Aerosols play a crucial role in cloud and fog formation as cloud condensation nuclei (CCN), yet their characteristics and activation behavior over the complex terrain of the Liupan Mountain (LM) area remain poorly understood. Based on long-term in situ observations in the LM area, this study systematically analyzed submicron aerosol size distribution, activation behavior, and their impacts on cloud and fog microphysics. The results show that submicron aerosol number concentration (N<sub>a</sub>) peaks in late autumn and late spring and exhibits an afternoon unimodal distribution, whereas mass concentration (M<sub>a</sub>) reaches its maximum in early August, mainly contributed by accumulation mode particles. The aerosol size distribution exhibits marked weather-dependent shifts, transitioning from bimodal new-particle formation on clear days to accumulation-mode growth under foggy conditions and Aitken-mode dominance after wet scavenging on rainy days. Importantly, N<sub>a</sub> varies synchronously with CCN number concentration (N<sub>CCN</sub>), and accumulation mode particles serve as the dominant CCN source. Consequently, N<sub>a</sub> is positively correlated with fog droplet number concentration (N<sub>f</sub>) but negatively correlated with droplet size and liquid water content (LWC), leading to a “many but small” fog droplet spectrum under high aerosol loading. Observations of a typical fog event reveal a two‑way feedback. Aerosols supply CCN for fog formation, while the activation, growth, and evaporation of fog droplets in turn modify aerosol concentration and size distribution. These findings provide critical in situ evidence for understanding mountain fog formation and the indirect effects of aerosols.