acid compounds emitted into the atmosphere which then return to the surface either in the form in which they were discharged or as new compounds formed by reaction in the atmosphere. Includes dry deposition, usually of sulphur and nitrogen oxides near the source, and wet deposition which follows when acids are washed from the atmosphere by precipitation, matieres polluantes acides, d'origine atmosphérique, qui se sont déposées par voie humide ou par voie seche