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A collector for sampling airborne particles
71
Discussions of Papers
A COLLECTOR
FOR SAMPLING
AIRBORNE
PARTICLES
I WOULD like to comment on the Technical Note by LANEand CLARK(1966). The louvred collector which they describe will no doubt be 100 per cent retentive of falling dust, but there is a major source of error which is not brought out in their description. If the collector is placed in the surface of the earth in an area which is itself 100 per cent retentive it will no doubt work satisfactorily. If it is placed on a surface where dust can be re-entrained when the wind rises, it will tend to collect not only dust particles which fall directly upon it but also particles which have originally landed elsewhere and arrive at the collector on their first, second or fiftieth bounce. It was pointed out by LUCAS(1957) that m-entrained dust travels in the main close to the ground and that a collector a few feet above the surface of the earth is very much less affected by re-entrained dust than a collector at ground level The ground level cohector can have a result an order of magnitude greater than the collector a few feet above the surface of the earth. D. H.
Central Efectrfcity Research ~baratories, Cfeeve Road, Leatkerkead, Surrey, Engfand.
LUCAS
REFERENCES LANEW. B. and CLARKD. E. Jr, (1966) A collector for sampling deposited airborne particles. Air & Wat. Poffut. ht. J. 10, 627-628. LUCASD. H. (1957) Certain aspects of the deposition of dust. J. brsr. Fuel. 30,623-627.
Authors’ reply :
WE HAVE never observed an order-of-magnitude error in particle collection because of re-entrainment, although some combination of particle sixe, particle concentration, total mass, wind speed, surface characteristics, and duration of exposure would no doubt give such a result. Re-entrainment of particles was not significant in the collections which we made in Nevada. The collectors were never dug-in so that they were flush with the ground and in most cases they were on small wooden platforms to facilitate making a level exposure. In Costa Rica we made routine ground level collections and post collections at a height of 5 ft at the same location. We found that the elevated collections were sometimes smaller than the corresponding ground collections, and that the ratio of the post collection to the ground collection was inversely related to wind speed. Stanford Research Institute, Menlo Park, Cafrfonia 94025, U.S.A.
W. B. LANEand D. E. CLARK,JR.
Discussions of Papers
A COLLECTOR
FOR SAMPLING
AIRBORNE
PARTICLES
I WOULD like to comment on the Technical Note by LANEand CLARK(1966). The louvred collector which they describe will no doubt be 100 per cent retentive of falling dust, but there is a major source of error which is not brought out in their description. If the collector is placed in the surface of the earth in an area which is itself 100 per cent retentive it will no doubt work satisfactorily. If it is placed on a surface where dust can be re-entrained when the wind rises, it will tend to collect not only dust particles which fall directly upon it but also particles which have originally landed elsewhere and arrive at the collector on their first, second or fiftieth bounce. It was pointed out by LUCAS(1957) that m-entrained dust travels in the main close to the ground and that a collector a few feet above the surface of the earth is very much less affected by re-entrained dust than a collector at ground level The ground level cohector can have a result an order of magnitude greater than the collector a few feet above the surface of the earth. D. H.
Central Efectrfcity Research ~baratories, Cfeeve Road, Leatkerkead, Surrey, Engfand.
LUCAS
REFERENCES LANEW. B. and CLARKD. E. Jr, (1966) A collector for sampling deposited airborne particles. Air & Wat. Poffut. ht. J. 10, 627-628. LUCASD. H. (1957) Certain aspects of the deposition of dust. J. brsr. Fuel. 30,623-627.
Authors’ reply :
WE HAVE never observed an order-of-magnitude error in particle collection because of re-entrainment, although some combination of particle sixe, particle concentration, total mass, wind speed, surface characteristics, and duration of exposure would no doubt give such a result. Re-entrainment of particles was not significant in the collections which we made in Nevada. The collectors were never dug-in so that they were flush with the ground and in most cases they were on small wooden platforms to facilitate making a level exposure. In Costa Rica we made routine ground level collections and post collections at a height of 5 ft at the same location. We found that the elevated collections were sometimes smaller than the corresponding ground collections, and that the ratio of the post collection to the ground collection was inversely related to wind speed. Stanford Research Institute, Menlo Park, Cafrfonia 94025, U.S.A.
W. B. LANEand D. E. CLARK,JR.