Differential Impact of Agricultural and Vehicular Ammonia Emissions on Atmospheric PM_2.5 in Beijing-Tianjin-Hebei

Agriculture and vehicles are important ammonia emissions sources in rural and urban areas, respectively. A comparative study of their environmental impacts is of great significance for the scientific formulation of emission reduction strategies. However, there is currently a lack of research on the differential impacts of agricultural and vehicular NH₃ emissions. This study established a NH₃ emission inventory with 1 km resolution for fertilizer, livestock and vehicles based on the emission factor method, and analyzed their spatial and temporal distribution characteristics. Further, WRF-CAMx-PSAT, combined with Brute-Force method was used to simulate the emission reduction scenarios of agricultural and vehicular NH₃ in January and June 2017 to investigate the differential effects of different NH₃ emission sources on atmospheric PM_2.5. Results show that the ammonia emissions from fertilizer, livestock and vehicles in Beijing-Tianjin-Hebei were 31.0×10⁴ t, 44.7×10⁴ t, and 0.8×10⁴ t, respectively, in 2017. From the perspective of emission reduction effect, when agricultural and vehicular NH₃ emission was gradient reduced by 10%, 30%, 50%, 70%, and 100%, respectively, NH₄⁺ showed a nearly linear change, however, SO₄^2- and NO₃^- showed a non-linear decreasing trend. Influenced by emissions, the average change rates (-16.3%, -4.3%, and -37.3%) of NH₄⁺, SO₄^2-, and NO₃^- concentrations under the agricultural NH₃ reduction scenarios were significantly higher than those (-1.7%, -0.2%, and -4.3%) under the vehicular NH₃ reduction scenarios. However, from the perspective of emission reduction efficiency, the opposite phenomenon was found. To analyze the emission reduction efficiency, the NH₃ emission reduction air quality improvement efficiency index was proposed and defined as the change in PM_2.5 concentration under the unit (10⁴ tons) of NH₃ emission reduction. By comparison, it is found that the air quality improvement efficiency of vehicular NH₃ emission reduction was significantly higher than that of agricultural NH₃ emission reduction, with a difference of 1.0-3.2 times and 1.2-2.3 times in January and June, respectively. The results of the study can provide scientific basis for the optimal prevention and control of air pollution.