Cars’ ammonia may sabotage tailpipe gains

Most people—even pollution experts—cite livestock waste as the leading source of urban, airborne ammonia. While that may have been true even a few years ago, a California study now indicates that cars have usurped this notorious distinction.

If confirmed nationally, the findings would show that vehicles make an unexpectedly large contribution to visibility-robbing haze. The good news, according to this study, is that only 10 percent of cars emit 66 percent of the ammonia. Identifying and correcting whatever distinguishes these heavy polluters could yield big gains, concludes the study’s leader, Marc M. Baum of the Oak Crest Institute of Science in Baldwin Park, Calif.

Ammonia fosters a chemical transformation of acidic combustion gases, such as nitric acid, into dust-size airborne particles, such as ammonium nitrate. Less than a micrometer in diameter, such particles can be inhaled deeply and aggravate lung and heart disease (SN: 1/31/98, p. 68: https://www.sciencenews.org/sn_arc98/1_31_98/fob1.htm). Moreover, because these small particles pick up water, they can create a smoglike haze, explains Michael R. Hoffmann of the California Institute of Technology in Pasadena.

Ammonia’s propensity to spawn such airborne particulates led governments to require factories to control ammonia emissions. Many use the alkaline chemical to neutralize acidic gases in smokestacks. No comparable regulations limit ammonia emissions from vehicles, however. In fact, Baum notes, since the advent of tailpipe catalytic converters, pollution scientists have written off cars as ammonia sources.

Until 2 years ago. That’s when Glen R. Cass, then at Caltech, measured copious ammonia coming from vehicles passing through a Los Angeles tunnel. His data indicated that the metropolitan area’s traffic spewed more than 25 tons of ammonia per day—about the same amount as area livestock did. The finding that vehicles were emitting some 61 milligrams of ammonia per kilometer of travel surprised pollution analysts.

A study by Robert A. Harley of the University of California, Berkeley and his team now confirms the Cass result. These researchers sampled air in a different local tunnel as some 60,000 cars passed through. They found an average ammonia emission by the vehicles of 49 mg/km, or 1 teaspoon per 100 miles traveled. These data will be published soon in Environmental Science & Technology.

Baum worries, however, that the latest tunnel measurements may underestimate the problem. At the American Chemical Society meeting in Washington, D.C., this week, he reported results from 4,500 vehicles cruising up a freeway ramp. Unlike the tunnel studies, his research used remote sensing to analyze exhaust from individual vehicles. This allowed him to show that a few cars emit much ammonia, others almost none.

Overall, Baum found, vehicles on average spewed 138 mg/km—about twice the amount of ammonia that Cass had measured 7 years ago. The finding confirms what his team observed in a pilot study of just 20 cars, reported in the July 1 Environmental Science & Technology. In that small study, catalytic converters quashed nitrogen oxides, as intended, in two-thirds of the cars. However, half of these cars emitted ammonia, which is integral to making particulate pollutants.

Concludes Baum, “The catalyst seems the key to the [ammonia] problem.”

Cass, now at the Georgia Institute of Technology in Atlanta, agrees: “It’s clear that not all catalyst systems are performing as they are supposed to.” With three large studies indicating cars as a significant source of urban ammonia, he demands, “what, if anything, will be done about it?”