4.—(1) Subject to sub paragraph (2) below, an annual mean is a mean which is calculated on a yearly basis, yielding one annual mean per calendar year.
(2) The annual mean for a particular substance at a particular location for a particular calendar year is–
(a)in the case of lead, the mean of the daily levels for that year;
(b)in the case of nitrogen dioxide, the mean of the hourly means for that year;
(c)in the case of PM10, the mean of the 24 hour means for that year.
(3) For the purpose of the calculation of an annual mean for lead, the daily level for lead at a particular location for a particular day is the level at which lead is recorded as being present in the air at that location during the week in which the day occurs on the basis of a continuous sample of air taken throughout that week (each day in that week therefore being attributed with the same daily level).
(4) For the purpose of sub paragraph (3) above, “week” means a complete week beginning on a Monday, except that it also includes any period of less than seven days from the beginning of the calendar year until the first Monday in that year or from the beginning of the last Monday in the calendar year to the end of that year.