Modeling of aerosol property evolution during winter haze episodes over a megacity cluster in northern China: Roles of regional transport and heterogeneous reactions

Abstract

<p><strong>Abstract.</strong> Regional transport and heterogeneous reactions played crucial roles in haze formation over a megacity cluster centered on Beijing. In this study, the updated Nested Air Quality Prediction Model System (NAQPMS) and the HYSPLIT Lagrangian trajectory model were employed to investigate the evolution of aerosols – in terms of the number concentration, size distribution, and aging degree – in Beijing during six haze episodes between November 15 and December 15, 2016, as part of the Air Pollution and Human Health–Beijing (APHH-Beijing) winter campaign of 2016. The model exhibited reasonable performance not only for mass concentrations of PM<sub>2.5</sub> and its components in Beijing but also for the number concentration, size distribution, and aging degree. We discovered that regional transport played a nonnegligible role in haze episodes, with contributions of 14 %–31 % to the surface PM<sub>2.5</sub> mass concentration. The contribution of regional transport to secondary inorganic aerosols was larger than that of regional transport to primary aerosols (30 %–63 % vs. 3 %–12 %). The chemical transformation of SO<sub>2</sub> in the transport pathway from source regions to Beijing was the major form of SO<sub>4</sub><sup>2−</sup> regional transport. We also found that sulfate formed outside Beijing from SO<sub>2</sub> that was emitted in Beijing; this sulfate was then blown back to Beijing and considerably influenced haze formation. In the transport pathway, aerosols underwent aging, which altered the mass ratio of coating to black carbon (RBC) and the size distribution of number concentrations. During the episodes, the geometric mean diameter (GMD) increased from less than 100 nm at the initial site to approximately 120 nm at the final site (Beijing), and RBC increased from 2–4 to 4–8. These changes would affect regional radiation and climate. In haze episodes with high humidity, the average contributions of gas and aqueous chemistry, heterogeneous chemistry, and primary sulfate emission were comparable. Primary emissions had the greatest impact under light to moderate pollution levels, whereas heterogeneous chemistry had a stronger effect under high pollution levels.</p>

Effects of Regional Transport on Haze in the North China Plain: Transport of Precursors or Secondary Inorganic Aerosols

“…The composition and phase state of an inorganic aerosol system was calculated using a thermodynamic equilibrium model (ISORROPIA v1.7) (Nenes et al, 1998). Further technical details about the NAQPMS can be found in Li et al (2012, 2011) and Du et al (2019).…”

Size-resolved characterization of organic aerosol in the North China Plain: new insights from high resolution spectral analysis

“…In addition, the aging processes during the transport in the atmosphere is another reason for the increased size at high PM levels because highly polluted periods were oen found to be largely inuenced by regional transport. 54,55 As shown in Fig. S12, † the differences of OA loading between relatively clean days and moderately polluted conditions (35 mg m À3 < NR-PM 1 < 70 mg m À3 ) peaked at $300 nm, which was smaller than that between relatively cleaner days and seriously (NR-PM 1 > 70 mg m À3 ) polluted conditions ($480 nm), highlighting that particles with relatively small sizes play an important role in the formation of moderate pollution compared to severe pollution.…”

Measurement report: On the contribution of long-distance transport to the secondary aerosol formation and aging

“…For the rural GZB transport sector, the fraction of sulfate and nitrate largely decreased to 19 % and 11 % of PM 2.5 , respectively, consistent with lower SO 2 (3.2 ± 2.5 µg m −3 ) and NO 2 (27.8 ± 10.3 µg m −3 ) in the rural GZB transport sector, which was about half of that in the three urban transport sectors (6.3-7.3 µg m −3 for SO 2 and 44.7-51.3 µg m −3 for NO 2 ). The high fraction of sulfate in the BTH transport sector was supported by high concentrations of SO 2 and sulfate in the BTH region and central China region (Du et al, 2019;Chen et al, 2020;Li et al, 2021a;Sun et al, 2022). It was further supported by high sulfur conversion efficiency (SOR), for which the BTH transport sector showed the highest SOR of 0.58, followed by the northern China transport sector (0.52), the urban GZB transport sector (0.49) and the rural GZB transport sector (0.44) (Fig.…”

“…Some modeling studies reported that heterogeneous chemistry during transport was identified as the dominant factor during haze episodes in megacities (Li and Han, 2016;Li et al, 2017) and were further supported by the observations. Du et al (2019) reported that the chemical transformation from SO 2 to sulfate was the major source of sulfate in Beijing. Li et al (2021b) suggested that the pollution in winter in Beijing was largely affected by regional transport, and water vapor during transport of the air mass greatly increased the SIA proportion.…”