An ultrafine sea-salt flux from breaking waves: Implications for cloud condensation nuclei in the remote marine atmosphere

摘要

Sea-salt aerosol (SSA) is an important constituent of natural marine aerosol to which anthropogenic aerosols must be compared when assessing their climatic influence. Size distributions of particles, produced by bubbles from coastal oceanic breaking waves, were found to have sizes as small as 0.01 μm, with 60% smaller than 0.1 μm diameter. The thermal stability of these particles and their growth factor measured under increasing humidity indicate that most are sea salt. These SSA size distributions were used in conjunction with the measured number flux for bubbles from coastal breaking waves to develop a new sea-salt source function. This source function and the associated SSA flux compare favorably with previously published estimates available for sizes larger than 0.5 μm but extend the source function down to much smaller sizes. When this SSA flux is applied to oceanic whitecaps that have a highly nonlinear dependence on wind speed, it implies strong regional and temporal differences in the open ocean surface number flux. In marine regions with little continental impact we estimate that this SSA flux can contribute ～5–90% of the marine cloud condensation nuclei (CCN), with the rest accounted for by the flux of aerosol entrained from the free troposphere. These two fluxes are large enough to account for commonly observed aerosol and CCN concentrations in the clean MBL without requiring a nucleation source in the MBL. These observations have significant implications for modeling marine aerosol concentrations and evolution.