Marine atmospheric aerosols remain a major uncertainty in climate models. We report gas and particle phase measurements of two important aerosol forming vapours, sulfuric acid and methanesulfonic acid in the Indo-Pacific. We find that the latter typically dominates over sulfuric acid and contributes significantly to free tropospheric aerosol. Deep convection transports the precursor of both acids to the upper troposphere, providing a key aerosol source in the pristine remote Pacific Ocean.