The importance of vehicle emissions as a source of atmospheric ammonia in the megacity of Shanghai

Agricultural activities are a major source contributing to NH3 emissions in Shanghai and most other regions of China; however, there is a long-standing and ongoing controversy regarding the contributions of vehicle-emitted NH3 to the urban atmosphere. From April 2014 to April 2015, we conducted measurements of a wide range of gases (including NH3) and the chemical properties of PM2.5 at hourly resolution at a Shanghai urban supersite. This large data set shows NH3 pollution events, lasting several hours with concentrations 4 times the annual average of 5.3aEuro-A mu gaEuro-m(-3), caused by the burning of crop residues in spring. There are also generally higher NH3 concentrations (meanaEuro-A+/-aEuro-1aEuro-sigma) in summer (7.3aEuro-A+/-aEuro-4.9aEuro-A mu gaEuro-m(-3); n = 2181) because of intensive emissions from temperature-dependent agricultural sources. However, the NH3 concentration in summer was only an average of 2.4aEuro-A mu gaEuro-m(-3) or 41aEuro-% higher than the average NH3 concentration of other seasons. Furthermore, the NH3 concentration in winter (5.0aEuro-A+/-aEuro-3.7aEuro-A mu gaEuro-m(-3); n = 2113) was similar to that in spring (5.1aEuro-A+/-aEuro-3.8aEuro-A mu gaEuro-m(-3); n = 2198) but slightly higher, on average, than that in autumn (4.5aEuro-A+/-aEuro-2.3aEuro-A mu gaEuro-m(-3); n = 1949). Moreover, other meteorological parameters like planetary boundary layer height and relative humidity were not major factors affecting seasonal NH3 concentrations. These findings suggest that there may be some climate-independent NH3 sources present in the Shanghai urban area. Independent of season, the concentrations of both NH3 and CO present a marked bimodal diurnal profile, with maxima in the morning and the evening. A spatial analysis suggests that elevated concentrations of NH3 are often associated with transport from regions west-northwest and east-southeast of the city, areas with dense road systems. The spatial origin of NH3 and the diurnal concentration profile together suggest the importance of vehicle-derived NH3 associated with daily commuting in the urban environment. To further examine vehicular NH3 emissions and transport, sampling of the NH3 concentration was performed in (from the entrance to the exit of the tunnel) and out (along a roadside transect spanning 310aEuro-m perpendicular to the tunnel) of a heavily trafficked urban tunnel during the spring of 2014. NH3 concentrations in the tunnel exit were over 5 and 11 times higher than those in the tunnel entrance and in the ambient air, respectively. Based on the derived mileage-based NH3 emission factor of 28aEuro-mgaEuro-km(-1), a population of 3.04 million vehicles in Shanghai produced around 1300aEuro-t NH3 in 2014, which accounts for 12aEuro-% of total NH3 emissions in the urban area. Collectively, our results clearly show that vehicle emissions associated with combustion are an important NH3 source in Shanghai urban areas and may have potential implications for PM2.5 pollution in the urban atmosphere.