While people typically blame Southern California's smog on automobiles, a new study suggests that cows may be just as responsible, if not more so. A large fraction of the region's smog, especially the particles smaller than 2.5 micrometers in diameter, is ammonium nitrate. Those particles form in the atmosphere when ammonia, which is generated by cars with certain types of catalytic converters and by bacteria that consume cattle waste, reacts with nitrogen oxides that are produced in large quantities in automobile emissions. Data gathered during low-altitude flights in and around the Los Angeles basin in May 2010 suggest that the region's 9.9 million autos generate about 62 metric tons of ammonia each day. However, ammonia emissions from dairy farms  in the eastern portion of the basin—home to about 298,000 cattle—range between 33 and 176 metric tons per day, researchers report in Geophysical Research Letters. Ammonia emissions from the dairy farms are concentrated, boosting atmospheric levels of the gas to more than 100 times background levels, so efforts to curb the farms' emissions (perhaps by feeding the animals different diets) might reduce smog more than those targeting cars.
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