Observing Aerosol Direct and Indirect Effects from In Situ Spectral Radiometric Measurements Fonya Nzeffe*, Everette Joseph*, and Qilong Min** * Howard University ** State University of New York at Albany Aerosols influence the Earth radiation balance and thus the climate through direct interaction with solar radiation, and by indirect effect on cloud life cycle and radiative properties. The latter effect is believed to be the most dominant effect of aerosols on climate, and estimated in the 3rd IPCC assessment to produce a global mean radiative forcing of as much as -2 Wm-2. This estimate, however, is highly uncertain; thus, considerable research effort continues to be directed towards better understanding of the aerosol indirect effect. A recently developed method for observing cloud and aerosol optical and microphysical properties from spectral irradiance measurements at the surface is applied to the 2 sites with different aerosol concentration and composition to assess the influence of natural and anthropogenic CCN on cloud radiative properties. In particular, cloud optical depth and effective radius for both warm and ice clouds are simultaneously retrieved at both sites. Furthermore, information on aerosol composition and microphysical properties are derived from in situ measurements and laboratory analyses. Observations are use from the Atmospheric Radiation Measurement site in Oklahoma, and the Howard University atmospheric measurement site in Beltsville Maryland. The latter site is currently being established through a cooperative agreement between NOAA/NWS and Howard University (NOAA/Howard University Center for Atmospheric Sciences; NCAS). A comprehensive set of surface-based instruments (spectral radiometers, microwave radiometer and a LIDAR) are being deployed there to observe cloud and aerosol properties, fluxes, surface energy budget, soil properties, boundary layer (tower and upper air sounding, and water vapor LIDAR), and atmospheric chemistry (gas phase and aerosols). A contrasting picture of the extent to which changes in aerosols properties (like concentration) influence cloud properties is expected from this study.