Daily patterns of the multi-modal structure of the particle number size distribution in Milan, Italy

Aerosol number concentration and size distributions in the 10–20,000 nm size range were measured using a Differential Mobility Particle Sizer (DMPS) and an Optical Particle Counter (OPC) at an urban background site. Daily patterns of the total particle (TP), ultrafine (UFP, Dp < 100 nm) and submicron (SMP, 100 < Dp < 1000 nm) fine particle concentration are analysed both on seasonal (cold and warm season) and weekly (weekdays and weekends) basis. TP number concentrations are in the orders of 104 cm−3, with UFPs and SMPs accounting for 80% and 20%, respectively; the contribution of supermicron particles to TP number is negligible. The multi-modal structure of particle number size distributions (NSD) is investigated by fitting a multi-lognormal model to the observed size distributions in the submicron size range, where almost 100% of the TPs fall. Three lognormal components (1 located in the Aitken mode size range and 2 in the accumulation mode range) are usually present in the fitted model and the corresponding modal parameters and their daily pattern have been estimated. The Aitken mode particles increase simultaneously with primary traffic emissions, peaking on the morning rush hour and dominating the NSD still during the afternoon and on the evening rush hour. The smaller accumulation mode particles are influenced by the condensation and coagulation processes induced by the daily patterns of the boundary layer and of ambient temperature, becoming more and more important from the evening rush hour throughout the night and providing the largest contribution to TP number at late night.

► Seasonal and day-of-the-week features of particle number size distribution (NSD).
► Original procedure for multi-lognormal model fitting to the observed NSD.
► Daily time-resolved description of the multimodal behaviour of the NSD.
► Influence of traffic emission on particle NSD at an urban background site.