The total rate at which a solid, liquid or gaseous pollutant is emitted into the atmosphere from a given source; usually expressed as mass per unit time. Primary emissions are those substances which are emitted directly to the atmosphere (e.g. NO, SO₂, etc.), while secondary emissions are formed from the primary emissions through thermal or photochemical reactions (e.g. ozone, aldehydes, ketones, sulfuric acid, nitric acid, etc.). The point or area from which the discharge takes place is called the source; the area in which the emission or its transformed products (e.g. in the case of aerosols, acidic deposition, etc.) may be deposited is called the receptor area or sink. Emission may be applied to noise, heat, etc., as well as pollutants.