It is shown that for a given mobility distribution in an air sample a single universal characteristic curve can be obtained for all Gerdien condensers. An apparatus has been developed which can measure the mobility spectrum and concentration of small, intermediate, and large atmospheric ions simultaneously in much shorter time. Sources of error and some precautions taken to minimize these errors in this apparatus are described. Some representative results showing the validity and usefulness of the apparatus are described. ¿American Geophysical Union 1991