Comparative multi-model study of PM2.5 acidity trend changes in ammonia-rich regions in winter: Based on a new ammonia concentration assessment method.

ABSTRACT

Air quality in ammonia-rich regions such as Zhengzhou is improving year by year, however, fine particulate matter (PM2.5) 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 PM2.5, thus achieving standards in ammonia-rich regions, including Zhengzhou.