Acid deposition is a general name for a number of phenomena, namely acid rain, acid fog and acid mist. This means it can imply both wet and dry (gaseous) precipitation. Acid deposition is a rather well known environmental problem, for example acid fog killed several thousand people in London in 1952.
Acid deposition is concerned with long-range rather than local effects. Pollutants are mixed in the atmosphere and therefore usually cannot be attributed to any local source. Pollutants are generally more dispersed and of lower concentrations than local ground level pollutants.
Acid deposition typically has a pH below 4, but this may be as low as 1.5 under seriously acidic conditions. It primarily consists of two types of compounds, namely sulphuric acid (H2SO4) and nitric acid (HNO3).