Acid fog, generally referring to mist-like acidic substances. The particles of acid fog in the air are very small, smaller than those of water fog, but more humid than smoke, making it a substance between smoke and water fog, with strong corrosive properties. Most of them originate from sulfur oxides and nitrogen oxides emitted from the combustion of fossil fuels, with some also coming from vehicle exhausts (mainly nitrogen oxides). The main methods to control acid fog emissions include liquid absorption, solid adsorption, filtration, electrostatic fog removal, mechanical fog removal, and covering methods.