Characterization of aerosol acidity at a high mountain site in central eastern China

► Hourly data of water-soluble ions in PM2.5 was used to evaluate the aerosol acidity. ► PM2.5 was weakly acidic at Mt. Tai, with higher acidities in summer than in spring. ► Secondary organic aerosol formation events were investigated in more detail. ► Aerosol acidity promoted the serious secondary aerosol pollution at Mt. Tai. ► Aqueous phase reactions enhanced the secondary aerosol formation.