*3.3. Synoptic Patterns and Their Impacts on PM2.5 Spatial Distribution* 3.3.1. Identified Synoptic Patterns

By implementing the PTT classification method, the synoptic patterns in SCB and the surrounding area were classified into nine types in total, according to the daily average geopotential heights at 850 hPa, from 1 December 2015 to 30 November 2019. Among these patterns, the last three types occurred in less than 10 days and the first six types occurred in more than 98.8% of the days accumulatively. Hence, only the first six patterns were analyzed in this study, and their occurring days are shown in Table 2.


**Table 2.** The occurring days of 6 synoptic patterns in four seasons from 2016 to 2019.

The first synoptic pattern occurred in 432 days, out of 1443 days in total, which was much more than other types. The occurring days of Types 2–6 were 286, 252, 242, 172 and 59 days, respectively. Seasonally, Type 1 and Type 2 almost evenly occurred throughout the four seasons. The occurrence frequencies of Type 3 were highest in winter, followed by autumn and spring, but only occurred on 3 days in summer. On the contrary, the occurrence frequencies of Type 4 were relatively higher in summer and autumn, and close to zero in winter and autumn. Type 5 occurred more frequently in spring and summer than the other two seasons, while Type 6 almost occurred only in winter. Therefore, we mainly discussed Type 1, Type 2, Type 3 and Type 6 in winter, Type 1, Type 2, Type 3 and Type 5 in spring, Type 1, Type 2, Type 4 and Type 5 in summer, and Type 1, Type 2, Type 3 and Type 4 in autumn.

The spatial distribution characteristics of 850 hPa potential heights for six synoptic patterns are shown in Figure 5. In Type 1, the SCB was controlled by a weak low-pressure system, noted as weak low-pressure type. When Type 2 occurred, the SCB was under the control of a weak high-pressure system and uniform pressure fields (weak high-uniformpressure type). In Type 3, there was a strong high-pressure center to the north of SCB, noted as northern high-pressure type. Type 4 could be summarized as eastern weak high-pressure type for the existence of a weak high-pressure center to the east of SCB. Similarly, Type

5 could be named as southwestern weak low-pressure type for the existence of a weak low-pressure center to the southwest. Type 6 was also characterized with the control of a weak low-pressure system, just as Type 1, but collocating a high-pressure system to the north of SCB, noted as weak low-pressure with northern high-pressure type.

**Figure 5.** The distribution of geopotential heights at 850 hPa (white isolines) and the horizontal wind fields at 850 hPa (colored shadings and black vectors) for the six synoptic patterns in SCB.

Figure 5 presents the wind fields at 850 hPa for the six synoptic patterns in SCB. In Type 1, the regions to the east of SCB prevailed southerly winds. Air masses originated in southern China, such as Guangxi and Guangdong provinces, and entered the SCB through the southeastern edge of the basin (the south part of CQ). This air flows towards the north through the central and eastern basin, and turns west in the western basin. Affected by the topography, the wind speeds in the western basin were relatively low. In Type 2,

the uniform pressure fields led to extremely calm winds in the basin. The northeast part of the basin was controlled by the westerly or northwesterly air flows. Type 3 presented prevailing strong northerly winds in the areas to the east of SCB. These strong northerly winds entered SCB through the high mountains in the north and caused a relatively strong wind zone in the northern basin and western basin. The cyclonic circulation converged in the southeastern basin and led to a calm wind zone there. In Type 4, easterly winds invaded SCB through the eastern part of the basin area and turned south in the western part of the basin. This circulation pattern made the wind velocity in the southern part of the basin extremely low. In Type 5, although southerly winds prevailed in the area to the southeast of SCB, the air masses from the south could not cross over the mountains in the southeastern edge of the basin. Hence, the basin was mainly affected by the northerly air flows and created similar circulations as Type 3. The circulation features in Type 6 were similar to those in Type 5 but the wind speeds were relatively lower.
