Factors affecting the different growth rates of PM2.5：Evidence from composition variation, formation mechanisms, and importance analysis of water-soluble inorganic ions with case study in northern China

Abstract

PM_2.5 affects air quality, therefore, understanding the mechanism of PM_2.5 growth is essential to figure out mitigation measures. Hourly real-time concentrations of water-soluble inorganic ions (WSIIs), including anions and cations, in fine particulate matter (PM_2.5) were measured in Baoji, northwest China. During the winter monitoring period, the concentrations of PM_2.5 and most WSIIs exhibited similar trends. Mass proportions of SNA [i.e., sulfate (SO₄²⁻), nitrate (NO₃⁻), ammonium (NH₄⁺)] in PM_2.5 gradually increased with air deterioration, while equivalent ratios of anions to cations also increased. The heterogeneous aqueous reactions and/or gas-phase homogeneous reactions promoted the formation of secondary inorganics, especially during the haze events. Rapid transformations of primary gaseous precursors to secondary pollutants could lead to the substantial formation of SO₄²⁻ and NO₃⁻. In terms of particle growth rate, the mass proportions of SNA in PM_2.5 decreased from General Growth (GG) to Explosive Growth (EG) events. Furthermore, the particle growth rates did not coincide with the pollution levels, while it occurred most frequently during the Transition Period, instead of the Polluted Period. The diurnal variation of SNA at different PM_2.5 growth rates has been discussed. The results of the Random Forest (RF) model showed that RH was an important factor for EG of PM_2.5, while low RH was a reliable reason for the relatively low mass proportion of SNA. The results of this study could advance our understanding of particle growth and provide scientific evidence to support the establishment of unique air quality control measures under different pollution scenarios in Fenwei Plain, China.