Abstract: Exploring the chemical characteristics of different scales of droplets is an important issue in further in-depth research of fog microphysics and chemistry. From December 2016 to January 2017, a comprehensive fog observation experiment was carried out in Lushan. The three-stage Caltech Active Strand Cloud Collector (CASCC 3_stage) was used for size resolved fog sampling. 50% droplet size cuts at 22, 16 and 4 μm diameter are featured in stages one, two and three, respectively. A total of 73 samples were collected during three fog events. The pH, conductivity (EC) and concentration of nine water-soluble inorganic ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, Cl^-, NO₂^-, NO₃^-, SO₄^2-) were quantitatively obtained from the three-stage fog water. The results show that the fog water in Lushan was seriously acidified in the winter, with a pH of 3.96-5.82 and the samples with pH < 5.6 account for 98.6% of the total. Moreover, heavy acid samples (pH < 4.5) of 4-16 μm small droplets account for up to 54.2% of the total, and the small droplets are more acidic and conductive. NH₄⁺, Ca²⁺, NO₃^-, SO₄^2- are the main ionic components of the fog water, NH₄⁺+Ca²⁺+NO₃^-+SO₄^2- account for 83.8%, 88.0% and 88.7% of the total ion concentration (TIC) in the three-stage fog water, respectively. Combined with three fog processes, TIC, NH₄⁺, K⁺, NO₃^- and SO₄^2- are enriched in small droplets of 4-16 μm, showing size-dependent characteristics. 86% of the fog water samplesSO₄^2-/NO₃^- is between 0.5-3.0, which is a mixed acidification of sulfuric acid and nitric acid. The main reasons for the acidification of the fog water are the imbalance between alkaline buffer and acid component and possible contribution of organic acid. There are some differences in the chemical characteristics between different fog processes at the same observation point. In the second event (14:00 BT 25 Dec-21:00 BT 26 Dec), TIC, NH₄⁺, Ca²⁺, NO₃^- and SO₄^2- are enriched in droplets of 16-22 μm. This phenomenon may be the result of a combination of lower aerosol concentration, temperature drop, sampling interval, pollutant gas and aerosol transport in the fog region. Backward trajectory clustering and potential source contribution function (PSCF) show that the air masses affecting Lushan during the observation period all came from the west, and local air mass from northern part of Hunan accounted for 68.99% of the total trajectory. PM_2.5, SO₂, NO₂ have similar spatial distribution of potential source areas, which are mainly located in neighboring provinces such as Hubei, Hunan, southwestern Anhui and northern Jiangxi. Short-distance transport dominated pollutants transport.