Washington, D.C.--For years, cows have been getting a bad rap on air pollution. The problem is that waste from bovines and other livestock produces ammonia, a compound that contributes to the smog that chokes southern California. But new research presented at the American Chemical Society meeting here yesterday suggests that cars, not cows, are southern California's primary source of ammonia. The finding, researchers say, could prompt federal regulators to consider placing curbs on ammonia from car exhaust.
Ammonia (NH3) reacts readily with airborne sulfuric acid and nitric acid, both of which are formed by reactions of other tailpipe gases. The result is ammonium nitrate and ammonium sulfate, molecules that condense with others of their kind, plus water, to form tiny particles in the air that absorb sunlight and block visibility--smog.
Recent studies suggested that vehicles in southern California produce only 61 milligrams of ammonia per kilogram of exhaust, a number that would make the cumulative effect of southern California's cars only half that estimated to come from livestock. But the previous studies shared a common problem, says Marc Baum, a chemist at the Oak Crest Institute of Science, a nonprofit research center in Baldwin Park, California: They tracked car exhaust drifting out of highway tunnels, and the researchers were concerned that ammonia could react with the walls of the tunnel or other gases and thus not show up on detectors.
So Baum and his colleagues built a laser-based remote sensor that can track the exact amounts of a wide range of gases coming from each vehicle's tailpipe. The team then set up their device by the side of a highway onramp and checked the emissions from 4500 vehicles. To his surprise, Baum found an average of 138 milligrams of ammonia per kilogram of exhaust--more than twice as high as previous estimates. Not all cars were equal offenders, however. About two-thirds of the ammonia came from just 10% of the cars.
The findings could cause air quality officials to rethink antismog rules. "Ammonia is a source that hasn't been factored in the past," says Michael Hoffman, a chemist at the California Institute of Technology in Pasadena. So the new results, he says, "are very significant." But doing away with ammonia could create other challenges, Hoffman notes. Ammonia helps neutralize sulfuric acid and other acids in air pollution, so removing it from southern California skies could raise the risk of acid rain.