Sub-Micron Sea-Salt Particle Flux From Laboratory Foams

Sea-salt particles are generated, in part, by bursting bubbles entrained in whitecap foams. Remote marine aerosol can include a significant number of sub-micron sea-salt particles (Mason, 2001). For example, O`Dowd and Smith (1993) found that remote marine aerosol number concentrations in the particle size range 0.1--3.0 μ m were dominated by sea-salt particles for moderate-to-high wind speeds. Sub-micron sea-salt particles may be a significant source of cloud condensation nuclei in clean maritime air (Mason, 2001) and may also contribute to air-sea fluxes of latent heat (Andreas, 1992). Estimates of sub-micron sea-salt fluxes vary by an order of magnitude (Reid et al., 2001). Here we present laboratory measurements of the size and flux of sub-micron sea-salt particles from foams like those present on the oceans. These experiments were designed to examine the effect of seawater organic content, the spatial variation of which may account for the disparity between experimental measurements. Seawater samples were collected off the Scripps Institute of Oceanography Pier. Experiments were completed within 24-48 hours of seawater collection. Bubbles were generated using an air diffuser in a clean glass column filled with seawater (diameter = 15 cm, height = 40 cm). In order to study the influence of organics, sea spray was generated from seawater solutions containing a range of organic concentrations: artificial seawater (3.5 % salinity, no organics), filtered seawater, artificial seawater containing 1% filtered seawater, artificial seawater containing 10% filtered seawater, and unfiltered seawater. The sea spray was dried and subsequently sampled using a TSI (St. Paul, MN) Model 3080 Scanning Mobility Particle Sizer in order to measure the size distribution. For each seawater solution, the measured size distributions were monodisperse and the mean dry particle diameter were approximately the same ranging from 109--117 nm. The total number flux of sea spray was also independent of organic content. At a wind speed of 20 m s^-1, the flux of sub-micron sea-salt particles per ocean surface area according to our experimental data is 6\times10⁵ m^-2 s^-1, or 30 times that predicted by the exponential model proposed by Mason (2001). However, the experimentally measured number flux is several orders of magnitude below earlier estimates made by Blanchard (1969). These experiments suggest that spatial variation in seawater organic content does not account for the large discrepancies in experimental measurements of sub-micron sea-salt flux.