6.—(1) The National Assembly must classify each zone in relation to each of the relevant pollutants according to whether ambient air quality in that zone for that pollutant is required to be assessed by—
(a)measurements;
(b)a combination of measurements and modelling techniques; or
(c)by the sole use of modelling or objective estimation techniques.
(2) Measurements must be used to assess ambient air quality in relation to a relevant pollutant in a zone if—
(a)the zone is an agglomeration;
(b)the levels of that pollutant in the zone are between the relevant limit values and upper assessment thresholds; or
(c)the levels of that pollutant in the zone exceed the limit values for that pollutant.
(3) A combination of measurements and modelling techniques may be used to assess ambient air quality in any zone in relation to a relevant pollutant where the levels of that pollutant over a representative period are below the relevant upper assessment thresholds.
(4) Where the levels of a relevant pollutant in any zone over a representative period are below the relevant lower assessment thresholds, the sole use of modelling or objective estimation techniques for assessing levels of that pollutant is permissible unless—
(a)the zone is an agglomeration; and
(b)the pollutant being assessed is sulphur dioxide or nitrogen dioxide.
(5) The upper and lower assessment thresholds for the relevant pollutants are to be determined in accordance with Schedule 2.
(6) Where a zone is classified in relation to a pollutant under paragraph (1)(a), modelling techniques may be used for supplementing the measurements taken in order to provide an adequate level of information on ambient air quality in relation to a relevant pollutant in a zone.
(7) The National Assembly may also designate a zone classified under this regulation in relation to a relevant pollutant as follows:
(a)where the relevant pollutant is sulphur dioxide, the zone may be designated under this paragraph M1 if the limit values are exceeded in the zone owing to concentrations of sulphur dioxide in ambient air due to natural sources;
(b)where the relevant pollutant is PM10, the zone may be designated—
(i)under this sub-paragraph M2 if, due to natural events, concentrations of PM10 in the ambient air are significantly in excess of normal background levels from natural sources;
(ii)under this sub-paragraph M3 if, due to the resuspension of particulates following the winter sanding of roads, concentrations of PM10 in the ambient air are significantly in excess of normal background levels from natural sources.