Measurements of Aerosol Size Distributions in the Lower Troposphere over Northern Europe

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Abstract
Airborne measurements of particle size distributions were made at several altitudes within and above the mixing layer at sites near Ahlhorn and Meppen (West Germany), Rodby (Denmark), and Bruz (France). The distributions were measured over the range 0.2–5.9 μm in particle radius using a Royco model 220 particle counter. The experimental data gathered at the sites were analyzed in terms of volume-size distributions instead of the commonly used particle number distributions. The volume distribution plots were found to be bimodal with the accumulation mode centered at a mean radius in the range 0.26–0.49 μm and the coarse particle mode existing beyond 1.0 μm. The data show the accumulation mode is well defined by a log-normal distribution. The values of the measured volume scattering coefficient and mean particle radius of the accumulation mode increase as the maximum particle volume of the mode increases. The existence of an accumulation mode was almost always confined to the mixing layer. It is interesting that haze layers above the mixing layer were found to have a distinct coarse particle mode but, generally, no distinct accumulation mode. The total concentration of particles in each of the two modes appear related for larger values of concentration in the coarse particle mode.

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