The role of traffic emissions from weekends’ and weekdays’ fine PM data in Milan

A receptor-oriented and a source-oriented approach are applied to assess the contribution of traffic emissions to concentration levels of fine particulate matter in Milan (Italy). Both the approaches are based on the comparison between seasonal weekends’ and weekdays’ data. Relative differences observed between Sundays’ and weekdays’ PM10 concentrations are analysed in terms of their probability distributions for the cold and warm season separately. As a consequence of the reduced circulating traffic flows, Sundays’ concentration levels are on average about 20% lower, but an increase of concentration levels is sometimes observed under unfavourable meteorological conditions. The effect of the reduced traffic on Sundays is assessed also in terms of particles’ number concentration. The results obtained by the analysis of weekly PM10 concentration data are compared to information derived from the emission inventory for traffic. From observed reduction of Sundays’ traffic flow, the corresponding expected variation in primary PM10 emissions from traffic is calculated accounting for both exhaust and non-exhaust contribution. Seasonal probability distribution of PM10 emission variations are obtained by implementing the COPERT III methodology in a Monte Carlo simulation: on Sundays estimated values of PM10 emissions always result lower than on weekdays and on average about 50% less. Based on estimated reductions for PM10 traffic emission and on observed reductions for PM10 ambient air concentrations, traffic emissions appear responsible for about 50% of the PM10 concentration levels in the urban area. This contribution is both due to exhaust and non-exhaust emissions: depending on site exposure to the traffic source, out of the 50% contribution of total traffic emissions to PM10 concentrations, a 4–40% share is estimated to come directly from the exhaust, whereas the remaining share derives from non-exhaust emissions and resuspension of soil dust.