Comparative multi-model study of PM_2.5 acidity trend changes in ammonia-rich regions in winter: Based on a new...

Air quality in ammonia-rich regions such as Zhengzhou is improving year by year, however, fine particulate matter (PM_2.5) pollution is serious in winter. Aerosol acidity (pH) affects every aspect of the surrounding particle composition and environment. Thermodynamic models of gaseous and particulate composition datasets can provide pH estimates. Nevertheless, for ammonia-rich regions in the presence of prolonged NH₃ deficiency, the thermodynamic model is limited in calculating pH by using only datasets composed of the particulate phase. In this study, an NH₃ concentration calculation method was established via SPSS-coupled multiple linear regression to simulate the trend of NH₃ concentration over a long period of time and to assess the long-term pH value in ammonia-rich regions. The reliability of this method was verified using multiple models. The range of NH₃ concentration change from 2013 to 2020 was found to be 4.3-68.6 μg·m^-3, and the range of pH change was 4.5-6.0. The pH sensitivity analysis indicated that decreasing aerosol precursor concentrations and variations in temperature and relative humidity were the driving factors for aerosol pH changes. Therefore, policies to reduce NH₃ emissions are becoming increasingly necessary. This study provides a feasibility analysis for reducing PM_2.5, thus achieving standards in ammonia-rich regions, including Zhengzhou.