The Production and Dispersal of Maritime Aerosol

Abstract

Field experiments concerned with the characteristics and variability of maritime aerosol have been conducted, over several years, at a coastal site on the island of South Uist, in the Outer Hebrides. In the primary experiments aerosol size distributions (radii r 0.08 to 16 μm) and basic meteorological parameters weremeas- ured from a 10m tower, for both maritime and continental air-streams. In all cases the volumetric aerosol loading V (μm³cm⁻³) increased rapidly with wind- speed U (ms⁻¹ ) accordingto

$$\ln (V) = aU + b,$$

where, for the larger aerosol in maritime air, a∼0.2 and b∼4. The size distributions could be sub-divided into two ranges separated at r∼0.3μm, the lower corresponding to long-range aerosol and the higher to locally produced aerosol. In maritime air, this latter category is produced by bubble bursting.

In subsidiary field experiments evidence was found for: the production at the ocean surface of substantial quantities of aerosol in the radius band 20 to 60μm; a decrease of aerosol concentrations Nand loading with altitude; an inverse relationship between N and mixing depth; an insensitivity of aerosol characteristics to the range of relative humidity experienced at South Uist-but not at an alternative site; and clear evidence from chemical and microscopic examination that the aerosol in maritime air-streams arises principally from the ocean.

Laboratory experiments revealed the sensitivity of bubble size-distributions to temperature and salinity. An assessment of the distribution of maritime aerosol loading over the North Adantic indicates considerable spatial and temporal variability.