DEPOSITION OF AIR‐BORNE LYCOPODIUM SPORES ON PLANE SURFACES

Abstract

The deposition of Lycopodium spores on sticky surface traps, including vertical and horizontal microscope slides and Petri dishes as used in routine aerobiological survey, was studied in a small wind tunnel at wind speeds from approximately 0.5–9.5 m./sec.Deposition results from several processes acting singly or in combination. The pattern of deposit on a microscope slide orientated at angles varying from o to 90° to the wind, with gravity either positive, neutral or negative, indicates that, except on a surface parallel to the wind at the lowest wind speed, sedimentation under gravity plays a minor part in deposition on plane surfaces. As the wind speed is increased the deposit is decreased because of ‘edge shadow’. At the highest wind speed the deposit is as large on the lower as on the upper surface of the horizontal slide, which suggests deposition by turbulence. Deposition by impaction against a vertical strip increases with wind speed, and efficiencies observed are lower than for cylinders of the same diameter. The deposit on slides inclined at angles of 45° or less to the wind direction is increased by impaction of an ‘edge drift’. The interaction of these various deposition processes on mean deposition at different angles and wind speeds gives a series of curves with a maximum at 90° at 9.5 m./sec. and with minima at 0, 90 and 180° at wind speeds lower than 9.5 m./sec. At these lower wind speeds there are two maxima in the range 20–70° and 135–150° respectively.Deposits on Petri dishes show rim effects differing at different wind speeds. These can be eliminated by sinking the dish below a flat surface.Blow‐off from non‐sticky surfaces is least at about 45°, and greatest at 0°.Spore concentration in air is difficult to estimate from the deposits on planesurface traps, because horizontal traps under‐record at medium wind speeds, and because vertical traps are very sensitive to changes in wind speed. Power‐operated suction traps are to be preferred when data on spore concentrations are required.