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 PM_2.5 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 5.3 µg m⁻³, caused by the burning of crop residues in spring. There are also generally higher NH₃ concentrations (mean ± 1 <i>σ</i>) in summer (7.3 ± 4.9 µg m⁻³; <i>n</i> = 2181) because of intensive emissions from temperature-dependent agricultural sources. However, the NH₃ concentration in summer was only an average of 2.4 µg m⁻³ or 41 % higher than the average NH₃ concentration of other seasons. Furthermore, the NH₃ concentration in winter (5.0 ± 3.7 µg m⁻³; <i>n</i> = 2113) was similar to that in spring (5.1 ± 3.8 µg m⁻³; <i>n</i> = 2198) but slightly higher, on average, than that in autumn (4.5 ± 2.3 µg m⁻³; <i>n</i> = 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 310 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 28 mg km⁻¹, a population of 3.04 million vehicles in Shanghai produced around 1300 t NH₃ in 2014, which accounts for 12 % 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 PM_2.5 pollution in the urban atmosphere