The prediction of global scale climate changes needs a good estimation of the radiative forcing, which is influenced by several atmospheric parameters, such as meteorological variables, gas concentrations and aerosols. Among these, the most uncertain quantity is the aerosol load that influences the energy exchanges between the solar radiation and the Earth-atmosphere system by different processes. In this paper we take advantage of the aerosol facilities available at the Department of Physics of the University of Lecce, Italy, to track the aerosol load and calculate the aerosol radiative forcing in some selected days with a time resolution of a few hours.

The measurement site (40.33° N, 18.10° S) is located in the suburbs of Lecce, in a rural area, in the Salentum peninsula. The site is located in the central-eastern Mediterranean basin and is exposed to aerosol transport from many sources. The radiative forcing in the mediterranean region has been proved to be the one of the highest in the world in summer time.

In the site, different system for the study of atmospheric aerosols are present. In this paper we will use measurements mainly by the following systems:

a) a lidar system based on a XeF excimer laser (355 nm). We can measure, elastic backscattering during daytime and Raman backscattering from nitrogen and water vapor ( only at nighttime). After polarization of the laser output, the elastic backscattering can be polarization resolved to detect particles non sphericity both at nighttime and daytime.

At nighttime the extinction can be directly measured, thus the lidar ratio profile can be determined. This information can be exploited to infer aerosol properties and better determine the daytime extinction profile that are needed for radiative forcing calculations.

b) a commercial photometer from Cimel, that can measure sun and sky radiance at 8 different wavelengths. From these measurements the optical thickness and the microphysical properties of the aerosol column can be derived.

c) a Vaysala radiosounding system for pressure, temperature, humidity vertical profiles.

Ancillary meteorological measurements are also available and the site is close (35 km) to the meteorological station of Brindisi where systematic radiosoundings are performed.

Different projects aimed to a global characterization of the radiative properties of the aerosol load are currently in progress. In particular, our site participate to the solar photometer global network, AERONET, and the European lidar network EARLINET. The interaction of the two instruments is particularly fruitful because they are in some sense complementary: the solar photometer measures calibrated quantities averaged on the atmospheric column, while the lidar measures altitude resolved quantities which are often not calibrated. Furthermore the sun photometer, being robotized, can acquire data during daytime continuosly with an high temporal resolution.

Sun photometer measurements are obviously unavailable in the presence of clouds, and lidar measurements also suffer the presence of clouds; however, in some cases, clouds base height and thickness can be measured. Such information is important for the calculation of radiative forcing.

In the two stream model used for the radiative forcing calculation the atmosphere is modelized in 20 homogeneous plane parallel layers which were chosen to approximate a vertically inhomogeneous atmosphere; half of the layers are located below 5 km of altitude. Radiative fluxes are calculated at the boundary of the different layers.

In this paper, photometer measurements, like optical depth and particle microphysical characterization, are coupled with the vertical distribution of the aerosol obtained by lidar measurements to give an estimation of the instantaneous aerosol radiative forcing. Lidar and photometric measurements are used to describe at best the radiative properties of each layer. Other input parameters needed are :

a) atmospheric pressure, temperature and humidity that can be obtained by radiosoundings.

b)The surface albedo that is obtained by the MODIS instrument data onboard of the satellite Aqua and Terra.

c)Vertical profiles of the main trace gases concentrations, such as ozone, N2O, CO and CH4 are those from the US Air Force Geophysics Laboratory mid latitude standard atmosphere.

Backtrajectories analysis supplied by Aeronet are also used to better infer the variation of the vertical profile of the aerosol properties.

We will consider two days in which both photometer and lidar measurements are available all along the day. The aerosol forcing will be calculated using the time resolution of the lidar and sun photometer measurements. A typical time interval is of about one hour.

The first date which is considered is October 12, 2005. The available measurements available in this day are: polarization resolved elastic backscattering from 0800 UT , photometer measurements all along the day (with a gap from 0830 to 1130 UT) , a local radiosounding and 2 radiosoundings from Brindisi.

The backtrajectories analysis shows a transport from eastern Europe at all altitudes.

As a second day we will consider a Saharan dust transport event. Such events are quite common in the Mediterranean basin and the presence of Saharan dust surely influences the radiative forcing values observed in this area. The day examined is 18 July 2005. In this case the transport of dust is visible from satellite images and analysis of backtrajectories, and it can be seen that the dust plume arrives over the site in the afternoon. Radiative forcing calculation in this case should take into account the change in the nature of the aerosol during the day.

Results on the dependence of the radiative forcing on aerosol type and distribution will be analyzed and discussed.