Measurements of aerosol sulfuric acid in the free troposphere were performed in the vicinity of Tenerife, Canary Islands (28 ¿N, 16 ¿W), in July 1997. These measurements were carried out on board a Dutch Cessna Citation II research aircraft within the framework of the second Aerosol Characterization Experiment (ACE 2). We used the Volatile Aerosol Component Analyzer for the detection of sulfuric acid (H2SO4). Vertical profiles between 2 and 13 km altitude were obtained. Typically, H2SO4 mixing ratios ranged between 10 and 120 pptv. Between 4 and 6 km altitude a distinct H2SO4 aerosol layer was observed repeatedly with H2SO4 mixing ratios of up to 550 pptv. The measurements are in agreement with total aerosol mass concentrations inferred from simultaneous measurements of aerosol size distributions using two condensation particle counters, a differential mobility analyzer, and an optical aerosol counter. At altitudes above 4 km the predominant aerosol component was sulfuric acid, frequently correlated with ozone, suggesting photochemical air pollution as a common source. Sulfur dioxide measured by chemical ionization mass spectrometry technique revealed typical mixing ratios between 10 and 60 pptv at altitudes above 6 km and up to 200 pptv in the lower troposphere. ¿ 2001 American Geophysical Union