Marine aerosols: A review

The background aerosol in the boundary layer over the remote oceans is not aged continental aerosol but, rather, is largely of marine origin. Total particle concentrations are quite uniform throughout the tropical trade wind regions and normally are in the range of 100–300 cm−3. Precipitation reduces particle concentrations, but there is apparently an in situ source of small particles which allows particle concentrations to recover to their normal background level. The fine particle mode (r < 0.3 μm), which comprises 90–95% of the particles but only about 5% of the total mass, cosists primarily of non-sea-salt sulfate (nss-sulfate). There is considerable evidence that nss-sulfate, which is present in concentrations ranging from about 0.2 to 1.5 μg m−3, is formed by gas-to-particle conversion of the oxidation products of organosulfur gases (principally DMS) emitted by the ocean. The principal gas-to-particle conversion mechanisms are particle formation by homogeneous nucleation of low-volatility gas-phase reaction products, condensation of these products on existing particles, and SO2-to-sulfate conversion in cloud droplets. The submicron portion of the particle size distribution is bimodal with peaks at 0.03 μm and 0.1 μm radius. The peak at 0.1 μm is believed to be due to the growth of CCN-sized particles as a result of incloud SO2-to-sulfate conversion. It has been speculated that the sea-to-air flux of DMS affects the number of CCN and thereby affects cloud droplet size, cloud albedo and, consequently, climate. Coarse particles (r > 0.5 μm) are composed primarily of sea salt. The concentration of sea salt shows a strong dependence of wind speed and ranges from about 2 μg m−3 to as much as 50 μg m−3 or more at wind speeds in excess of 15 m s−1. The background coarse mode also contains smaller amounts of nitrate and mineral dust. The concentration of each of these components is normally less than 5% of the mass of sea salt, although dust concentrations can occasionally equal the sea salt loading during fresh intrusions of continental dust. Nitrate is formed by gas-to-particle conversion but the relative importance of the ocean, the stratosphere and lightning as a source of the nitrogen-containing precursor gases remains uncertain. Since nitrate is not found on the fine mode particles, it probably does not result from condensation of gas-phase reaction products or from aqueous-phase oxidation of NOx in cloud droplets.