Abstract:

On the basis of the pollution index data of five cities in the Central Plain of Shaanxi Province Guanzhong Plain, China, from 2014 to 2019 and weather research and forecasting model simulation data from the same period, this paper screened the typical years of pollution, defined the weather processes of areal pollution and heavy areal pollution, classified the stream fields over the Central Plain of Shaanxi, and studied the relationships between PM_2.5 and stream field types. Results show the following: (1) According to the airstream fields near the ground, the stream fields of heavy pollution weather can be classified into four major categories: the east airstream (A-C), the west and south airstream (D-F), the non-steering north airstream (G), and the weak airstream field (H). According to the occurrence time and location, the stream fields of heavy pollution weather can be classified into eight minor categories. The east airstream can be classified into northeast airstream (A), northeast airstream at night (B), and northeast airstream around Weinan (C). The west and south airstream can be classified into southwest and south airstream (D), the steering north airstream (E) and the airstream come from Baoji (F). (2) In the processes of areal heavy pollution, most of the airstreams are classified under the east airstream, with a frequency of 59.8%, with types B and C having a frequency of more than 25.0%. The probability of occurrence of different types of airstream field changes greatly day and night. In the daytime, type C appears most frequently, with a frequency of nearly 40.0%. At night, type B appears most frequently, with a frequency of nearly 50.0%, followed by types E and F. (3) PM_2.5 varies greatly in different airstream fields. The concentration of PM_2.5 is higher when the airstream field is type D or E. Mountain-valley wind appearing in airstream field type A and C or B, D and E easily forms vortex flow in the Central Plain of Shaanxi, resulting in pollutant accumulation, increased PM_2.5 concentration, and regional heavy pollution weather. In 2017, the type H airstream field appeared 83 times, and the average concentration of PM_2.5 was 203.4 μg·m^-3. In winter, stable weather, mountain-valley wind, and pollutant transportation from the north are the main causes of heavy pollution, in addition to the increase in pollutant emissions caused by heating.

Key words: Guanzhong region in Shaanxi Province, low-level streamline field typing, weather process of regional heavy pollution