An instrument for the direct in situ measurement of aerosol sulfuric acid in the atmosphere has been developed, characterized and calibrated. The instrument termed Volatile Aerosol Component Analyzer (VACA) utilizes chemical ionization mass spectrometry technique for the detection of sulfuric acid. Prior to the mass spectrometric identification the sample flow is heated, and sulfuric acid is evaporated from the aerosol. The VACA system is characterized as a function of temperature, residence time in the flow reactor, aerosol size, and chemical composition. A unit consisting of an aerosol generator, a differential mobility analyzer, a hygrometer, and a condensation nuclei counter is used to calibrate the VACA instrument. ¿ 2001 American Geophysical Union