## 3 Results  
### The Fitting Result of \(E_{ext}(RH)\) and Statistics Analysis of \(f(RH)\)  
In this study, \(\rm E_{ext}(RH)\) fitting results for five different urban sites were selected for analysis, which are Chengdu Lingjianglu (built-up area), Leshan Jiajiangxian (near the river), Yibin Xingwenerzhong (densely vegetated area), Bazhong Tongjiangzhongxue (near the river), and Chongqing Guanyinqiao (commercial area), and the fitting results are shown in Figure 3 and Figures S1-S4. In general, (1) the fitting ability of the model was strong at all five stations, with r around 0.9 at the stations of Leshan Jiajiang, Yibin Xingwen, and Chongqing Guanyinqiao; only in February, June, and December was the r less than 0.8 at the station of Bazhong Tongjiang, and the r was less than 0.8 at the station of Chengdu Linjiang only in July and August. (2) While the RH value is small, \(\rm E_{ext}(RH)\) grows slowly with RH, but the fitted curve is flat while RH > 80%, and the \(\rm E_{ext}(RH)\) shows a fast-rising trend.  
The fitting results of the five stations have obvious differences: the fitted curve of the Chengdu Lingjianglu station is basically the same from January to December and shows a slow rise when the relative humidity is higher, indicating that the aerosol hygroscopic growth characteristics of this station are weak, and its aerosol sources and environmental conditions vary less with the season [54], which may be related to the location of the selected station in a built-up area. The differences in hygroscopic growth characteristics between the months are large at Leshan Jiajiang and Bazhong Tongjiang, which are located close to the river. The fitted curve of Leshan Jiajiang shows a slow rise from January to May but a sharp rise from June to October, indicating that the hygroscopic growth characteristics vary greatly with the seasons, and there is almost no relative humidity less than 40% in July and August. For Bazhong Tongjiang, the fitted curve shows a sharp and steep rise in all 12 months. The hygroscopic growth characteristics of Yibin Xingwen, located in a densely vegetated area, and Chongqing Guanyinqiao Station, located in a commercial area, are relatively similar, where the fitted curves both show a slow rise from January to December with no obvious deliquescence.  
Using Equation (5), f(90%) from January to December was calculated for Chengdu Linjianglu, Leshan Jiajiangxian, Yibin Xingwenerzhong, Bazhong Tongjiangzhongxue, and Chongqing Guanyinqiao, respectively. The corresponding f(90%) could not be calculated because of the missing value of relative humidity less than 40% in December at Chongqing Guanyinqiao station and in July and August at Leshan Jiajiang station, as shown in Table 1. Comparison of different stations showed that the hygroscopic growth factors varied widely among stations, the variation of f(90%) at the same site in different months was large. There are 3.91 in November and 1.70 in February at Yibin Xingwenerzhong, and 5.02 in February and 1.13 in October at Leshan Jiajiangxian. The f(90%) of Chongqing Guanyinqiao is the highest (low) at 5.35 (2.46). There are small variations both Chengdu Linjianglu and Bazhong Tongjiangzhongxue stations. The f(90%) at Chongqing Guanyinqiao and Leshan  
Figure 3: \(\mathrm{E_{ext}(RH)}\) fitting Chengdu Lingjianglu station. The red solid lines are fitting lines of the scattered dots.  
Jiajiangxian stations are significantly higher than other stations, which the reason is that the main source of aerosols in commercial areas is secondary pollutants (such as sulfates) from vehicle emissions, which are water-soluble ions with strong hygroscopic properties. Leshan Jiajiangxian site is located between the built-up area and the river, and the aerosol is relatively complex.