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.

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 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.

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.

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

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.

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

It is important to understand the relative contribution of primary and secondary particles to regional and global aerosol so that models can attribute aerosol radiative forcing to different sources. In large-scale models, there is considerable uncertainty associated with treatments of particle formation (nucleation) in the boundary layer (BL) and in the size distribution of emitted primary particles, leading to uncertainties in predicted cloud condensation nuclei (CCN) concentrations. Here we quantify how primary particle emissions and secondary particle formation influence size-resolved particle number concentrations in the BL using a global aerosol microphysics model and aircraft and ground site observations made during the May 2008 campaign of the European Integrated Project on Aerosol Cloud Climate Air Quality Interactions (EUCAARI). We tested four different parameterisations for BL nucleation and two assumptions for the emission size distribution of anthropogenic and wildfire carbonaceous particles. When we emit carbonaceous particles at small sizes (as recommended by the Aerosol Intercomparison project, AEROCOM), the spatial distributions of campaign-mean number concentrations of particles with diameter >50 nm (N50) and >100 nm (N100) were well captured by the model (R2>0.8) and the normalised mean bias (NMB) was also small (-18% for N50 and -1% for N100). Emission of carbonaceous particles at larger sizes, which we consider to be more realistic for low spatial resolution global models, results in equally good correlation but larger bias (R2>0.8, NMB = -52% and -29%), which could be partly but not entirely compensated by BL nucleation. Within the uncertainty of the observations and accounting for the uncertainty in the size of emitted primary particles, BL nucleation makes a statistically significant contribution to CCN-sized particles at less than a quarter of the ground sites. Our results show that a major source of uncertainty in CCN-sized particles in polluted European air is the emitted size of primary carbonaceous particles. New information is required not just from direct observations, but also to determine the "effective emission size" and composition of primary particles appropriate for different resolution models.

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.