Uptake coefficients of O3 on various model compounds of sea salts at ambient temperature were measured using a Knudsen cell reactor coupled to a differentially pumped quadrupole mass spectrometer. Uptake of O3 was below the detection limit (<1¿10-5) for pure alkali metal bromides, whereas reactive uptake with Br2 formation was observed on two types of synthetic sea salts (Aqua Ocean and Instant Ocean) and commercial natural sea salt. Measured uptake coefficients were 10-2-10-3, (1.4¿1.1)¿10-3 and (9.7¿4.6)¿10-4 for Aqua Ocean, Instant Ocean and natural sea salt, respectively. These uptake coefficients were about three orders of magnitude larger than that on bulk bromide solution estimated from known liquid phase reactions. The results imply that minor components enhance reactivity of O3 on synthetic and natural sea salts. This heterogeneous reaction is potentially an important Br source in the marine boundary layer on a global scale as well as in the polar troposphere. ¿ 2000 American Geophysical Union