In the frame of the MIUR-AEROCLOUDS project (Study of Direct and Indirect Aerosol Effects on Climate), night-time and daytime size-segregated aerosol samples were collected concurrently at five different sites (near-city, urban, rural, marine and mountain background sites). The paper reports on the daily evolution of the main aerosol chemical characteristics as a function of particle size in different environments over the Italian Peninsula, spanning from the Po Valley to the south Tyrrhenian coast. Two 4-day intensive observation periods (IOPs) were undertaken in July 2007 and February 2008, under meteorological conditions typical of the summer and winter climate for Italy. In the summer IOP, under stable atmospheric conditions, at the low-altitude continental sites the diurnal evolution of the planetary boundary layer (PBL), induces an atmospheric dilution effect driving the particulate matter (PM) concentrations, while, at the mountain site, it determines the upward motion of polluted air masses from the Po Valley PBL in daytime. The fine fraction was dominated by ammonium salts and carbonaceous matter (water-soluble organic matter, WSOM, and water-insoluble carbonaceous matter, WINCM). High concentrations of ammonium sulphate and WSOM due to enhanced photochemical activity constituted the background aerosol composition over the whole country, whereas, ammonium nitrate and WINCM were more associated to local emissions (e.g. urban site with concentrations peaking in the finest size range due to strong local traffic-related sources of ultrafine particles). During the winter IOP in the Po Valley, the shallow PBL depths and low wind velocity, especially at night, favoured the condensation of semi-volatile species (i.e. organic matter and ammonium nitrate), causing the high fine PM concentration observed at ground level.

The Differential Mobility Particle Sizer (DMPS) was installed at ICO-OV in November 2005, in collaboration with Kuopio University (Finland). The system include a Differential Mobility Analyzer (DMA) to select a narrow particle size followed by a condensation particle counter (TSI model 3010) to count individual particles: by setting different voltages in the DMA, particles of different mobility are selected and their concentration are measured.

Aerosol concentration and size distribution of particles with optical diameter between 0.30 and 20 µm have been continuously recorded in 15-size channel by using an optical particle counter (OPC) Mod. GRIMM 1.108.

Aerosol concentration of particles with optical diameter between 0.30 and 20 µm has been continuously recorded in 15-size channel by using an OPC Mod. GRIMM 1.108.

Aerosol number concentration are continuously recorded at the ICO-OV from March 2008 by using a Condensation Particle Counter (TSI model 3772).

Aerosol size distribution from 10 nm to 500 nm is measured by aDifferential Mobility Particle Sizer (DMPS) installed at ICO-OV in November 2005.

A M9003 integrating nephelometer (ECOTECH) measures the aerosol scattering coefficient at 525 nm since May 2007.

The Condensation Particle Counter (TSI model 3772) was installed at ICO-OV in March 2008, calibrated from TSI Laboratories. Once per year the instrument is used for a multi-ay intercomparison with the CPC running at the "Nepal Climate Observatory - Pyramid" GAW global station.

An integrating nephelometer (M9003 ECOTECH) is used to measure the aerosol scattering coefficient at 525 nm since May 2007.

Particle size distribution in the range 0.3 < Dp < 20 µm, has been analysed from August 2002 to July 2006 at the GAW Station of Mt. Cimone (44.10 N, 10.42 E; 2165 m asl) in the northern Italian Apennines. The seasonal aerosol number size distribution, characterized by a bimodal shape, showed a behaviour typical for background conditions, characterized by highest values in summer and lowest in winter. The seasonal and diurnal variations of the larger accumulation mode (0.3 < Dp < 1 µm average values: 26.15 cm- 3) and the coarse mode (1 < Dp < 20 µm, average value: 0.17 cm-3) particle number concentrations (N0.3–1 and N1–20, respectively) exhibited a seasonal cycle with the highest values in spring–summer and the lowest value in autumn–winter. Except in winter, N0.3–1 showed a clear diurnal variation with high values during day-time. N1–20 showed a less marked diurnal variation (but with higher variability), suggesting the influence of non-continuous sources of coarse particle (i.e. Saharan dust events). Since July 2005, continuous measurement of black carbon (BC) concentrations was also available at the measurement site. On average low BC concentrations were recorded (average value: 0.28 µg m-3) even if a few events of high concentrations were recorded both in warm and cold season. Apart from wet scavenging processes which strongly affected aerosol concentrations, combined analysis of N0.3–1, BC, meteorological parameters and air mass back-trajectories, suggests that the transport of polluted air masses from the lower troposphere (by local, regional or long-range transport) represents an important mechanism favouring N0.3–1 and BC increases at Mt. Cimone. In particular, a trajectory statistical analysis for the period July 2005–July 2006 allowed the identification of the main source regions of BC and N0.3–1 for Mt. Cimone: north Italy, west Europe and east Europe.