For the high PM2.5
concentrations (i.e., the atmospheric particles with aerodynamic diameters equal to or less than 2.5 µm) in ambient atmosphere [1
] and its negative effects on human health [3
], causes [7
], and the impacts on environment and climate [9
] have been intensively studied in recent years.
Anthropogenic emissions of air pollutants are the primary driver for the increase in PM2.5
], and unfavorable meteorological conditions are the major driver to exacerbate air pollution [13
]. Four regions are closely associated with heavy air pollution on account of their high emission intensity—the North China Plain (NCP) [16
], the Yangtze River Delta (YRD) [18
] in East China, the Pearl River Delta (PRD) [20
] in South China, and Sichuan Basin (SCB) [22
] in Southwest China. Adverse meteorological conditions, including weak surface winds, temperature inversion layer, and low boundary layer height, are conducive to the formation of air pollution [25
]. The Chinese government has implemented a series of tailored strategies to restrict pollutant emissions [26
], Clean Air Action for instance, to improve the atmospheric environment, or utilized wind corridors to better disperse air pollutants [28
]. Nonetheless, compared to the anthropogenic emissions and meteorological conditions, less attention was paid to the effects of topography on air pollution.
In fact, terrain could change the meteorological conditions further influencing the air quality. Atmospheric circulation changes induced by climate warming of Tibetan Plateau (TP) result in the high frequency of air pollution in central-eastern China [30
]. Moreover, it also accentuates PM2.5
accumulations to the SCB with the impact of TP on mid-latitude westerly winds [31
]. The thermal effect of the Loess Plateau can modulate the boundary layer structure to suppress the mixed layer growth and then increase air pollutants [32
]. Meanwhile, the topographic effects of SCB—a unique deep basin over China intensifies the haze pollution owing to the reducing wind speed and boundary layer height, as well as the raising air temperature and humidity [31
]. The lack of ventilation in the Valley of Mexico leads to high levels of air pollutants [33
]. However, previous studies have not comprehensively explored the ‘sub-basin’ effects on air pollution, which should also be taken into account for polices of mitigating air pollution.
The Twain-Hu basin (THB), characterized by a sub-basin terrain, with altitudes of ~200 m in a.s.l. over central China (Figure 1
a), has become a new center of air pollution [34
] due to its rapid urbanization and dense population. Given the important role of topography in air quality, the reasons of PM2.5
pollution over the sub-basin terrain of THB, should be worth investigating. Moreover, due to THB’s unique geographical location (Figure 1
), air pollutants of the NCP can be transported easily to the THB driven by prevailing northerly winds of East Asian monsoons in the wintertime [35
]. Meanwhile, polluted parcels would be trapped over THB given several mountains locating at the south of THB. Thus, the extent of the terrain impacts on the air pollutants, with regard to the regional transport and local pollution, needs a quantitative assessment. Moreover, how the sub-basin terrain affecting the thermal and dynamic conditions of meteorology and its relationship to air pollution also need to be further investigated.
This paper is organized as follows: Section 2
introduces observation data, method, and numerical experiments conducted in this paper; Section 3
describes the overall heavy air pollution event happened in the THB; Section 4
presents the results of the terrain effects on changes of the air pollutants and meteorology. Section 5
manifests the brief conclusions.
3. Descriptions of a Heavy Air Pollution Episode
As shown in Figure 2
, the THB region suffered a heavy air pollution from 4 January to 10 January 2019. All eight cities experienced high PM2.5
concentrations exceeding 150 μg m−3
with longer duration time for the western cities (XY, JM, JZ, and CD in Figure 1
) compared with those in the eastern cities (SZ, XG, WH, and XN in Figure 1
). During the episode process, the mean PM2.5
concentrations in the western and eastern cities were 189.8 and 106.3 μg m−3
, respectively, both well above the ‘polluted’ threshold of 75 μg m−3
following the ambient air quality standards of China (GB3095-2012). In addition to investigate the terrain effect on the overall PM2.5
concentrations, how sub-basin terrain affecting the regional transport of PM2.5
and its local pollution needs a quantitative evaluation, considering the special geographical location of THB.
From 4 January to 10 January 2019, the THB experienced a heavy air pollution event, which was more serious in the western cities, with an average PM2.5 concentration of 189.8 μg m−3, compared with the eastern cities (the average PM2.5 of 106.3 μg m−3). In this study, four simulation experiments—including E1 (control experiment, CE), E2 (transport experiment, TE), E3 (filling-hgt experiment, FHE), and E4 (filling-hgt & transport experiment, FHE&TE)—were conducted by WRF-Chem model to evaluate the effects of the sub-basin topography on the air quality over THB, central China.
The comparison of E1 and E3 simulations revealed that the sub-basin terrain totally contributed 5.2% PM2.5 worsening the air quality of typical 8 cities over THB. In particular, such terrain can contribute 12.0% to the PM2.5 concentrations in the western cities, but slightly mitigated the air pollution in eastern cities with the contribution of −4.6 %. Vertically, the terrain effects were more substantial at around 0.9 km with the peak PM2.5 difference of 15.2 μg m−3, contributing about 28.1 %.
Differences of the E2 and E4 experiments indicated that the sub-basin terrain was conducive to the PM2.5 accumulation of RT with the average contribution of 39.1 %, which was more significant in the western cities (48.6 %) compared with the eastern cities (29.0 %), while the THB’s topography could mitigate the local PM2.5 pollution by 57.0% over THB via the increased PBLH and VC. However, the air quality was still exacerbated with high levels of PM2.5 persisting over THB, suggesting the outstanding important role of deteriorated PM2.5 of regional transport by sub-basin terrain over the THB.
This study revealed that the meteorology altered by topography can alleviate the local PM2.5 pollution over THB. However, a substantial increase in PM2.5 concentrations exceptionally reflected the important role of sub-basin topography driving increasing PM2.5 of regional transport to worsen the air quality over the THB.
Note that in this study near-surface PM2.5 concentrations were resulted from the combined action of physical and chemical processes (e.g., advection, dry deposition, turbulent diffusion, cloud and aerosol chemistry, wet scavenging, etc.) without separating every process to evaluate their difference of contribution to PM2.5 before and after filling the sub-basin terrain, which could be further studied.