Astroclimatology of Paranal

A CLIMET (CI-8060) airborne particle counter has been operated for seven months from Dec. 92 to May 93 at Paranal. The device counts particles at six ranges (0.3, 0.5, 0.7, 1.0, 5.0 and 10 micron) simultaneously. An analysis of the height distribution of particles larger than 0.5 micron over 6 levels from 1 to 10m above ground is presented on the figure below, out of 3000 samples of 10 minutes each, taken at night from February to May 1993. A 40% increase is seen at ground level during events with the lowest aerosol concentration in which cases locally produced contamination is prevailing. With increasing particle concentration, the effect of height becomes marginal showing that the site is then in free air conditions: it can be seen on the plot that during events with median to high concentration, the increase of aerosol density is negligible from 10 to 5m and of the order of only 10% from 5m to 1m above ground.
Compared to the standard definition of indoor cleanliness in use in industry, the site is better than class 30,000 for particles larger than 1 micron and around class 100,000 for sizes down to 0.3 micron.

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As can be seen on the figure below, the density of particles of diameter larger than 0.5 micron, measured at 10m above ground over 20mn averages, shows very large variations with a time constant of several days. The theory says that particles of diameter between 0.5 and 1 micron, when carried aloft with the wind, fall 1m in a time varying from days to one hour, which means that the aerosol content measured in the Paranal area is affected by sources located up to several hundred kilometers away. In this range of size one may find human made (mines) as well as natural (pollen, bacteria, sea salt) aerosols. It is most probable that whirlwinds, a thermal phenomenon particular to the desert areas, are mainly responsible for the noticeable increase of Paranal particles density during the warmer (and more humid) summer months.

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