Member States shall establish, starting in 1990 and for each subsequent year until and including 2003, a complete emission inventory for existing plants covering SO₂ and NO_x:

• on a plant by plant basis for plants above 300 MWth and for refineries;

• on an overall basis for other combustion plants to which this Directive applies.

The methodology used for these inventories shall be consistent with that used to determine SO₂ and NO_x emissions from combustion plants in 1980.

The results of this inventory shall be communicated to the Commission in a conveniently aggregated form within nine months from the end of the year considered. The methodology used for establishing such emission inventories and the detailed base information shall be made available to the Commission at its request.

The Commission shall organise a systematic comparison of such national inventories and, if appropriate, shall submit proposals to the Council aiming at harmonising emission inventory methodologies, for the needs of an effective implementation of this Directive.