*3.2. Relationship between SR, Precipitation, and Particle Mass Concentration*

The scavenging ratio *SR* indicates precipitation effects on the particle mass concentration. Figure 4 shows that the *SR* is positively correlated with accumulative precipitation. The data corresponding to the position of 5 mm on the X-axis are the *SR*s of the precipitation process between 0 and 5 mm; similarly, the position of 10 mm is the average *SR* of the precipitation process with precipitation of 5–10 mm in Figure 4. From the figure, we can see that it increases faster when accumulative precipitation is below 15 mm and more slowly when accumulative precipitation is above 15 mm. The *SR* of PM10 is higher than the *SR* of PM2.5 under the same accumulative precipitation, and when accumulative precipitation is above 50 mm, the precipitation *SR*s of PM2.5 and PM10 are about 50% and 60%, respectively.

**Figure 3.** Effects of different rain intensities (*RI*s) and particle mass concentrations on scavenging efficiency (*SE*).

**Figure 4.** Relationship between precipitation and scavenging rate (*SR*) about the particle mass concentrations ((**a**), PM10; (**b**), PM2.5). (The solid line is the arithmetic mean of the *SR*, and the shaded area is the 25% and 75% percentile values of all the individual cases).

The precipitation effect on the removal of particles is not only related to precipitation but also related to the particle mass concentration before precipitation starts. Figure 5 shows the effect of the particle mass concentration on the *SR* under different precipitation volumes. We can see that the *SR* and particle mass concentration before the rain are positively correlated, the arithmetic mean *SR* of precipitation at any level is above zero while the PM2.5 concentration is higher than 50 μg m<sup>−</sup>3, and the average *SR* of precipitation at almost any level is less than zero while the PM2.5 concentration is lower than 20 μg m−<sup>3</sup> (which means precipitation had a very limited scavenge). In addition, the *SR* increases faster with the increase in the particle mass concentration when the particle mass concentration is below 50 μg m<sup>−</sup>3, and it increases more slowly when the particle mass concentration is higher than 50 μg m<sup>−</sup>3.

Assuming that the *SR*s of PM2.5 and PM10 are, respectively, *SRpm*2.5 and *SRpm*10, the particulate matter concentration before precipitation is *C*, and the process of rainfall is *P*, the quadric surface fitting is performed on the segmentation statistical results in Figure 5 (not for all samples, but for the classification analysis of samples as shown in Figure 5), and the results are as follows:

$$SR\_{pm2.5} = -66.6 + 1.4323P + 1.4241C - 0.0148P^2 - 0.0053C^2 + 0.0032P \times C \tag{8}$$

$$SR\_{pm10} = -85.04 + 1.2770P + 1.8157C - 0.0126P^2 - 0.0070C^2 + 0.0024P \times C \tag{9}$$

**Figure 5.** Effect of particle concentration on *SR* under different precipitation volumes. Rain ranges: 0–1, 1–5, 5–10, 10–20, 20–30, 30–50, >50 mm.

The Adj. R-Square of the two equations is 0.87 and 0.90, respectively, which means that the deviation between the fitting data and the statistical data is small, and the fitting effect is good.

#### *3.3. Region Difference of SR*

In this section, it is discussed whether there are any differences between different regions for the precipitation, *RI*, particle mass concentration, and *SR* in Jiangsu. Figure 6 shows the relationship among precipitation, particle mass concentration, and *SR* in Jiangsu. The average concentration of PM2.5 in the 10 inland cities was 51.0 μg m<sup>−</sup>3, and the average concentration of PM10 was 80.5 μg m<sup>−</sup>3, higher than the average concentration of PM2.5 in the coastal areas, which was 40.4 μg m<sup>−</sup>3, and the average concentration of PM10, which was 63.0 μg m<sup>−</sup>3. The *SR* of coastal areas is less than the *SR* of inland Jiangsu, which is consistent with the distribution of the particle mass concentration because the inland concentration is higher than the coastal concentration. However, precipitation is also an important factor. The increase in the *RI* and mean precipitation accumulation was beneficial to the increase in the *SR*. The *SR* in coastal areas is relatively low because the concentration of particulate matter is lower than that in inland areas. The higher the precipitation, the higher the *SR* and *SE*. Therefore, the precipitation distribution center in south Jiangsu shows the *SR* is higher in southwest Jiangsu.


**Table 1.** Relationships between precipitation, *RI*, particle mass concentration, and *SR* in Jiangsu from 2013 to 2017 (data are consistent with Figure 6).

**Figure 6.** Relationships between precipitation, *RI*, particle mass concentration, and *SR* in Jiangsu from 2013 to 2017 (the data in Table 1). (The average PM concentration is based on the mean hourly particulate concentration of all stations in each city over a five-year period. The average values of precipitation and the precipitation rate are the average values of all precipitation processes in the region in 5 years. Three colors in the map: red, the southern inland area; blue, coastal areas; and orange, the northern inland area.)

In this section, it is discussed why the *SR* of PM2.5 in the northern coastal area of Jiangsu is higher than that in the southern coastal area though the PM2.5 concentration and average precipitation in the northern coastal area are less than those in the southern coastal area. This is due to the influence of the *RI*, which is larger in the northern coastal area. The *SR* of PM10 is less affected by the *RI* compared with the *SR* of PM2.5. Therefore, the *SR* of PM10 in southeast Jiangsu is higher than that in the northeast region in spite of the larger *RI* in the northeast. Therefore, the *SR* of PM2.5 is more affected by the *RI*. Precipitation with a low *RI* has almost no *SE* on PM2.5. Therefore, continuous drizzle can cause a large amount of precipitation over a long time period but cannot effectively reduce the concentration of PM2.5. Since low-*RI* precipitation has some *SE* on PM10, more precipitation (meaning high-*RI* precipitation or continuous drizzle) can reduce the PM10 concentration effectively.
