I will present and discuss recent research progresses on atmospheric acidity (including the acidity of aerosol and cloud water, usually described by the pH metric). Particulate sulfur is a key chemical component of aerosols especially in polluted environments such as the Chinese haze. Atmospheric acidity affects the formation of sulfate and organic sulfur compounds in aerosols, partly due to the dissociation of sulfur dioxide in its water solution. Thus, these two topics are closely related with each other. Since no operational techniques exist for direct measurements, estimates of aerosol water acidity are usually based on thermodynamic modeling which considers the balance between acidic and basic species in the atmosphere. In the first half of the presentation, recent progresses in thermodynamic modeling and buffer theory from the research community will be discussed. In the second half, the impacts of atmospheric acidity on the aqueous chemistry of sulfate and hydroxymethanesulfonate, a key organic sulfur species, are presented, using North China winter haze as a case study.