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The focal length method of measuring liquid aerosol droplets
907
Discussions
AUTHORS
REPLY
We agree with Dr McKee that measurements of smoke opacity are, for certain purposes, useful and that in the field they are more readily obtained than mass emission rates. What we do question is the use of the “visibility” criterion forming the basis of the European emission standard for diesel smoke, which relates only to considerations of visual amenity, without recourse to health or other environmental effects. Not onlv does this lack current relevance, it is atso incompatible w&h the newly adopted European air quality criteria. if recognition were given to the need to reconcile emission and air quahty criteria, as happened in the U.S.A. at least a decade ago (NAS, 1974) there can be little doubt but that the visibility criterion would be found inadequate and, since it admits no need to limit emisstons beyond the point at which they are overtly visible at source, an alternative criterion would have to be found. One obvious alternative candidate for consideration is the control of the massemission rate. While we recognise that the appii~atlon of any quantitative check in the field presents technicai dithculties, this would be far less the case in the context of ‘type’ testing of production line engines on a test rig. with which we are here primarily concerned. The debate over the need for a size-specific particulate standard is. as Dr McKee acknowledges, less relevant in the case of diesel emissions than for some other sources. Nonetheless. we would detract from the suggestion that particles having an aerodynamic diameter in excess of 5 pm have no health effects. Particles of diameters very much greater than 5 pm are effectively deposited in nose and mouth (e.g. Ogden and Birkett, 197X), and their toxicity will depend. ~nicr &LI. on their solubility in body fluids. As for the changing relative importance of vehicular and stationary sources of smoke in London, this may be attributable as much to socio-economic and political factors as to pollution control measures. The acid test ofcontrol strategies in London. and perhaps also in Houston, will likely come in the next decade or so if the forecast mcrease in coal usage materializes (CEE. 1981) and when the days of increasingly plenttful natural gas and low sulphur oil have passed,
REFERENCES
Commission on Energy and the Environment (1981)Coaland the Enr~ronmenr. H.M.S.O., London. National Academy of Sciences (1974) Report by the Coordinating Committee on Air Quality Studies, Air Quahty and Automobtle Emission Control&The Kelationship of Emissions to Ambient Air Quality. Vol. 3. No. 93.-24. Ogden T. L. and Birkett J. L. (1978) Antt. occup. Hyy. 21, 41 50
THE FOCAL LENGTH METHOD OF MEASURING LIQUID AEROSOL DROPLETS* Sizing fine liquid droplets deposited on a slide and then calculating their smaller diameters, when airborne, was described by Liu rr crl. (1982); the method depends on the angle of contact being known. A better way. for which knowledge of the refractive index of the liquid is needed, instead of the angle of contact, was worked out at the Chemical Defence Experimental Establishment at Porton, England in 1940and has been used extensively since then. The glass surface has usually been polished with aerosol OT (di-2ethylhexyl sodium sulphosuccinate) which ensures consistent spreading of many oils. By examination under a microscope illuminated with parallel light the focal length of the droplet lens is easily measured with the fine adjustment and there is no need to try and determine the thickness of the lens. To avoid having to calculate the spread ratio, tables were prepared by Bexon and Ogden (1974) from which the ratio can be read for refractive indices from 1.2 to 1.7 either when the drop is on the top of the slide or when it hangs underneath. A table of contact angles is included as a check on the spreading. Aerosol LuhorororJ nepurtmenl of‘Chemi.srr!, University of Essex, Wloenhar Park Colrhester CO4 3SQ. U.K.
C’. N. Dhvrc-s
REFERENCE
Bexon R. and Ogden T. L. (4974) 1. tlarosol
Sci. 5, SO9 ~524.
STREET LEVEL VERSUS ROOFTOP SAMPLING: CARBON MONOXIDES AND AEROSOL IN NEW YORK CITY* The results reported by Bauman ct al. (1982) on the verttcal distribution ofselected components of atmospheric poilutton are quite consistent with a previous long term study we conducted using samplers separated by 43 floors at the Midtown Manhattan N.Y.U. Medical Center, (19761978). and by 13 floors at the DOE Environmental Measurements Laboratory (1968-1970) (Lioy pi ui., 4980). Those results showed that there was a statisttcally significant dilference between the rooftop and ground level concentrattons ofTSP. Pb and Mn measured at the NYU station and of theTSP and Pb measured at the DOE station. In each case. the ratto ot roof to ground ranged between 70 and 85”,,. In contrast. vanadium showed no significant dtfference between the roof and ground which reflects its being associated primarily with rooftop sources. During the DOE sampling period. tht: Mn levels were the sa,me at the roof and ground. This contrast could beexplained by the fact that in 4968 1970, the Mn was being emitted prtmarily by coal burning facihttes which are no longer operational. However, in the later period of study manganese was an additive in gasolme and showed patterns similar IO those of lead.
* Liu 13. Y. H., Pui D. Y. H. and Wang X.-Q. (1982) Atmospheric Emironment 16, 563.-567. * Bauman S. E., Williams E. T., Finston H. L.. Ferrand E. F. and Sontowskt J. A~m~s~h~r~~ Environment (1982) 16, 2489- 2496.
Discussions
AUTHORS
REPLY
We agree with Dr McKee that measurements of smoke opacity are, for certain purposes, useful and that in the field they are more readily obtained than mass emission rates. What we do question is the use of the “visibility” criterion forming the basis of the European emission standard for diesel smoke, which relates only to considerations of visual amenity, without recourse to health or other environmental effects. Not onlv does this lack current relevance, it is atso incompatible w&h the newly adopted European air quality criteria. if recognition were given to the need to reconcile emission and air quahty criteria, as happened in the U.S.A. at least a decade ago (NAS, 1974) there can be little doubt but that the visibility criterion would be found inadequate and, since it admits no need to limit emisstons beyond the point at which they are overtly visible at source, an alternative criterion would have to be found. One obvious alternative candidate for consideration is the control of the massemission rate. While we recognise that the appii~atlon of any quantitative check in the field presents technicai dithculties, this would be far less the case in the context of ‘type’ testing of production line engines on a test rig. with which we are here primarily concerned. The debate over the need for a size-specific particulate standard is. as Dr McKee acknowledges, less relevant in the case of diesel emissions than for some other sources. Nonetheless. we would detract from the suggestion that particles having an aerodynamic diameter in excess of 5 pm have no health effects. Particles of diameters very much greater than 5 pm are effectively deposited in nose and mouth (e.g. Ogden and Birkett, 197X), and their toxicity will depend. ~nicr &LI. on their solubility in body fluids. As for the changing relative importance of vehicular and stationary sources of smoke in London, this may be attributable as much to socio-economic and political factors as to pollution control measures. The acid test ofcontrol strategies in London. and perhaps also in Houston, will likely come in the next decade or so if the forecast mcrease in coal usage materializes (CEE. 1981) and when the days of increasingly plenttful natural gas and low sulphur oil have passed,
REFERENCES
Commission on Energy and the Environment (1981)Coaland the Enr~ronmenr. H.M.S.O., London. National Academy of Sciences (1974) Report by the Coordinating Committee on Air Quality Studies, Air Quahty and Automobtle Emission Control&The Kelationship of Emissions to Ambient Air Quality. Vol. 3. No. 93.-24. Ogden T. L. and Birkett J. L. (1978) Antt. occup. Hyy. 21, 41 50
THE FOCAL LENGTH METHOD OF MEASURING LIQUID AEROSOL DROPLETS* Sizing fine liquid droplets deposited on a slide and then calculating their smaller diameters, when airborne, was described by Liu rr crl. (1982); the method depends on the angle of contact being known. A better way. for which knowledge of the refractive index of the liquid is needed, instead of the angle of contact, was worked out at the Chemical Defence Experimental Establishment at Porton, England in 1940and has been used extensively since then. The glass surface has usually been polished with aerosol OT (di-2ethylhexyl sodium sulphosuccinate) which ensures consistent spreading of many oils. By examination under a microscope illuminated with parallel light the focal length of the droplet lens is easily measured with the fine adjustment and there is no need to try and determine the thickness of the lens. To avoid having to calculate the spread ratio, tables were prepared by Bexon and Ogden (1974) from which the ratio can be read for refractive indices from 1.2 to 1.7 either when the drop is on the top of the slide or when it hangs underneath. A table of contact angles is included as a check on the spreading. Aerosol LuhorororJ nepurtmenl of‘Chemi.srr!, University of Essex, Wloenhar Park Colrhester CO4 3SQ. U.K.
C’. N. Dhvrc-s
REFERENCE
Bexon R. and Ogden T. L. (4974) 1. tlarosol
Sci. 5, SO9 ~524.
STREET LEVEL VERSUS ROOFTOP SAMPLING: CARBON MONOXIDES AND AEROSOL IN NEW YORK CITY* The results reported by Bauman ct al. (1982) on the verttcal distribution ofselected components of atmospheric poilutton are quite consistent with a previous long term study we conducted using samplers separated by 43 floors at the Midtown Manhattan N.Y.U. Medical Center, (19761978). and by 13 floors at the DOE Environmental Measurements Laboratory (1968-1970) (Lioy pi ui., 4980). Those results showed that there was a statisttcally significant dilference between the rooftop and ground level concentrattons ofTSP. Pb and Mn measured at the NYU station and of theTSP and Pb measured at the DOE station. In each case. the ratto ot roof to ground ranged between 70 and 85”,,. In contrast. vanadium showed no significant dtfference between the roof and ground which reflects its being associated primarily with rooftop sources. During the DOE sampling period. tht: Mn levels were the sa,me at the roof and ground. This contrast could beexplained by the fact that in 4968 1970, the Mn was being emitted prtmarily by coal burning facihttes which are no longer operational. However, in the later period of study manganese was an additive in gasolme and showed patterns similar IO those of lead.
* Liu 13. Y. H., Pui D. Y. H. and Wang X.-Q. (1982) Atmospheric Emironment 16, 563.-567. * Bauman S. E., Williams E. T., Finston H. L.. Ferrand E. F. and Sontowskt J. A~m~s~h~r~~ Environment (1982) 16, 2489- 2496.