The duration of high NO2 and CO concentrations in an urban atmosphere

With the principal aim of providing a more powerful assessment of exposure to polluted atmospheres, 5 years of hourly concentration data recorded at 19 monitoring stations located in the Milan area are analysed, taking into account their time structure, in order to generate statistics and simple empirical models for describing the parameters of duration events of concentrations continuously exceeding given thresholds. In particular, the main statistics of the number and duration of events for CO and NO2 were described by simple models, parametrized in terms of the arithmetic average concentration of the annual time series. These findings are particularly useful for the potential utilisation of mean concentration values over long periods in roll-back models, applied to stable pollutants for evaluating the reduction of emissions required for attaining specific objectives like the compliance with standards or, as in this case, the control of critical durations. For the area considered, the roll-back approach can also be extended to evaluate the reduction of emissions on NO2 through the utilisation of a previously developed NO2–NOx statistical link.