In situ continuous hourly observations of wintertime nitrate, sulfate and ammonium in a megacity in the North China plain from 2014 to 2019: Temporal variation, chemical formation and regional transport

Nitrate (NO₃^-), sulfate (SO₄^2-) and ammonium (NH₄⁺) in airborne fine particles (PM_2.5) play a vital role in the formation of heavy air pollution in northern China. In particular, the increasing contribution of NO₃^- to PM_2.5 has attracted worldwide attention. In this study, a highly time-resolved analyzer was used to measure water-soluble inorganic ions in PM_2.5 in one of the fastest-developing megacities, Tianjin, China, from November 15 to March 15 (wintertime heating period) in 2014-2019. Severe PM_2.5 pollution episodes markedly decreased during the heating period from 2014 to 2019. The highest concentrations of NO₃^- and SO₄^2- were recorded in the heating period of 2015/2016. Afterwards, NO₃^- decreased from 2015/2016 (20.2 ± 23.8 μg/m³) to 2017/2018 (11.6 ± 14.8 μg/m³) but increased with increasing NO_x concentrations during the heating period of 2018/2019. A continuous decrease in the SO₂ concentration led to a decrease in SO₄^2- from 2015/2016 (16.8 ± 21.8 μg/m³) to 2018/2019 (6.5 ± 8.9 μg/m³). The NO₃^- and SO₄^2- concentrations increased as the air quality deteriorated. However, the proportion of NO₃^- and SO₄^2- in PM_2.5 slightly increased when the air quality deteriorated from moderate pollution (MP) to severe pollution (SP) levels. The average molar ratios of NH₄⁺ to [NO₃^-+2 × (SO₄^2-)] were 1.7, 0.9, 1.2, 1.2 and 1.5 for the heating periods of 2014/2015, 2015/2016, 2016/2017, 2017/2018 and 2018/2019, respectively, most of which were higher than 1.0, thus revealing an overall excess of NH₄⁺ during the heating periods. However, the molar equivalent ratios of [NH₄⁺] to [NO₃^-+2 × (SO₄^2-)] were less than 1 under increasing PM_2.5 pollution. The molar equivalent ratios of [NO₃^-]/[SO₄^2-] were positively correlated with those of [NH₄⁺]/[SO₄^2-]. When the molar equivalent ratios of [NH₄⁺]/[SO₄^2-] were more than 1.5, those of [NO₃^-]/[SO₄^2-] increased from close to 1 to higher values, indicating that the dominance of NO₃^- formation played an important role. The results of nonparametric wind regression exhibited distinct hot spots of NO₃^-, SO₄^2- and NH₄⁺ (higher concentrations) in the wind sectors between NE and SE at wind speeds of approximately 6-21 km/h. The southern areas in the North China Plain and parts of the western areas of China contributed more NO₃^-, SO₄^2- and NH₄⁺ than other areas to the study site. The abovementioned areas were also characterized by a higher contribution of NO₃^- than of SO₄^2- to the study site and by NH₄⁺-rich conditions. In summary, more efforts should be made to reduce NO_x in the Beijing-Tianjin-Hebei region. This study provides observational evidence of the increasingly important role of nitrate as well as scientific support for formulating effective control strategies for regional haze in China.