Abstract
Abstract. Meteorological conditions within the planetary boundary layer (PBL) are closely governed by large-scale synoptic patterns and play important roles in air quality by directly and indirectly affecting the emission, transport, formation, and deposition of air pollutants. Partly due to the lack of long-term fine-resolution observations of the PBL, the relationships between synoptic patterns, PBL structure, and aerosol pollution in Beijing have not been well understood. This study applied the obliquely rotated principal component analysis in T-mode to classify the summertime synoptic conditions over Beijing using the National Centers for Environmental Prediction reanalysis from 2011 to 2014, and investigated their relationships with PBL structure and aerosol pollution by combining numerical simulations, measurements of surface meteorological variables, fine-resolution soundings, the concentration of particles with diameters less than or equal to 2.5 µm, total cloud cover (CLD), and reanalysis data. Among the seven identified synoptic patterns, three types accounted for 67 % of the total number of cases studied and were associated with heavy aerosol pollution events. These particular synoptic patterns were characterized by high-pressure systems located to the east or southeast of Beijing at the 925 hPa level, which blocked the air flow seaward, and southerly PBL winds that brought in polluted air from the southern industrial zone. The horizontal transport of pollutants induced by the synoptic forcings may be the most important factor affecting the air quality of Beijing in summer. In the vertical dimension, these three synoptic patterns featured a relatively low boundary layer height (BLH) in the afternoon, accompanied by high CLD and southerly cold advection from the seas within the PBL. The high CLD reduced the solar radiation reaching the surface, and suppressed the thermal turbulence, leading to lower BLH. Besides, the numerical sensitive experiments show that cold advection induced by the large-scale synoptic forcing may have cooled the PBL, leading to an increase in near-surface stability and a decrease in the BLH in the afternoon. Moreover, when warm advection appeared simultaneously above the top level of the PBL, the thermal inversion layer capping the PBL may have been strengthened, resulting in the further suppression of PBL and thus the deterioration of aerosol pollution levels. This study has important implications for understanding the crucial roles that meteorological factors (at both synoptic and local scales) play in modulating and forecasting aerosol pollution in Beijing and its surrounding area.
Highlights
Beijing, located on the North China Plain (Fig. 1), is the center of politics, culture, and economics in China
Type 1, 4, and 5 synoptic patterns associated with southerly planetary boundary layer (PBL) winds, high CLD, low boundary layer height (BLH), and high RH2 in Beijing favor the occurrence of heavy aerosol pollution in summer
With the weakest pressure gradient over Beijing (Fig. 5f), the large-scale synoptic forcing of Type 6 to the Beijing area may be overwhelmed by the local atmospheric circulations in the idealized simulations with intense insolation, which may lead to the failure simulation of the PBL structure for Type 6
Summary
Beijing, located on the North China Plain (Fig. 1), is the center of politics, culture, and economics in China. Since the Bohai Sea is located ∼ 150 km to the southeast of China (Fig. 1), a sea breeze (Miller, 2003) can be established and penetrate inland to Beijing This can play a role in affecting the PBL structure and air quality (Liu et al, 2009; Sun et al, 2013; Miao et al, 2015a, b). The importance of the PBL structure for air pollution has been widely recognized (Wang et al, 2014; Miao et al, 2015b; Tang et al, 2016), more investigations are warranted concerning (1) the crucial factors affecting the development of the PBL and (2) the relationships between largescale synoptic forcing and the structure of the PBL They are yet to be fully understood, partly due to the lack of long-term fine-resolution PBL observations (Liu and Liang, 2010).
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