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To study the formation and approaches to controlling secondary nitrate in PM, the ionic compositions of PM, pH of aerosols, variations in NH-NH and HNO-NO concentrations, and the joint NH/HNO 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 was 58μg·m, and the main ionic compositions of PM were nitrate (NO), ammonium (NH), sulfate (SO), Cl, and K with corresponding mass percentages of 18.4%, 11.6%, 10.3%, 3.3%, and 2.6%, respectively. Concentrations of PM 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 NH(g) (gas NH) and HNO(g) (gas HNO) were 16.7μg·mand 1.2μg·m, respectively. The concentrations of NH(g) were relatively higher from April to September and lower from October to February of the following year. HNO(g) concentrations did not show any clear monthly pattern. Except during the summer, NH(g) concentrations were higher in the morning and evening, and HNO(g) concentrations were higher during the day. No clear linear relationships were observed between the concentrations of NH(g) and NH nor the concentrations of HNO(g) and NO at different pH levels. Higher concentrations of NO and NH were observed in the morning and evening, while no linear relationships were observed between the pH and concentrations of NH(g)-NH and HNO(g)-NO. The joint NH/HNO sensitivity regime map showed that most of the points were located in the HNO sensitive region with some in the NH & HNO sensitive region. In spring, autumn, and winter, most of the points were located in the HNO sensitive region while in summer, a significant quantity of the points were located in the NH & HNO sensitive region. Therefore, the precursors of HNO (such as NO) should be controlled in the spring, autumn, and winter, and attention should be given to the control of the precursors of HNO (NO) and NH in the summer to effectively control nitrate and ammonium aerosols in PM in Tianjin.
