Water-soluble ions in PM2.5 were serially on-line monitored using the MARGA sampling and measurement system in Chengdu in 2019. Pollution characteristics of water-soluble ions were analyzed using the meteorological monitoring data. The results show that variations in the concentrations of eight water-soluble ions were consistent with the variations in PM2.5 in Chengdu. The annual average mass concentration of the total water-soluble ions was (20.2±12.7) μg ·m-3, accounting for 48.6% of the PM2.5 mass, which indicates that water-soluble ions were the major components of PM2.5. The mass concentrations of all the ions were in the order of NO3- > SO42- > NH4+ > Cl- > Ca2+ > K+ > Mg2+ > Na+. The annual average mass concentration of secondary ions was (20.2±12.7) μg ·m-3, accounting for 87.2% of total water-soluble ions. The concentrations of total water-soluble ions in different seasons were in the order of winter > spring ≈ autumn > summer. Monthly variations in total water-soluble ion concentrations followed a U-shaped curve; mass concentrations were the highest in January and December and lowest from June to August. Monthly variations in the concentrations of NO3-, SO42-, NH4+, Cl-, Na+, and K+ were consistent with the total water-soluble ion concentrations, while the concentrations of Ca2+ and Mg2+ were the highest in June. Visibility declined with the increase in the concentration of water-soluble ions, especially secondary water-soluble ions regardless of the rainfall. Light rain (accumulated rainfall in 24 h <10 mm) had no scavenging effect on water-soluble ions, while moderate and heavy rainfall had a significant effect. There was a significant positive correlation between NO3-, SO42-, and NH4+ (all the correlation coefficients were over 0.7), indicating that the mechanisms of evolution of secondary water-soluble ions in the atmosphere are highly similar to each other. The annual mean values of SOR and NOR were 0.42 and 0.12, respectively, which were negatively correlated with temperature and O3 and positively correlated with humidity, indicating that the main source of SO42- was heterogeneous oxidation reactions in the liquid phase and the main source of NO3- was heterogeneous oxidation reactions at night. The annual mean values of CE/AE and NR were 1.2 and 1.1, respectively, indicating that most aerosols in the study area were relatively alkaline. The atmospheric environment of Chengdu is rich in ammonia; thus, (NH4)2 SO4 and NH4NO3 were the main forms of secondary ions.
Keywords: Chengdu; PM2.5; meteorological parameters; secondary ions; water-soluble ions.