Particle size distributions of methanesulfonate in the tropical pacific marine boundary layer

Fourteen high-volume cascade impactor samples were collected during a January-February, 1990, research cruise in the tropical Pacific from Panama to 180°. Aqueous extracts of the samples were analyzed for methanesulfonate (MSA), sulfate, and the seasalt tracer ion magnesium. The majority of MSA size distributions showed no pronounced maximum on submicrometer particles, as has been observed elsewhere. Analysis of the data indicated that MSA was distributed essentially uniformly with the effective surface area of particles >0.5 m in radius, which were primarily seasalt. Relatively less MSA was found in smaller particles which were primarily sulfate. These results are consistent with those from theoretical and laboratory experimental studies reported in the literature which suggest that MSA produced from photochemical oxidation of dimethylsulfide condenses on pre-existing particles in strong preference to nucleating into new particles. This implies that MSA may not contribute appreciably to enhancing cloud condensation nucleus populations in the remote tropical marine atmosphere.