Atmospheric ammonia is a precursor to the formation of fine particulate matter, which contributes to human health problems and decreased visibility. Atmospheric ammonia may also be transported through wet and dry deposition to water bodies such as the Chesapeake Bay, contributing to excess nutrient loadings that cause eutrophication. Vehicle exhaust contains ammonia, created by a reaction of NOx with H2 in the three-way catalytic converter. Ammonia emissions were measured using a mass balance on the Fort McHenry Tunnel, Baltimore, Maryland. Atmospheric concentrations of gas-phase and particulate-phase ammonium were measured during 2003-2004 using denuders and filter packs. The average vehicle (veh) emission rate for these studies was 8.1 ± 4.3 mg NH3-N veh-1 km-1. We estimate the annual emission of ammonia from vehicles to be 151 metric tons NH3-N yr-1 in Baltimore City and County and 707 metric tons NH3-N yr-1 in all of Maryland.