Abstract
ABSTRACT This study investigates the vertical thermal and dynamic structure of the atmosphere on synoptic and local scales from a three-dimensional perspective and its contribution to haze formation in Beijing during autumn. On a synoptic scale, the occurrence of heavy haze corresponds either to significant horizontal pollutant transport by southerly winds or to strong atmospheric vertical stability with downward air motion at altitudes below ~1500–3000 m; hence, meteorological parameters measured below ~1500 m serve as better indicators of pollutant transport and dispersion than surface observations. When accompanied by increased southerly winds, the upward air motion between the ground and altitudes above ~1500 m can transport pollutants from surrounding areas to Beijing, resulting in a rapid increase in PM2.5 (within several hours) despite weak winds on the surface, which confirms the possibility of regional transport during stagnant surface conditions and its potential role in haze formation. Additionally, the mountain-plain breeze in the Beijing-Tianjin-Hebei (BTH) region during autumn drives strong local wind circulation, influencing the cumulative stage of haze episodes in this season. During the daytime, this breeze pushes pollutants to areas along the mountains and then to Beijing, resulting in a day-by-day increase in pollution. (By contrast, winter haze episodes arise from the accumulation of local pollution under stable meteorological conditions.) The combination of easterly winds and local topography can induce the formation and dissipation of haze, with the orographic effect propelling the haze into higher layers that host the transport of PM2.5 to the southeast. Afterward, southerly winds carry this pollution plume back along the mountain front, where it merges with surface pollutants through vertical mixing, finally this mixed plume arrived Beijing and contributes to the development of the next haze episode.
Highlights
Beijing, the capital city and the political and cultural center of China, suffers from frequent haze pollution in recent years, which has become one of the greatest issues of concern for the public and government (Zhao et al, 2011; Tao et al, 2014; Han et al, 2016; Huang et al, 2018)
When accompanied by increased southerly winds, the upward air motion between the ground and altitudes above ~1500 m can transport pollutants from surrounding areas to Beijing, resulting in a rapid increase in PM2.5 despite weak winds on the surface, which confirms the possibility of regional transport during stagnant surface conditions and its potential role in haze formation
Using comprehensive observational vertical data from four severe haze episodes that occurred sequentially in Beijing and the surrounding areas during autumn 2014 as well as model simulations of the relevant mechanisms, we analyzed the vertical thermal and dynamic structure of the atmosphere on synoptic and local scales to evaluate its effect on haze formation processes, including the regional transport and accumulation of pollutants, from a three-dimensional perspective
Summary
The capital city and the political and cultural center of China, suffers from frequent haze pollution in recent years, which has become one of the greatest issues of concern for the public and government (Zhao et al, 2011; Tao et al, 2014; Han et al, 2016; Huang et al, 2018). Hua et al (2016) suggested that vertical meteorological and optical profiles are better to identify regional transport than merely ground-based observations They analyzed three PM2.5 pollution episodes occurred sequentially in Beijing in November 2014 and confirmed the important influence of regional transport. In Zhong et al (2018), with ground and vertical meteorological data, 12 persistent heavy aerosol pollution episodes in Beijing were comprehensively analyzed and divided into two stages: the transport stage, characterized by pollutants being transported from the southern source region, and the cumulative stage, dominated by stable atmospheric stratification featuring slight or calm southerly winds, nearground anomalous inversion, and moisture accumulation. Yang et al (2015) and Xu et al (2017) analyzed the characteristics of chemical compositions of aerosols, unfavorable meteorological conditions, and the impact of regional transport for these four haze episodes They indicated that secondary transformation, low PBLH, and increased water vapor were very important for haze formation. It was found that pollutants were aloft within the higher atmosphere (caused by the coupling effect of meteorology and topography) and may turn out to be the reason for the quick formation of the haze episode at the surface
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