Characteristics, regional transport and control strategies of atmospheric ammonia in urban Taiyuan, Fenwei Plain, China

Abstract

Ammonia (NH₃) is a significant precursor for secondary inorganic aerosol, in order to better study the impacts of NH₃ on PM_2.5 pollution in Fenwei Plain in China, hourly-resolved NH₃ and water-soluble ions (WSI) were measured at an urban site in Taiyuan from 1 December 2021 to 30 November 2022. Hourly NH₃ concentrations ranged from 0.7 to 40.2 μg m⁻³, with an average concentration of 10.2 ± 5.0 μg m⁻³. Due to the impacts of meteorology and emission sources, NH₃ exhibited apparent seasonal variations: summer > autumn > spring > winter. Diurnal variations of NH₃ concentrations showed higher values during the daytime except in autumn. Cluster analysis of backward trajectories suggested that the southern short-distance air mass from Taiyuan Basin had the highest concentrations of TNHx (NH₃+NH₄⁺) and PM_2.5. The analysis by conditional probability function and weighted concentration weighted trajectory function showed the rough consistency between the distribution of the TNHx and PM_2.5 in four seasons. The analysis of hourly excess NH₃ showed that Taiyuan's atmosphere was always ammonia-sufficient. SOR (nSO₄²⁻/(nSO₄²⁻ + nSO₂)) and NOR (nNO₃^-/(nNO₃^- + nNO₂)) increased with NHR (nNH₃/(nNH₄⁺+nNH₃); n denotes the molar concentration) and RH in four seasons, indicating that the gas-particle conversion of NH₃ promoted the formation of SO₄²⁻ and NO₃⁻ under high RH condition. The critical total ammonia concentrations (CTACs) in spring, summer, autumn, and winter were 63 %, 61 %, 60 %, and 53 %, respectively. Considering the current difficulty in reducing NH₃ and WSI concentration decreased linearly with the reduction of TNO₃ (NO₃⁻ + HNO₃), controlling NOx emissions is more effective for PM_2.5 pollution mitigation in Taiyuan.