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

Abstract

Abstract. 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 dataset shows NH3 pollution events, lasting several hours with concentrations four 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 ± 1σ) in summer (7.3 ± 4.9 μg m-3; n = 2181) because of intensive emissions from temperature-dependent agricultural sources. However, the NH3 concentration in summer was only an average of 2.4 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; n = 2113) was similar to that in spring (5.1 ± 3.8 μg m-3; n = 2204) but slightly higher, on average, than that in autumn (4.5 ± 2.3 μg 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 310 m perpendicular to the tunnel) of a heavily trafficked urban tunnel during the spring 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 PM2.5 pollution in the urban atmosphere.

Assessing Contributions of Agricultural and Nonagricultural Emissions to Atmospheric Ammonia in a Chinese Megacity

“…61 From a climate perspective, differences in temperature and other meteorological parameters (e.g., precipitation, wind speed, planetary boundary layer) over the Shanghai region are minor. 36 Interestingly, the lowest NH 3 concentrations at urban Shanghai sites were not observed in the winter, while the NH 3 difference between summer and winter is much lower at urban sites than at rural sites in our dataset (Fig. 3).…”

“…17 Previous observations have also shown coincident enhancements of NH 3 and carbon monoxide (CO) in the Shanghai urban atmosphere. 36 Following a stable period of NH 3 concentrations between 22:00 and 5:00 (5.7 ± 0.1 μg m -3 ), the maximum NH 3 concentration occurs in the morning rush hour (7.0 μg m -3 , 10:00), 22% higher than the overnight level. In Fig.…”

“…44 For example, the region has continuously experienced double-digit growth in auto sales since 2009. 36 The expanding motor vehicle population in its cities, in turn, is reshaping the urban atmospheric composition. 59,60 Meanwhile, the vast rural areas of the YRD region are dominated by fluvial plains with fertile soil, and abundant production of rice and tea.…”

Tall Tower Vertical Profiles and Diurnal Trends of Ammonia in the Colorado Front Range

“…This will likely cause some of the impacts of temperature to be observed some time after the temperature‐driven emissions actually occur. A lack of positive correlation of NH 3 with temperature has been observed previously in similar locations surrounded by intensive agriculture where source region impacts dominate (Burkhardt et al, ; Chang et al, ; Horvath & Sutton, ; Pryor et al, ; Sharma et al, ; Vogt, Held, & Klemm, ).…”

Potential Sources and Formations of the PM2.5 Pollution in Urban Hangzhou

“…The height of 500m is chosen here as the averages of nighttime and daytime boundary heights of cities were mostly around or higher than 500 m [10,25,26], and air pollutants are well mixed below this layer. The height of 500 m is frequently chosen as the receptor height for backward air trajectories [14,[27][28][29][30].…”