[Formation and Prevention of Secondary Nitrate in PM_2.5 in Tianjin]

To study the formation and approaches to controlling secondary nitrate in PM_2.5, the ionic compositions of PM_2.5, 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 PM_2.5 was 58μg·m^-3, and the main ionic compositions of PM_2.5 were nitrate (NO₃^-), ammonium (NH₄⁺), sulfate (SO₄^2-), Cl^-, and K⁺ with corresponding mass percentages of 18.4%, 11.6%, 10.3%, 3.3%, and 2.6%, respectively. Concentrations of PM_2.5 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 NO) should be controlled in the spring, autumn, and winter, and attention should be given to the control of the precursors of HNO₃ (NO) and NH₃ in the summer to effectively control nitrate and ammonium aerosols in PM_2.5 in Tianjin.