacturing, forestry, and agriculture . In 2004, transportation accounted for approximately 40 percent of total greenhouse gas emissions in California . About 80 percent of that came from road transportation . </P> <P> Population growth increases air pollution, as more vehicles are on the road . California's large population significantly contributes to the high amount of smog and air pollution in the state . In 1930, California's population was less than six million people and the total registered vehicles were two million . </P> <P> California has a unique topography which contributes to some of the problems; the warm, sunny climate is ideal for trapping and forming air pollutants . On hot, sunny days, pollutants from vehicles, industry, and many products may chemically react with each other . In the winter, temperature inversions can trap tiny particles of smoke and exhaust from vehicles and anything else that burns fuel . This keeps pollution closer to the ground . </P>

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