Directive 2001/81/EC
"Directive 2001/81/EC of the European Parliament and the Council on National Emission Ceilings for certain pollutants (NEC Directive) sets upper limits for each Member State for the total emissions in 2010 of the four pollutants responsible for acidification, eutrophication and ground-level ozone pollution (sulphur dioxide, nitrogen oxides, volatile organic compounds and ammonia), but leaves it largely to the Member States to decide which measures – on top of Community legislation for specific source categories - to take in order to comply."
"The national emission ceilings in Annex I shall have as their purpose to meet broadly the following interim environmental objectives, for the Community as a whole, by 2010:
(a) Acidification
The areas where critical loads are exceeded shall be reduced by at least 50 % (in each grid cell) compared with the 1990 situation.
(b) Health-related ground-level ozone exposure
The ground-level ozone load above the critical level for human health (AOT60=0) shall be reduced by two-thirds in all grid cells compared with the 1990 situation. In addition, the ground-level ozone load shall not exceed an absolute limit of 2,9 ppm.h in any grid cell.
(c) Vegetation-related ground-level ozone exposure
The ground-level ozone load above the critical level for crops and semi-natural vegetation (AOT40=3 ppm.h) shall be reduced by one-third in all grid cells compared with the 1990 situation. In addition, the ground-level ozone load shall not exceed an absolute limit of 10 ppm.h, expressed as an exceedance of the critical level of 3 ppm.h in any grid cell."(http://ec.europa.eu/environment/air/pollutants/ceilings.htm)