The adjoints of the GEOS-Chem Chemical Transport Model and a comprehensive cloud droplet parameterization are coupled to study the sensitivity of cloud droplet number concentration (Nd) over US regions and Central Europe to global emissions of anthropogenic fine mode aerosol precursors. Simulations reveal that the Nd over the midwestern and southeastern US is mostly sensitive to SO2 emissions during August, and to NH3 emissions during February. Over the western US, Nd is mostly sensitivity to SO2 and primary organic aerosol emissions. In Central Europe, Nd is most sensitive to NH3 and NOx emissions. As expected, local emissions strongly affect Nd; long-range transport, however, is also important for the western US and Europe. Emissions changes projected for the year 2050 are estimated to have the largest impacts on cloud albedo and Nd over Central Europe during August (42% and 82% change, respectively) and western US during February (12% and 36.5% change, respectively). © 2012. American Geophysical Union. All Rights Reserved.