Characteristics and formation mechanism of continuous hazes in China: a case study during the autumn of 2014 in the North China Plain

Abstract

Abstract. Four extreme haze episodes occurred in October 2014 in the North China Plain (NCP). To clarify the formation mechanism of hazes in autumn, strengthened observations were conducted in Beijing from 5 October to 2 November. The meteorological parameters, satellite data, chemical compositions and optical properties of aerosols were obtained. The hazes originated from the NCP, developing in the southwest and northeast directions, with the highest concentration of PM2.5 of 469 μg m−3 in Beijing. The NCP was dominated by a weak high pressure system during the haze episode, which resulted in low surface wind speed and relatively stagnant weather. Moreover, the wind slowed down around Beijing city. The secondary aerosols NO3− was always higher than that of SO42−, which indicated the motor vehicles played a more important part in the hazes in October 2014, even though the oxidation rate from SO2 to SO42− was faster than that of NOx to NO3−. Sudden increases of the concentrations of organic matter, Cl− and BC (black carbon) before each haze episode implied that regional transport of pollutants by biomass burning was important for haze formation during autumn. A satellite map of fire points and the backward trajectories of the air masses also indicated this pollution source. The distinct decrease in the PBL (planetary boundary layer) height during four haze episodes restrained the vertical dispersion of the air pollutants. Water vapor also played a vital role in the formation of hazes by accelerating the chemical transformation of secondary pollutants, leading to hygroscopic growth of aerosols and altering the thermal balance of the atmosphere.

Chemical characterization and source identification of PM<sub>2.5</sub> at multiple sites in the Beijing–Tianjin–Hebei region, China

“…The large increase in the concentrations and contributions of SNA during the haze process was observed in many previous studies and is mainly attributed to enhanced secondary conversions via enhanced heterogeneous reactions under relatively high humidity conditions during the haze periods ( Fig. S6; averaging 67 % during haze episodes and 37 % during clean periods in summer in Beijing, as measured in this study; Huang et al, 2016;Yang et al, 2015). The degree of the secondary formation of sulfate and nitrate is commonly estimated using the sulfur oxidation ratio (SOR = n-SO 2− 4 /n-SO 2− 4 + n-SO 2 , where n refers to molar concentration) and the nitrogen oxidation ratio (NOR = n-NO − 3 /n-NO − 3 +n-NO 2 ), respectively X.…”

“…In this study, the SOR and NOR were significantly elevated during the increases in PM 2.5 concentrations in the summer period and remained at a high level during haze episodes in Beijing (averaging 0.92 for SOR and 0.38 for NOR), Tianjin (0.83 and 0.41),and Shijiazhuang (0.65 and 0.44). In autumn, Yang et al (2015) also revealed that the intense secondary formation of SNA contributed most to the formation of the hazes in October 2014, with the SOR and NOR values increasing considerably during the haze episodes. Similar variations were also observed in winter, but the formation of SNA in winter was much weaker than that in summer and autumn, with SOR values of only 0.18-0.35 and NOR values of 0.20-0.22 during the regional haze episode (13 to 15 January) at the three urban sites.…”

“…In addition, from the annual results, the mass ratio of NO 4 mass ratio has often been used as an indicator of the relative contributions of sulfur and nitrogen from mobile versus stationary sources to aerosol particles in the atmosphere (Arimoto et al, 1996;Cao et al, 2009;Han et al, 2016), as vehicle exhaust and coal combustion emissions are significant contributors of nitrate and sulfate, respectively . Therefore, the higher NO the predominance of motor vehicle emissions in the contributions to PM pollution (Han et al, 2016;Yang et al, 2015), while in Tianjin and Shijiazhuang, coal combustion may still play a dominant role. However, compared to the reported results, the NO 78 (2009Li et al, 2013) to 0.95 (this study).…”

“…The PM 2.5 concentrations in autumn were close to those in summer, but one extreme haze episode was recorded from 5 to 11 October in Beijing and Shijiazhuang, with the highest concentration of the daily PM 2.5 reaching 394 µg m −3 in Beijing and 460 µg m −3 in Shijiazhuang. The extreme haze episode in October 2014 was recorded and analyzed in depth by Yang et al (2015). 3.2 Chemical compositions of PM 2.5…”

The contribution of residential coal combustion to atmospheric PM<sub>2. 5</sub> in northern China during winter

“…Atmospheric NO − 3 and SO 2− 4 are mainly from secondary formation via heterogeneous, multiphase or gasphase reactions, which are dependent on the concentrations of their precursors (NO 2 and SO 2 ) and OH radicals, the surface characteristics and areas of particles, and RH (Ravishankara, 1997; Wang et al, 2013;Quan et al, 2014;Nie et al, 2014;He et al, 2014;Yang et al, 2015;. The remarkably higher concentrations of NO 2 , SO 2 and PM 2.5 at BD, WD and DBT compared to BJ (Table 2) densities, and thus residential coal combustion might also make an evident contribution to atmospheric pollutants at BJ.…”