Daily patterns of the multi-modal structure of the particle number size distribution in Milan, Italy G. Lonati a, * , M. Crippa b , V. Gianelle c , R. Van Dingenen d a Dipartimento di Ingegneria Idraulica, Ambientale, Infrastrutture viarie, Rilevamento, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy b Paul Scherrer Institute, 5232 Villigen, Switzerland c ARPA Lombardia, Viale F. Restelli 3/1, 20124 Milano, Italy d European Commission, DG Joint Research Centre, Institute for Environment and Sustainability, T.P. 290, 21020 Ispra (VA), Italy article info Article history: Received 29 April 2010 Received in revised form 28 January 2011 Accepted 2 February 2011 Keywords: Multi-modal size distribution Number concentration Fine particles Ultrafine particles Urban areas abstract Aerosol number concentration and size distributions in the 10e20,000 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, D p < 100 nm) and submicron (SMP, 100 < D p < 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 10 4 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 corre- sponding 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 domi- nating 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. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Atmospheric aerosol particles cause negative effects on human health (Pope, 2000; Oberdorster, 2001) and on environment, influencing the Earth’s radiation balance (Schwartz, 1996) and reducing the visibility (Finlayson Pitts and Pitts, 2000). Though it is not completely clear which fraction of particulate matter (PM) is responsible for the observed adverse health effects, the association between ambient PM and both respiratory and cardiovascular disorder has been long recognised (Dockery and Pope, 1994). Recently health-related concerns have focused also on ultrafine particles (UFPs, diameter D p less than 100 nm), based on the hypothesis that they might be most toxic and thus responsible for the observed adverse effects, due to their deep penetration into the respiratory track. Toxicological studies indicate that UFPs are stronger than larger particles in inducing cellular damage and suggestions are that the exposure to UFPs is associated with both respiratory and cardiovascular effects (Morawska et al., 2007). UFPs are both of primary and secondary origin, associated with both nucleation events caused by the dilution and cooling of exhaust combustion gases in the atmosphere and with chem- icalephysical transformations and reactions involving gaseous precursors (Kulmala et al., 2004). In ambient air UFPs are present in very high numbers, on average in the order of 10 4 e10 5 parti- cles cm 3 in urban areas, but contribute to the overall PM mass by only a few percents. Therefore, UFPs concentration is usually expressed as number concentration, with additional information provided by the number size distribution (NSD) of the particles. In this work particle number concentration levels and related NSDs observed at an urban background site in Milan are presented. Daily patterns for particle concentrations are analysed both on a seasonal basis (cold and warm season) and on a weekly basis, comparing weekday and weekend concentrations. Since Milan and its metropolitan area are a non-attainment area for current PM air quality standards, in recent past years several studies aimed to assess mass concentration levels (Vecchi et al., 2004; Giugliano * Corresponding author. Tel.: þ39 02 23996430; fax: þ39 02 23996499. E-mail address: giovanni.lonati@polimi.it (G. Lonati). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.02.003 Atmospheric Environment 45 (2011) 2434e2442