Romanian Reports in Physics, Vol. 57, No. 3, P. 426-435, 2005 ATMOSPHERIC AND EARTH PHYSICS EFFECTS OF THE ATMOSPHERIC AEROSOL ON THE OPTICAL PROPERTIES OF CLOUD GABRIELA IORGA * , SABINA STEFAN** * Department of Physics, Faculty of Chemistry, University of Bucharest, Regina Elisabeta Avenue, No. 4-12, 70346 - Bucharest, Romania ** Department of Atmospheric Physics and Earth Sciences, Faculty of Physics, University of Bucharest, P.O. Box MG-11, 077125, Bucharest-Magurele, Romania (Received June 22, 2005) Abstract.The work is focussed on the role that pre-cloud aerosol (number concentration and spectral dispersion) plays in cloud reflectivity. Rural and marine aerosols have been described by lognormal aerosol size distributions, and the cloud condensation nuclei were obtained based on Abdul-Razzak's activation parameterisation. Cloud albedo including dispersion in the scattering asymmetry factor and specified aerosol properties has been estimated. The sensitivity of cloud albedo to changes in cloud droplet number concentration in connection to the changes in liquid water content and the cloud droplet effective radius has been investigated. For the same liquid water path, clouds over rural areas have more and smaller cloud droplets and thus a higher cloud albedo than marine clouds. Key words: aerosol size distribution, indirect aerosol effect, effective radius, liquid water content, cloud albedo. Atmospheric aerosol and optical properties of clouds 1. INTRODUCTION The aerosol number or mass concentration is a key parameters in the studies of atmospheric radiative effects. The assessments of radiative effects of aerosols, especially of the indirect effect on the radiative balance of the Earth are highly uncertain (IPCC, 1996). Uncertainty arises in part, because of the differences in the assumed aerosol concentration and evaluation of the cloud droplets number concentration (CDNC). On the other hand, the uncertainty in the estimates of indirect radiative forcing results from the complexity of the relationship between the aerosol mass input to the atmosphere and both the radiative properties of clouds and cloud dynamic. In the IPCC (2001) is shown, comparatively with the estimates from the Second Assessment Report (SAR), the level of scientific understanding regarding to the global, annual mean radiative forcing (W/m 2 ) due to a number of agents for the period from pre-industrial (1750) to present (late 1990s, about 2000). It is striking that consideration of all of the estimates available since 1996 lead to the same best estimate (–0.4 W/m 2 ) and uncertainty (–0.2 to –0.8 W/m 2 ) range. Here are again emphasized the large uncertainties appearing from the fact that the indirect forcing due to tropospheric aerosols is poorly understood and the determining of the radiative forcing is a hard challenge. * Corresponding author. E-mail addresses: giorga@gw-chimie.math.unibuc.ro (G. Iorga), sabina_stefan@yahoo.com (S. Stefan)