13.—(1) The National Assembly must assess concentrations of pollutants in accordance with the methods required or, in the case of Group A pollutants and Group B pollutants, permitted by this regulation.
(2) In relation to Group A pollutants and Group B pollutants in cases where—
(a)the assessment concerns a Group A pollutant within an agglomeration; or
(b)concentrations of a pollutant have exceeded the upper assessment threshold,
assessment is required to be by means of fixed measurement.
(3) In cases where paragraph (2) does not apply and, in the case of nitrogen dioxide, subject to regulation 15(7), the National Assembly may use the following assessment methods in relation to Group A pollutants and Group B pollutants—
(a)a combination of—
(i)fixed measurement, and
(ii)modelling techniques,
provided that concentrations of the relevant pollutant have, over a representative period, been below the upper assessment threshold; or
(b)the sole use of either modelling or objective estimation techniques, provided that—
(i)paragraph (4) does not apply, and
(ii)concentrations of the relevant pollutant have, over a representative period, been below the lower assessment threshold.
(4) The National Assembly must not use the methods referred to at paragraph (3)(b) to assess nitrogen dioxide or sulphur dioxide within an agglomeration.
(5) For the purposes of paragraphs (2) and (3)—
(a)the upper and lower assessment thresholds are specified for Group A pollutants in Part 1 of Schedule 4 and for Group B pollutants in Part 2 of that Schedule; and
(b)the representative period is to be interpreted in accordance with Part 3 of that Schedule.
(6) The National Assembly must review the method by which Group A pollutants and Group B pollutants are assessed within each zone—
(a)in the event of significant changes in activities affecting concentrations of a pollutant within that zone; and
(b)in any event, at least once in every five years.
(7) The National Assembly must assess concentrations of ozone by fixed continuous measurement if, within the zone concerned, concentrations have exceeded a long-term objective during any of the previous five years of measurement.
(8) In cases where fewer than five years' data is available, the National Assembly may assess concentrations of ozone by combining the following—
(a)measurement campaigns of short duration at times and locations which it considers are likely to be typical of the highest pollution levels; and
(b)results from emission inventories and modelling.