Contributions of trans-boundary transport to summertime air quality in Beijing, China

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Abstract. Water vapor has been proposed to amplify the severe haze pollution in China by enhancing the aerosol–radiation feedback (ARF). Observations have revealed that the near-surface PM2.5 concentrations ([PM2.5]) generally exhibit an increasing trend with relative humidity (RH) in the North China Plain (NCP) during 2015 wintertime, indicating that the aerosol liquid water (ALW) caused by hygroscopic growth could play an important role in the PM2.5 formation and accumulation. Simulations during a persistent and heavy haze pollution episode from 5 December 2015 to 4 January 2016 in the NCP were conducted using the WRF-Chem Model to comprehensively quantify contributions of the ALW effect to near-surface [PM2.5]. The WRF-Chem Model generally performs reasonably well in simulating the temporal variations in RH against measurements in the NCP. The factor separation approach (FSA) was used to evaluate the contribution of the ALW effect on the ARF, photochemistry, and heterogeneous reactions to [PM2.5]. The ALW not only augments particle sizes to enhance aerosol backward scattering but also increases the effective radius to favor aerosol forward scattering. The contribution of the ALW effect on the ARF and photochemistry to near-surface [PM2.5] is not significant, being generally within 1.0 µg m−3 on average in the NCP during the episode. Serving as an excellent substrate for heterogeneous reactions, the ALW substantially enhances the secondary aerosol (SA) formation, with an average contribution of 71 %, 10 %, 26 %, and 48 % to near-surface sulfate, nitrate, ammonium, and secondary organic aerosol concentrations. Nevertheless, the SA enhancement due to the ALW decreases the aerosol optical depth and increases the effective radius to weaken the ARF, reducing near-surface primary aerosols. The contribution of the ALW total effect to near-surface [PM2.5] is 17.5 % on average, which is overwhelmingly dominated by enhanced SA. Model sensitivities also show that when the RH is less than 80 %, the ALW progressively increases near-surface [PM2.5] but commences to decrease when the RH exceeds 80 % due to the high occurrence frequencies of precipitation.

“…Figure 1 shows the WRF-Chem Model simulation domain, and Table 1 provides the model configurations. A detailed model description can be found in Wu et al (2019).…”

Section: Wrf-chem Model and Configurationmentioning

confidence: 99%

Is water vapor a key player of the wintertime haze in North China Plain?

Bei

et al. 2019

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“…In recent years, China has been suffering from severe haze pollution in winter -especially over the Beijing-Tianjin-Hebei (BTH) region (Guo et al, 2014;Chang et al, 2016). Observations have shown that OAs play a critical role in haze pollution in China (Xing et al, 2013;Sun et al, 2013;Huang et al, 2014;. Huang et al (2014) reported that OA accounted for 30 %-50 % of the PM 2.5 mass in four megacities in China during severe haze days, with the SOA contribution ranging from 44 % to 71 % in winter.…”

Section: Introductionmentioning

confidence: 99%

Wintertime secondary organic aerosol formation in Beijing–Tianjin–Hebei (BTH): contributions of HONO sources and heterogeneous reactions

Elser

et al. 2019

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Abstract. Organic aerosol (OA) concentrations are simulated over the Beijing–Tianjin–Hebei (BTH) region from 9 to 26 January 2014 using the Weather Research and Forecasting model coupled with chemistry (WRF-CHEM), with the goal of examining the impact of heterogeneous HONO sources on SOA formation and SOA formation from different pathways during wintertime haze days. The model generally shows good performance with respect to simulating air pollutants and organic aerosols against measurements in BTH. Model results show that heterogeneous HONO sources substantially enhance near-surface SOA formation, increasing the regional average near-surface SOA concentration by about 46.3 % during the episode. Oxidation and partitioning of primary organic aerosols treated as semi-volatile dominate SOA formation, contributing 58.9 % of the near-surface SOA mass in BTH. Irreversible uptake of glyoxal and methylglyoxal on aerosol surfaces constitutes the second most important SOA formation pathway during the episode, with the SOA contribution increasing from 8.5 % under non-haze conditions to 30.2 % under haze conditions. Additionally, direct emissions of glyoxal and methylglyoxal from residential sources contribute about 25.5 % of the total SOA mass on average in BTH. Our study highlights the importance of heterogeneous HONO sources and primary residential emissions of glyoxal and methylglyoxal to SOA formation over the BTH region in winter.

“…3b). In winter, the insolation is weak in the north of China, unfavorable for the O 3 photochemical production, so the O 3 level is substantially influenced by the boundary conditions Wu et al, 2017). Hence, one of possible reasons for the O 3 underestimation might be from the uncertainty in the O 3 boundary conditions.…”

Section: Model Performancementioning

confidence: 99%

Does afforestation deteriorate haze pollution in Beijing–Tianjin–Hebei (BTH), China?

Long

Bei

et al. 2018

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Abstract. Although aggressive emission control strategies have been implemented recently in the Beijing-TianjinHebei area (BTH), China, pervasive and persistent haze still frequently engulfs the region during wintertime. Afforestation in BTH, primarily concentrated in the Taihang and Yan Mountains, has constituted one of the controversial factors exacerbating the haze pollution due to its slowdown of the surface wind speed. We report here an increasing trend of forest cover in BTH during 2001-2013 based on long-term satellite measurements and the impact of the afforestation on the fine-particle (PM 2.5 ) level. Simulations using the Weather Research and Forecast model with chemistry reveal that afforestation in BTH since 2001 has generally been deteriorating the haze pollution in BTH to some degree, enhancing PM 2.5 concentrations by up to 6 % on average. Complete afforestation or deforestation in the Taihang and Yan Mountains would increase or decrease the PM 2.5 level within 15 % in BTH. Our model results also suggest that implementing a large ventilation corridor system would not be effective or beneficial to mitigate the haze pollution in Beijing.