Profiling of aerosol concentrations, particle size distributions and relative humidity in the atmospheric surface layer over the North Sea

Abstract

Results are presented from measurements in the lower 15 m of the marine atmospheric surface layer, of sea spray particle size distributions, intensities of light scattered by these particles in the near-forward direction, and relative humidity. Particle size distributions were measured with a Rotorod rotating impaction sampler. Variations of particle total area concentrations were determined from measurements of forward scattering intensities with an angular scattering device (further referred to as an optical scatterometer). This latter instrument was used to measure both time-dependent variations and mean values, at fixed heights between 1 and 15 m above the mean sea surface and wave-following at heights between 0.5 and 3 m. The paper is focussed on the comparison of the profile data obtained with the Rotorod and the optical scatterometer. These instruments are based on different physical principles. The shapes of the particle forward-scattering profiles are in good agreement with the particle concentration profiles measured with the Rotorod, provided that ambient conditions do not change during the consecutive measurements. The observed trends in the particle profiles and in the humidity profiles are also similar, reflecting the interaction between the relative humidity and sea-spray droplets. Results from the time-dependent measurements, which have been published elsewhere, are briefly summarized in the context of the analysis of the profile measurements. They show that fluctuations of particle total area concentrations are correlated with the motion of the waves under the sampler. Simultaneous time-series measurements of relative humidity do not reveal a distinct wave influence. This suggests that water vapour is transported by turbulent (molecular) diffusion on a time scale that is much faster than the time scale for particle transport. This is due to the inertia of the largest particles, which respond much slower to changes in the air flow than passive molecular tracers. These results are significant in the context of humidity transport and the constant flux assumption in the surface layer. They are also important for the insights they may provide as to flow interaction with the waves in the open ocean, using the aerosol as a tracer. This requires a comprehensive theoretical analysis which is outside the scope of this paper.