## 2 Materials and Methods  
### Observation  
Atmospheric observations were conducted at the Mt. Fuji Research Station (MFRS) (35.36\({}^{\circ}\) N, 138.73\({}^{\circ}\) E, altitude 3776 m) at the summit of Mt. Fuji (Figure 1a) from 18 July to 20 August of 2014. More details on the MFRS can be found elsewhere (e.g., [22; 23]). Briefly, the MFRS consists of five buildings: Building 1, building 2, building 3, building 4 and a temporary building (Figure 1b). Aerosol observations were conducted at buildings 1 and 3 during summer only, when the commercial power supply was available. The instrumental setup used in this study is shown in Figure 1c. Ambient air was drawn from an air inlet installed on the 2 nd floor of building 1 using a conductive silicon tube (I.D. 3/8 in.) and introduced into a diffusion dryer filled with silica gel to dehumidify the sample air down to a relative humidity (RH) of 30% or less. Temperature and RH of the sampled air both upstream and downstream of the diffusion dryer were measured by a temperature and humidity sensor (HygroPalm23, Rtoronic AG). After the diffusion drying, the size distribution of aerosol particles ranging in size from 10 to 500 nm in diameter was determined by a scanning mobility particle size (SMPS; SMPS3034, TSI Inc., Shoreview, MN, USA). Number concentration of total aerosol particles (condensation nuclei, CN) (\(N_{\text{CN}}\)) was calculated by the sum of the aerosol concentrations of particles in the range of 10 to 500 nm obtained by the SMPS. The particle loss at the diffusion dryer was calculated to be about 7% for the minimum size bin of the SMPS (the diameter midpoint is 10.4 nm), and less than 3% above 20 nm in diameter. The number size distributions were corrected on the basis of this calculation. Note that SPMS data are missing from 18:00 LT (LT = UTC + 9 h) 30 July to 07:00 LT 31 July, from 23:00 LT 1 August to 12:00 LT 3 August, and from 07:00 LT 7 August to 11:00 LT 14 August, due to maintenance and instrumental failure. Simultaneously, number concentrations of CCN (\(N_{\text{CCN}}\)) at 6 different SSs (0.02, 0.06, 0.16, 0.21, 0.36 and 0.55%) were measured using a CCN counter (CCNC, CCN-100, DMT Inc., Loveland, CO, USA). The CCN data obtained at the 0.02% and 0.06% SS conditions were not used for the following analysis because those SSs are below the operating limit (0.07%; [24]). The SSs were calibrated on-site by  
Figure 1: (**a**) Location of Mt. Fuji (altitude 3776 m) in Japan, (**b**) Schematic of the MFRS with locations of the instruments and (**c**) schematic diagram of the instrumentation system in building 1.  
using (NH4)2 SO4 particles. For the calibration, the (NH4)2 SO4 particles were generated by atomization of an aqueous solution of (NH4)2 SO4 (Kanto Chemical Co., Inc., Tokyo, Japan, >99.5%) and introduced to a diffusion dryer filled with silica gel to dehumidify the (NH4)2 SO4 aerosol to RH of 30% or less. A differential mobility classifier (Model 3081, TSI Inc., Minnesota, USA) was used to classify the particles in the range of 30 to 200 nm every 10 nm, and the \(N_{\rm CN}\) and \(N_{\rm CCN}\) of the monodisperse aerosol particles were measured by a condensation particle counter (Model 3025A, TSI Inc., Shoreview, MN, USA) and the CCNC, respectively, to obtain CCN activation spectra. For the calibration, a Kohler model similar to the Reference Kohler model described by [PERSON] and [PERSON] (2015) was applied. Number concentrations (\(N_{\rm CD}\)) and size distributions (from 2 to 50 \(\upmu\)m in diameter) of cloud droplets (CD) were measured by a fog monitor (FM, FM-100, DMT Inc., Loveland, CO, USA) which was installed outside of building 3. The most frequent wind direction at the summit of Mt. Fuji was west (2016). Therefore, the FM was installed with its inlet facing the west slope because orographic clouds are likely to be generated by the wind ascending the westerly slope.