Emission factor of ammonia (NH₃) from on-road vehicles in China: tunnel tests in urban Guangzhou

Ammonia (NH₃) is the primary alkaline gas in the atmosphere that contributes to formation of secondary particles. Emission of NH₃ from vehicles, particularly gasoline powered light duty vehicles equipped with three-way catalysts, is regarded as an important source apart from emissions from animal wastes and soils, yet measured emission factors for motor vehicles are still not available in China, where traffic-related emission has become an increasingly important source of air pollutants in urban areas. Here we present our tunnel tests for NH₃ from motor vehicles under 'real world conditions' in an urban roadway tunnel in Guangzhou, a central city in the Pearl River Delta (PRD) region in south China. By attributing all NH₃ emissions in the tunnel to light-duty gasoline vehicles, we obtained a fuel-based emission rate of 2.92 ± 0.18 g L⁻¹ and a mileage-based emission factor of 229.5 ± 14.1 mg km⁻¹. These emission factors were much higher than those measured in the United States while measured NO _x emission factors (7.17 ± 0.60 g L⁻¹ or 0.56 ± 0.05 g km⁻¹) were contrastingly near or lower than those previously estimated by MOBILE/PART5 or COPERT IV models. Based on the NH₃ emission factors from this study, on-road vehicles accounted for 8.1% of NH₃ emissions in the PRD region in 2006 instead of 2.5% as estimated in a previous study using emission factors taken from the Emission Inventory Improvement Program (EIIP) in the United States. (letter)