Formation and Prevention of Secondary Nitrate in PM2.5 in Tianjin

Abstract

To study the formation and approaches to controlling secondary nitrate in PM2.5, the ionic compositions of PM2.5, pH of aerosols, variations in NH3-NH4+ and HNO3-NO3- concentrations, and the joint NH3/HNO3 sensitivity regime map of ammonium nitrate were investigated based on high-resolution online monitoring data for an urban site in central Tianjin from 2018 to 2019. The results showed that the average concentration of PM2.5 was 58μg·m-3, and the main ionic compositions of PM2.5 were nitrate (NO3-), ammonium (NH4+), sulfate (SO42-), Cl-, and K+ with corresponding mass percentages of 18.4%, 11.6%, 10.3%, 3.3%, and 2.6%, respectively. Concentrations of PM2.5 and the main components were relatively high during the heating season and relatively low during the non-heating season. The aerosols were weakly acidity with an average pH of 5.21; pH was higher in spring and winter and lower in summer and autumn, and diurnal variations pH were lower in the morning (00:00-08:00) and slightly higher at other times. The concentrations of NH3(g) (gas NH3) and HNO3(g) (gas HNO3) were 16.7μg·m-3and 1.2μg·m-3, respectively. The concentrations of NH3(g) were relatively higher from April to September and lower from October to February of the following year. HNO3(g) concentrations did not show any clear monthly pattern. Except during the summer, NH3(g) concentrations were higher in the morning and evening, and HNO3(g) concentrations were higher during the day. No clear linear relationships were observed between the concentrations of NH3(g) and NH4+ nor the concentrations of HNO3(g) and NO3- at different pH levels. Higher concentrations of NO3- and NH4+ were observed in the morning and evening, while no linear relationships were observed between the pH and concentrations of NH3(g)-NH4+ and HNO3(g)-NO3-. The joint NH3/HNO3 sensitivity regime map showed that most of the points were located in the HNO3 sensitive region with some in the NH3 & HNO3 sensitive region. In spring, autumn, and winter, most of the points were located in the HNO3 sensitive region while in summer, a significant quantity of the points were located in the NH3 & HNO3 sensitive region. Therefore, the precursors of HNO3 (such as NOx) should be controlled in the spring, autumn, and winter, and attention should be given to the control of the precursors of HNO3 (NOx) and NH3 in the summer to effectively control nitrate and ammonium aerosols in PM2.5 in Tianjin.