Aerosol water content is an important component of aerosol radiative forcing, but the effect of water uptake on aerosols throughout the atmospheric column is not monitored at the present time. We present a technique for retrieving the volume fraction of water in atmospheric aerosols, and apply the technique to the AERONET database. We estimate that the volume fraction of water and the geometric hygroscopic growth factor (gHGF) can be retrieved to within 0.3 using this retrieval. The growth factors we obtain are consistent with published measurements, and indicate that aerosol water uptake is high in humid continental regions (gHGF
1.3 along the U.S. East Coast in August) and low in regions dominated by desert dust (gHGF
1.04 in Saudi Arabia).