We studied the hygroscopic behavior of atmospheric aerosols by using a novel approach, the combination of atomic force microscopy (AFM) with transmission electron microscopy (TEM) imaging of the same individual particles. By comparing the dimensions of hydrated and dry ammonium sulfate particles collected above the North Atlantic Ocean, we determined that particle volumes are up to four times larger under ambient conditions (as determined by AFM) than in the vacuum of a transmission electron microscope. We interpret these changes as resulting from the loss of water. Organic films on the particles may be responsible for the relatively large water uptake at low relative humidities. ¿ 1998 American Geophysical Union