Aerosol column optical depths derived from airborne Sun photometer and in situ measurements of aerosol properties in 14 vertical profiles off the mid-Atlantic coast of the United States in June show excellent agreement. Simultaneous measurements of the chemical compositions of the aerosol allows an assessment of the chemical apportionment of the aerosol column optical depths. The optical depths had essentially three chemical components, which, in order of descending average contributions, were condensed water, carbonaceous species, and sulfate. These results do not support the common assumption that sulfate dominates aerosol optical depths in polluted regions. ¿ 1997 American Geophysical Union