On-Road Measurement of Ammonia and Other Motor Vehicle Exhaust Emissions

Ammonia is the primary alkaline gas in the atmosphere and contributes to fine particle mass, visibility problems, and dry and wet deposition. The objective of this research was to measure ammonia and other exhaust emissions from a large sample of on-road vehicles using California phase 2 reformulated gasoline with low sulfur content (~10 ppm by weight). Vehicle emissions of ammonia, NOx, CO, and CO2 were measured in the center bore of a San Francisco Bay area highway tunnel on eight 2-h afternoon sampling periods during summer 1999. Ammonia concentrations were divided by total carbon (mainly CO2) concentrations to compute an emission factor of 475 ± 29 mg L-1 (95% C I.). The molar ratio of nitrogen emitted in the tunnel in the form of ammonia to that emitted in the form of NOx was 0.27 ± 0.01. Emissions of NOx and CO have been measured at this tunnel sampling location since 1994. From 1994 to 1999, emissions decreased by 41 ± 4% for NOx and 54 ± 6% for CO. These reductions include the impacts of turnover in the vehicle fleet and the use of reformulated gasoline. Between 1997 and 1999, when fuel properties did not change significantly, emissions of NOx and CO decreased by 26 ± 2% and 31 ± 3%, respectively. While use of three-way catalytic converters has contributed to decreases in NOx and CO emissions, their use, in combination with fuel-rich engine operation, is the likely cause of the ammonia emissions from motor vehicles observed during this study.