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Bruce W. Fitch
and
Ted S. Cress

Abstract

This is the second of two papers based on extensive airborne measurements of particle size distributions, taken within the lower troposphere at nine sites in western Europe. The first paper focused on the behavior of the volume (mass) mode measured in the particle radius range 0.25–0.49 μm and its relationship to the measured volume scattering coefficient. In this paper, volume distribution plots for particle radii 0.25–5.9 μm are shown to consist of three distinct modes, each of which is well fit by a log-normal distribution. The log-normal fit parameters do not all appear to be independent variables, but are predictable. Patterns in the behavior of the volume modes and their combinations are discussed as a function of altitude, season, relative humidity and site of measurement.

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Bruce W. Fitch
and
Ted S. Cress

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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Ted S. Cress
and
Douglas L. Sisterson
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Thomas P. Ackerman
,
Ted S. Cress
,
Wanda R. Ferrell
,
James H. Mather
, and
David D. Turner
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