Precipitation over mountain ranges is an important part of the Earth’s water cycle providing a major source of agricultural, domestic, and industrial water supply for areas downstream. Over many mountainous regions, such as those in central and eastern China, research has documented a significant reduction in precipitation in the past decades. Consistent with the increase of air pollution in these regions, many have linked the precipitation trend to the effect of aerosol particle on microphysics that suppresses warm rain. Yet, there are few rigorous quantitative investigations looking into the reasons for this precipitation reduction. Now, a research team led by Department of Energy scientists at Pacific Northwest National Laboratory found that human-caused pollution contributes about a 40% precipitation reduction over the Mt. Hua, China region during summer. Using an improved Weather Research and Forecasting model with online coupled chemistry (WRF-Chem), they conducted simulations at the convection–permitting scale to explore the major mechanisms governing changes in precipitation from orographic clouds in the mountainous area of Central China. They noted that the reduction was mainly associated with precipitation events linked with valley-mountain circulation in a mesoscale cold front event. In Part I of the paper, the team scrutinized the mechanism leading to a significant reduction for those cases associated with valley-mountain circulation. They found that the valley breeze becomes weakened by aerosol particles because the absorbing aerosols induce both warming aloft and cooling near the surface as a result of an aerosol-radiation interaction. The combination of a weakened valley breeze and reduced valley water vapor (caused by reduced evapotranspiration as a result of aerosol-induced surface cooling) significantly reduced both the water vapor transport from the valley to mountain, and the relative humidity over the mountain. The combined effects suppressed convection and precipitation over the mountain.