Aerosol-Cloud Interaction at the Summit of Mt. Fuji, Japan: Factors Influencing Cloud Droplet Number Concentrations  
[PERSON]  
1 Graduate School of Integrated Sciences for Life, Hiroshima University, Higashi-Hiroshima 739-8521, Japan 12 Faculty of Science Division, ITokyo University of Science, Tokyo 162-8601, Japan; [EMAIL_ADDRESS]  
[PERSON]  
3 Graduate School of Science, Tokyo University of Science, Tokyo 162-8601, Japan; [EMAIL_ADDRESS]  
[PERSON]  
3 Graduate School of Science, Tokyo University of Science, Tokyo 162-8601, Japan;  
[EMAIL_ADDRESS]  
[PERSON]  
4 Atmosphere and Ocean Research Institute, The University of Tokyo, Kashinya 277-8564, Japan;  
[EMAIL_ADDRESS]  
[PERSON]  
2 Faculty of Science Division, ITokyo University of Science, Tokyo 162-8601, Japan; [EMAIL_ADDRESS]  
Footnote 2: email: {kataoka,kataoka,  
With regard to the CCN activation of aerosol particles, both the particle size distribution and chemical composition of atmospheric aerosols have been measured at various locations (e.g., [4]), whereas the knowledge about \(SS\) is limited. Water vapor \(SS\) is an important parameter for CCN activation of aerosols and is necessary for numerical models predicting cloud/fog formation [5; 6; 7]. A critical degree of supersaturation (\(SS_{\text{c}}\)) can be determined for a single particle based on the Kohler theory. In the atmosphere, where the size and chemical compositions of aerosol particles are complex, a range of critical supersaturations is generated for a number of CCN. The \(SS\) at which the CCN concentration equals the cloud droplet number concentration is termed effective supersaturation (\(SS_{\text{eff}}\)) and is considered that the \(SS_{\text{eff}}\) exceeds the \(SS_{\text{c}}\) for all CCN active particles in the atmosphere [1]. Although direct measurement of the \(SS\) remains unattainable, it is possible to estimate the \(SS_{\text{eff}}\) indirectly from observation data. The reported values of the \(SS_{\text{eff}}\) have been limited to Europe [8; 9; 10; 11], and \(SS_{\text{eff}}\) values from Asian regions have been limited [12]. Observations using airplanes (e.g., [13]), balloons (e.g., [14]) or helicopters (e.g., [10]) are powerful approaches to observe both aerosol and cloud particles in the atmosphere directly. Although these observations by aircraft have provided important findings of aerosol-cloud interactions, it is difficult to get long-term observation data. At mountain sites, orographic clouds frequently form when air flows upward along a mountain slope and the air temperature drops below the dew point due to adiabatic expansion. Observations at mountain sites, therefore, make it possible to observe the CCN activate into cloud droplets on-site over a long time period. Several studies have reported CCN properties at mountain sites. For example, at Jungfraujoch (altitude 3580 m), Switzerland, the impact of the aerosol size distribution on cloud droplet properties [15], CCN activation over the period of one year [16], temporal variations of aerosol hygroscopicity in the free troposphere [17] and scavenge of black carbon in liquid-phase clouds [18] have been studied. [PERSON] et al. [19] reported changes in the hygroscopicity of aerosols through cloud-processing observed at the upwind and downwind sites of Mt. Schmucke (altitude 937 m), Germany. [PERSON] et al. [20] reported the impact of anthropogenic aerosols on CCN activation at Storm Peak Laboratory in Colorado (altitude 3210m), United States. [PERSON] et al. [9] compared CCN and actually activated particles inside a cloud by using a two-inlet system at Puy-de-Dome, France (altitude 1465 m), and [PERSON] et al. [21] characterized CCN at two different mountain sites in Asia, Doi Ang Kang (altitude 1536 m), Thailand and Mt. Lulin (altitude 2862 m), Taiwan. In this study, we observed aerosols and clouds at the summit of Mt. Fuji (altitude 3776 m; Figure 1a), Japan, at the highest atmospheric monitoring site downwind of East Asia, during the summer of 2014. Because the summit of Mt.