Detection of excess ammonia emissions from in-use vehicles and the implications for fine particle control

The emission rate for ammonia from in-use vehicles is calculated based on measurements made inside a Los Angeles roadway tunnel. Using fleet distributions by vehicle age and type, known catalyst distributions, and fuel economy by model year and vehicle type and attributing all ammonia to vehicles equipped with three-way catalysts or three-way catalysts plus oxidizing catalysts (dual-bed catalysts), we estimate an average ammonia emission rate of 72 mg km^-1 for these vehicles, or 61 mg km^-1 driven by the vehicle fleet as a whole. These emissions can emanate from vehicles running under rich air-fuel conditions, with three-way catalytic converters designed to reduce NO(x) to N₂ and O₂ in addition forming NH₃. Resulting calculations estimate ammonia emissions of 24-29 t of NH₃ day^-1 from the vehicle fleet for the South Coast Air Basin (SoCAB) that surrounds Los Angeles. This represents an increase in the daily emissions of ammonia attributable to motor vehicles in the SoCAB from 2% of basin wide emissions before the introduction of catalyst-equipped automobiles to 15% based on the current experiment. The air basin wide emission rate of ammonia from motor vehicles is compared to ammonia emissions from livestock waste decomposition at local dairies, and the implications for control of fine particle ammonium nitrate concentrations are discussed.