[Elimination of Japanese cedar (Cryptomeria japonica) aeropollen by rainfall]

The concentration of aeropollens of Japanese cedar decreases on rainy days, due to the rainfall removing pollens from the atmosphere. An analysis was performed of the concentration of pollen grains in rain water samples from Yokohama during the period February 10 to April 30, 1991. Sampling of rain water was performed using an automatic rain water collector, and an analysis performed on each mm of rainfall, from 1 mm to 5 mm. In this collector, 1 mm of rainfall corresponds to 100 ml of accumulated rain water. The pollens in rain water were concentrated by centrifugation to 1/200 volume, and pollen grain count was performed on a Fuchs. Rosenthal hemocytometer. Pollens of Japanese cedar were detected in the rain water during the period, 22 February to 19 April, and peaked on 22 March. The number of pollens at peak was 264,000 grains per 100 ml. The pollen count contained in the 2nd to 5th mm of rainfall compared to that in the first mm of rainwater was 19.4% in the 2nd mm of rainfall, 6.7% in the 3rd, 4.4% in the 4th and 2.3% in the 5th. From a log-log graph plot of this data, a regression line was obtained for a regression equation, y = 100.x-2.29. These data indicate that rainfall quickly clears pollens of Japanese cedar from the atmosphere. Examination of pollen obtained from the rain water, showed a few % of the pollen grains had ruptured, and on the surface of the cytoplasmic membrane of the ruptured pollen were attached suspended particulate matters from atmosphere.