3. Where in a given average exposure territorial unit, the average exposure reduction obligation laid down in Section 5 of Annex I is not achieved, Member States shall establish air quality plans for those average exposure territorial units that set out appropriate measures to achieve the average exposure reduction obligation and to keep the exceedance period as short as possible. Those air quality plans shall be established as soon as possible and no later than 2 years after the calendar year during which the exceedance of the average exposure reduction obligation was recorded.