The duration of PM10 concentration in a large metropolitan area

Annual ambient PM10 concentration time series recorded between 1991 and 2006 at 13 monitoring stations located within the “critical area” of Milan (Italy), encompassing the city and its metropolitan area, are analysed for the temporal duration of concentration levels. Duration episodes occur when the measured concentration is continuously higher than fixed concentration thresholds. For 20 concentration thresholds, ranging from 10 μg m−3 up to 200 μg m−3 by a 10 μg m−3 step, the total time of exceedance, the number of events of exceedance and the duration of the events of exceedance are evaluated on annual basis. Simple, empirically-derived expressions, formulated in order to describe these features of the observed durations, are presented. Since these formulations are parameterized in terms of the annual average PM10 concentration, they can be used to assess expected exposure to high-concentration levels for future scenarios characterized by lower annual average concentrations. A case-study for Milan urban area is presented, assessing the occurrence of duration episodes at annual PM10 concentration at the compliance level of the PM10 air quality limits, currently not-attained.