陕西关中重污染天气低空流场的分型研究

Low-level streamline field typing of heavy pollution weather of Guanzhong region in Shaanxi Province

利用陕西关中区域内五市(西安、咸阳、渭南、铜川、宝鸡)有PM_(2.5)监测数据以来的2014—2019年各市污染指数(AQI)筛选代表年,定义关中区域污染和区域重污染天气过程,结合陕西WRF(Weather research & forecasting model)模拟数据,对影响关中区域的流场分型,并重点研究了各流场分型与PM_(2.5)浓度的关系。结果表明:(1)根据关中区域低空流场,将重污染天气日流场分为:东部来流(A~C)、西部/南部来流(D~F)、北部来流非转向(G)和无显著流场(H)4个大类,根据出现时间和地点等,东部来流又分为东北来流(A)、东北来流夜间型(B)、渭南绕山来流(C),西部/南部来流分为西南/南部来流(D)、北部来流转向(E)、宝鸡来流(F),共8个类型分别记为A~H。(2)区域重污染过程中东部来流出现最多,频率达到59.8%,其中B、C型占比均达到25.0%以上;各流场分型昼夜变化较大,在昼间(08:00—19:00)流场中,出现最多的是C型流场,在夜间(20:00—07:00)流场中B、E、F型比较活跃,出现最多的是B型流场,另外A、D、G、H型夜间出现比昼间多。(3)关中区域重污染主要出现在冬季,除因供暖导致的污染物的排放量增加外,低空流场对污染物扩散影响显著,从流场分型上看,静稳天气、山谷风、北方污染物输送是导致关中区域重污染的主要原因。

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 203.4μg·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.