Control of combustion related ammonia emissions can be effective in mitigating PM2.5 pollution in two megacities in Sichuan Basin, Southwest China

Abstract

The elevated concentration of ammonium (NH₄⁺) in PM_2.5 is a significant contributor to haze formation. However, the reduction potential and sources of its precursor ammonia (NH₃) remain unclear, particularly in the Sichuan Basin, where limited attention has been given to these issues. Here, daily PM_2.5 samples were simultaneously collected from the cities of Chengdu and Chongqing between 1 September to 30 November 2017. The concentration of major water-soluble inorganic ions and the δ¹⁵N-NH₄⁺ value in PM_2.5 were measured. As a result, no significant difference in NH₄⁺ concentration was observed between Chengdu (4.8 ± 3.4 μg m⁻³) and Chongqing (4.5 ± 2.7 μg m⁻³). Although clear differences in the major chemical forms of NH₄⁺ in PM_2.5 were identified, the rapid increase in ammonium nitrate (NH₄NO₃) was a key factor in triggering haze formation in both cities. The reduction of NOx emission proved more effective than NH₃ in mitigation aerosol pollution in the short term. However, the decrease in NH₄NO₃ concentrations was more pronounced, particularly on haze days, when both NH₃ and NOx reductions were considered concurrently. The Bayesian Mixing Model (MixSIAR) revealed significant differences in the source-resolved structure of NH₄⁺ between Chengdu and Chongqing. Specifically, combustion and volatile sources accounted for 47.7 % and 52.3 % in Chengdu, while they accounted for 65.1 % and 34.9 % in Chongqing. However, combustion emissions remain a major source of NH₄⁺ in PM_2.5 during haze days in both cities, contributing over 65 % of NH₄⁺ concentrations. Based on these results, reducing combustion emissions is an effective strategy for mitigating haze pollution in Chengdu and Chongqing.