“Dome effect” of black carbon amplifies transboundary transport of haze in China

The formation mechanisms of regional long-lasting haze and the physical and chemical connections between different regions are unclear. We found that long-range transport and aerosol-boundary layer feedback may interact and amplify transboundary air pollution transport over a distance of 1000 km.

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Haze pollution currently is one of the most important environmental challenges in China. Understanding the formation mechanisms of haze needs cross-disciplinary efforts from the perspectives of atmospheric chemistry, atmospheric physics and meteorology as well as their interactions. As the three “golden cases” demonstrated in Fig. 1, our interest in this topic started from 2012 and has extended from city scale to regional scale in eastern China. 

Fig. 1. Time series of the PM2.5 concentrations at the SORPES station in Nanjing and the three “golden” cases to understand the air pollution-boundary layer -weather interactions.

In mid-June 2012, an intense haze pollution occurred in the Yangtze River Delta (YRD) region in eastern China. Our field measurements at the Station for Observing Regional Processes of the Earth System (SORPES) in Nanjing well reordered the evolution of chemical species and meteorological parameters during this event (1). Based on data analysis together with model simulations, we found that the mixed plumes from agricultural straw burning and fossil fuel combustions could cause positive feedback loops to enhance secondary aerosols formation and pollution accumulation in the planetary boundary layer, and further modify weather by changing temperature stratification and regional rainfall patterns (Fig. 2) (1-3). 

Fig. 2. Conceptual scheme showing how mixed biomass burning and urban plumes influence boundary layer and weather as well as atmospheric chemistry (Ding et al. (1)).

In December, most of the YRD cities as well as the eastern China suffered from persistent haze pollution. The main findings of the June 2012 event inspired us to come up with a new hypothesis that aerosol-boundary layer feedback induced by absorption aerosol, black carbon, might be important to enhance haze pollution in megacities. Using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), we examined the role of black carbon in modifying air temperature and boundary layer dynamics and confirmed a significant impact of black carbon in enhancing haze pollution in megacities cities (4). This process has been named as the “dome effect” of black carbon. Using 1-D modeling and field observation, we found the “dome effect” does play important roles in deteriorating haze pollution in cities in northern and eastern China (5, 6).

Fig. 3. Conceptual scheme showing how the “dome effect” of back carbon enhanced haze pollution in megacities (Ding et al.(4)).

However, previous studies mainly demonstrated the local effects at scales from a city to city clusters (e.g. the YRD). In the past few years, air pollution control in China has been implemented in an individual city or sub-regional scale city clusters, such as the two largest cities clusters, the Beijing-Tianjin-Hebei (BTH) and the YRD regions. Under such strict emission control measures, we did achieve considerable improvement of air quality in those regions. However, although the annual averaged concentration of PM2.5 has been substantially reduced (7), heavy haze pollution with PM2.5 concentrations over 300 µg m-3 still frequently engulfs megacities in both regions, such as the 2017/2018 cross-year haze event shown in Fig. 1.

By investigating the 2017/2018 haze event together with other 17 events since 2013 in northern and eastern China, we found that long-range transport and aerosol-boundary layer feedback may interact rather than act as two isolated processes as traditionally thought. This interaction can aggravate local accumulation of air pollutants and enhance secondary formation of haze, which then amplifies transboundary air pollution transport over a distance of 1000 km and boost long-lasting secondary haze from the BTH to the YRD regions (Fig. 4). We also found that an in-advance coordinated cross-regional reduction efforts between different regions, especially black carbon emission control, before the pollution episodes would efficiently achieve a better air pollution mitigation at regional scale.

Fig.4. A conceptual scheme of amplified transboundary transport of haze pollution by aerosol-PBL interactions (Huang et al. (8)).

Our study here provides an excellent example to highlight the importance of cross-disciplinary efforts especially that from both perspectives of physical meteorology and environmental chemistry, in understanding air pollution mechanisms, and also indicates the advantage of a synoptic-scale coordinated cross-regional reduction efforts for severe haze mitigation.

For more details about these studies, please read the paper, “Amplified transboundary transport of haze by aerosol-boundary layer interaction in China”, in Nature geoscience (8), as well as those listed in the references below.

References

[1] Ding, A., Fu, C. et al. Atmos. Chem. Phys.13,10545-10554 (2013).  

[2] Xie, Y., Ding, A. et al., J. Geophys. Res. 120, 24, 12679-12694 (2015).

[3] Huang, X. Ding, A. et al. Atmos. Chem. Phys.16, 10063-10082 (2016).

[4] Ding, A., Huang, X. et al. Geophys. Res. Lett. 43, 2873-2879 (2016).

[5] Wang, Z., Huang, X. & Ding, A. Atmos. Chem. Phys.18, 2821-2834 (2018).

[6] Huang, X., Wang, Z. & Ding, A. Geophys. Res. Lett.45, 16, 8596-8603 (2018).

[7] Ding, A., Huang X. et al., Atmos. Chem. Phys. 19, 18, 11791-11801 (2019)

[8] Huang, X., Ding, A. et al. Nat. Geos. (2020).

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Aijun Ding

Professor, Nanjing University

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