Size distributions of sea-source aerosol particles: A physical explanation of observed nearshore versus open-sea differences

Lognormal size distribution fits to aerosol probe data collected by the National Oceanic and Atmospheric Administration's King Air aircraft were obtained by a new computer fitting routine. The average fit to western Atlantic nearshore data (about 100 km off Newport News, Virginia) and open-sea data (upwind of Bermuda) showed a marked difference in the volume geometric median diameter (VGMD) of the coarse-particle (diameter range from 1 to 44 μm) sea-salt mode. Analysis of seven separate near-shore flights during offshore flow showed the average VGMD to be 8.1 μm, with a geometric median standard deviation (σ_g) of 2.1. Six separate open-sea flights showed the average VGMD to be 5.6 μm, with a σ_g of 1.7. It is shown that atmospheric state conditions, nearshore versus open sea, could not have caused the significant difference (at 99.6% confidence) between the nearshore and open-sea VGMD values. Use of in situ aircraft sampling and lognormal fitting, plus sampling during offshore flow conditions, gave rise to the ability to observe this difference. It is demonstrated, by considering both the developing wave under offshore flow conditions and the shoreline disturbed wave (i.e., wave age) condition, that sea-derived aerosol size distributions nearshore may often be expected to have more large aerosol particles present than sea-derived aerosol distributions over the open sea.