We measured aerosol methanesulfonic acid (MSA), Cl-, NO3-, SO4=, Na+, and NH4+ concentrations and size distributions and gaseous ammonia and nitric acid concentrations from Santa Maria, Azores during the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) experiment in June 1992. We also sampled some of these species in the free troposphere and marine boundary layer (MBL) from the National Center for Atmospheric Research's (NCAR's) Electra aircraft and at the surface from the R/V Oceanus. In relatively clean Atlantic marine air masses the mean (and standard error of the mean) observed MSA concentrations (27¿2 parts per trillion molar mixing ratio, ppt) were smaller than in continental polluted air masses (36¿6 ppt), reflecting either longer aerosol lifetimes in air with higher cloud condensation nuclei (CCN) concentrations or more active photochemical production in polluted air. Mean NH4+/non-sea-salt (nss) SO4= molar ratios in marine and continental air masses were 0.65¿0.05 and 1.3¿0.2, respectively, indicating that continental sulfate was neutralized by ammonia to a greater extent than marine sulfate was. Most of the NH4+ and nss SO4= was contained in submicrometer modes; Na+, Cl-, and NO3- were in supermicron modes; and MSA showed bimodal behavior, with major and minor peaks at 0.3--0.4 μm and 1.5 μm, respectively. The molar ratio NH4+/nss SO4= relative to size confirms that most ammonia vapor condensed on submicron nss SO4=. The average dry deposition fluxes for aerosol nss SO4=, MSA, NH4+ and NO3- during ASTEX/MAGE were 1.49, 0.09, 0.18, and 3.25 μmol m-2 d-1, respectively. ¿ American Geophysical Union 1996