Significant reduction in precipitation in the past decades has been documented over many mountain ranges such as those in central and eastern China. Consistent with the increase of air pollution in these regions, it has been argued that the precipitation trend is linked to aerosol microphysical effect on suppressing warm rain. Rigorous quantitative investigations on the reasons responsible for the precipitation reduction are lacking. Here in this study, we employed an improved Weather Research and Forecasting (WRF) model with online coupled chemistry (WRF-Chem) and conducted simulations at the convection–permitting scale to explore the major mechanisms governing changes in precipitation from orographic clouds in the Mountain (Mt.) Hua area in Central China. We find that anthropogenic pollution contributes to a ~ 40% reduction of precipitation over Mt. Hua during the one-month summer time period. The reduction is mainly associated with precipitation events associated with valley-mountain circulation and a mesoscale cold front event. In this Part I paper, we scrutinize the mechanism leading to significant reduction for the cases associated with valley-mountain circulation. We find that the valley breeze is weakened by aerosols due to absorbing aerosol induced warming aloft and cooling near the surface as a result of aerosol-radiation interaction (ARI). The weakened valley breeze along with reduced water vapor in the valley due to reduced evapotranspiration as a result of surface cooling significantly reduce the transport of water vapor from the valley to mountain and the relative humidity over the mountain, thus suppressing convection and precipitation in the mountain.