## 3 Results and Discussion
### CCN Properties and Their Relationship to Air Mass Origins  
The mean value of \(N_{\text{CN}}\) for the entire observation period was 527.7 cm\({}^{-3}\). By air mass origin, the mean values of \(N_{\text{CN}}\) of the Continental, Pacific and Others periods were 617.9, 391.5 and 524.5 cm\({}^{-3}\), respectively. The 25 th percentile, median, mean and 75 th percentile values of \(N_{\text{CN}}\) and \(N_{\text{CCN}}\) are summarized in Tables 1 and 2, respectively. The mean values of \(N_{\text{CN}}\), \(N_{\text{CCN}}\) and the activation ratio (\(N_{\text{CCN}}/N_{\text{CN}}\)) at each \(SS\) condition showed clear differences between the periods classified as Continental and Pacific; those values were significantly higher for the Continental air masses than for the air masses from the Pacific (Figure 5a,b). The same trend was observed in a previous study at the MFRS [32]. The mean values of \(\kappa\) were calculated by Equation (6), to be 0.25 to 0.28 at each \(SS\) condition over the analysis period. These values were similar to values observed at other mountain sites, Jungfraujoch (0.13 to 0.29 at \(SS\) 0.12% to 1.18%; [16]) and Storm Peak Laboratory (0.2 \(\pm\) 0.15 at \(SS\)s 0.07% to 0.72%; [20]). The recent study summarized aerosol chemistry at alpine sites and aircraft measurements and reported that the main components of submicron non-refractory aerosol in high-altitude sites are sulfate, ammonium and organics [33]. Additionally, it was reported that ammonium and sulfate are the main inorganic ion species in the aerosol collected at the summit of Fuji [34]. The \(\kappa\) values observed at MFRS during summer can be explained by the mixtures of inorganics (\(\kappa\) = 0.61 to 1.28; [27]) and organics (\(\kappa\) = 0 to 0.2; [35]). Black carbon (\(\kappa\cong 0\)) may affect the aerosol composition at MFRS especially when the airmass comes from the planetary boundary layer [36] although it could not be a major component. For the different air mass origins, the mean values of \(\kappa\) are 0.26 to 0.30 (Continental), 0.19 to 0.28 (Pacific) and 0.24 to 0.30 (Others) (Figure 5d). In the free troposphere, aerosol particles have a longer lifetime because they are less susceptible to deposition. Therefore, particles in the free troposphere are more aged and well-mixed than those in the planetary boundary layer (e.g., [33]). This could cause the relatively uniform chemical composition made up of mainly sulfate and aged (oxidized) organics regardless of airmass origins. Because smaller particles require larger \(SS\) to activate, each \(\kappa\) value at a certain \(SS\) reflects the chemical composition of the particles around \(d_{\text{a}}\). The \(\kappa\)  
Figure 4: Temporal variation of (**a**) air mass origins, (**b**) number size distributions (10–500 nm), (**c**) total particle concentrations (\(N_{\text{CN}}\)) and CCN concentrations (\(N_{\text{CCN}}\)) at each \(SS\) condition, and (**d**) Liquid water content (_LVC_). The green shading in (**d**) indicates cloud-shrouded periods. values are relatively uniform regardless of the particle size (Figure 5c,d), indicating that no size dependency of the chemical composition of the aerosol at the MFRS during summer exists. This result differs from other CCN studies at non-alpine sites (e.g., [37; 38; 39]), in which smaller particles were shown to be less hygroscopic, suggesting that the aerosol in the free troposphere is well-mixed compared to the aerosol sampled closer to the sources. The \(\kappa\) values are similar between air mass origins, while the particle size distribution is strongly dependent on the particle source (Figure 6). The mode diameter of the aerosol particles of Continental origin (58 nm) is significantly larger than that of Pacific origin (24 nm). It is thought that aging processes, such as condensation, coagulation and cloud processing, change number size distributions (e.g., [40]).