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 (SO42−) and ammonium (NH4+) in airborne fine particles (PM2.5) play a vital role in the formation of heavy air pollution in northern China. In particular, the increasing contribution of NO3− to PM2.5 has attracted worldwide attention. In this study, a highly time-resolved analyzer was used to measure water-soluble inorganic ions in PM2.5 in one of the fastest-developing megacities, Tianjin, China, from November 15 to March 15 (wintertime heating period) in 2014–2019. Severe PM2.5 pollution episodes markedly decreased during the heating period from 2014 to 2019. The highest concentrations of NO3− and SO42− 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 NOx concentrations during the heating period of 2018/2019. A continuous decrease in the SO2 concentration led to a decrease in SO42− from 2015/2016 (16.8 ± 21.8 μg/m3) to 2018/2019 (6.5 ± 8.9 μg/m3). The NO3− and SO42− concentrations increased as the air quality deteriorated. However, the proportion of NO3− and SO42− in PM2.5 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 × (SO42−)] 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 × (SO42−)] were less than 1 under increasing PM2.5 pollution. The molar equivalent ratios of [NO3−]/[SO42−] were positively correlated with those of [NH4+]/[SO42−]. When the molar equivalent ratios of [NH4+]/[SO42−] were more than 1.5, those of [NO3−]/[SO42−] 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−, SO42− 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−, SO42− and NH4+ than other areas to the study site. The abovementioned areas were also characterized by a higher contribution of NO3− than of SO42− to the study site and by NH4+-rich conditions. In summary, more efforts should be made to reduce NOx 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.