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The seasonal evolution of the tropospheric aerosol vertical distribution and of its optical properties is investigated using lidar and multi-filter rotating shadow-band radiometer (MFRSR) measurements collected throughout the period 2006-2009 in the urban environment of Rome. The evolution of the aerosol distribution is studied also in relation to long range transport of dust Hybrid Single-Particle Lagrangian Integrated Trajectory model backward trajectories are used to identify possible aerosol sources in remote regions.

Aerosol optical depth at 500 nm, tau, and Angstrom exponent, alpha, are derived from MFRSR measurements. The Angstrom exponent generally displays relatively high values, indicating the predominance of fine particle over the entire column. The average optical depth at 500 nm and Angstrom exponent over the whole period are 0.18 +/- 0.09 and 1.12 +/- 0.39, respectively. Cases affected by Saharan dust (class 1) are separated from those not influenced by dust (class 0) by using backward trajectories. The average values of tau and alpha are 0.17 +/- 0.08 and 1.17 +/- 0.36 for class 0, respectively, and 0.22 +/- 0.09 and 0.95 +/- 0.46 for class 1.

About 214 days of lidar measurements are selected for the analysis. The aerosol vertical distribution is influenced by dust events that induce a marked seasonal behaviour. Desert dust generally reaches higher altitudes than other aerosol types; the maxima altitudes are observed during Spring and Summer, when the monthly average altitude exceeds 5 km. The annual average occurrence of desert dust is 27%, with maxima in Spring and in the first part of Summer. The decrease in the dust event frequency observed in winter months is mainly linked to the seasonal behaviour of the synoptic circulation in the Mediterranean. According to the back-trajectories aerosols are primarily observed below 3 km altitude throughout the year when classified as not affected by desert dust. The extinction coefficient vertical profiles for the two classes show largest differences during Spring and Summer. The extinction for non-dust profiles decreases monotonically with altitude throughout the year, except in Summer. Conversely, the aerosol extinction coefficient shows a relative minimum at the lowest sounded altitude, always except in winter, for class 1 cases. A winter maximum of the aerosol is evidently present in winter in the lower troposphere. Using the MFRSR optical depth and the lidar profiles, the lidar ratio was derived. The overall average lidar ratio is 58 sr. (C) 2012 Elsevier B.V. All rights reserved.