AEROsol Impact on tropospheric CLOUDS – AEROCLOUDS

Atmospheric aerosols have a major impact on the Earth Climate through direct and indirect radiative forcing. While direct forcing is now fairly well constrained, indirect forcing by clouds remains a problematic issue for quantifying future climatic change [IPCC, 2001]. A major concern is related to the overall effects of increased emissions of anthropogenic aerosols and the subsequent modifications of cloud properties. Not only can aerosol particles modify cloud albedo by increasing the number of cloud droplets (Twomey effect) but they also potentially impact cloud dynamics through changing the cloud energy balance (semi-direct effect) and in turn, its geometry and radiative properties. This leads to a high uncertainty for estimating indirect radiative forcing of anthropogenic tropospheric aerosols (0 to -1.5 W m-2), the most uncertain of all other forcings. The difficulty for reducing this uncertainty is clearly linked to the accounting for both the Twomey effect and the cloud feedback. It has been recently shown from climate model simulations and satellite observations that the uncertainty linked to the boundary-layer clouds (BLC) radiative feedback is one of the major issues of climate change. It is thus a priority to better understand aerosol-cloud interaction processes for BLC. The processes involved in these effects make direct simulation rather complicated and it now clear that proper parameterizations will only be achieved by improving both direct field observations and process-oriented laboratory experiments. A certain number of international campaigns have been taken place over the last 10 years to assess the modulation of cloud albedo by aerosol particles. Most campaigns focused either on the Twomey effect (ACE-2) or on the semi-direct effect (BBC), showing clear correlations between aerosol physical properties (concentration, number and size) and cloud macro (liquid water path) and microphysical (droplet effective radius) properties (Snider et al., 2003). However, the understanding of many processes involved in the interaction between aerosols and clouds is still low (Lohman and Feichter, 2005), limiting the ability of current models to adequately forecast and quantify impacts of aerosols on cloud properties and cloud life cycle. The present project is, therefore, aiming at contributing to a significant reduction in the uncertainty in the quantification of the indirect effect of aerosols on climate. To meet this objective, the methodological approach of AEROCLOUDS is the acquisition of experimental data for quantifying both the Twomey effect and the semi-direct effect. For that purpose, AEROCLOUDS will be organized with two major scientific activities. 1- to improve our ability to understand which aerosols act as cloud condensation nuclei (CCN) 2- to assess the impacts of aerosol property changes on the dynamics and the optical properties of Boundary Layer clouds The first activity provides constrains to the second activity. Both activities are aimed to improve models and will be achieved through both experimental (including technological developments) and modeling approaches. The first activity is to improve our ability to predict the CCN fraction of an aerosol population. It is organized with 2 major tasks. The first task (task 1) is to identify and quantify chemical aging processes of atmospheric aerosols under the influence of controlled oxidants concentration and light conditions. The focus will be the understanding of the transformation processes in relation to the hygroscopic properties of the particles. In particular, we will study the interaction between specific oxidants and the hygroscopic properties of the particles. This task, leaded by partner 2 and involving partners 1 will provide constrains on the kinetics of natural and anthropogenic aerosol aging processes. The second task (task 2) is to quantify the evolution of the hygroscopic properties in the natural atmosphere. It is clearly complementar