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DISCUSSIONS LIDAR OBSERVATIONS

OF TROPOSPHERIC

AEROSOLS*

WE CONSIDER the preprint “Lidar Observations of Tropospheric Aerosols” by Gambling and Bartusek, a significant contribution to the application of lidar in atmospheric studies. We would, however, like to make one comment. The authors state (p. 184) that, in comparing their average turbidity profile with results of Elterman, “the comparison suggests a significantly greater optical turbidity in the lower atmosphere over Adelaide”. The computations of optical turbidity are particularly sensitive to the exclusion of /3”(h) in the transmission correction term (p. 182) and to the value of 0.25 selected for the aerosol phase function for backscatter (p. 184). Using values for molecular and aerosol extinction given by MCCLATCHEY et al. (1970), we find a decrease of as much as 10 per cent in the transmission correction term by including the aerosol extinction for the case of a hazy atmosphere at mid-latitude summer between 3 and 10 km. Inclusion of aerosol extinction would have decreased the optical turbidity particularly between 3 and 4 km. There is also some uncertainty as to the exact value of the aerosol phase function for backscatter. For example, BULLR~CH(1964) computed values as large as 044. Since the optical turbidity is inversely proportional to this factor, an increase in the value-of 0.25 would decrease the computed optical turbidity. Therefore. including- Bl(h) .-. . and increasing 0.25 could make the ontical turbidity between 3 and 4-km compatible with that given by Elterman. It should be noted, of course, that Elterman’s Standard Aerosol Atmosphere represents a most useful reference condition but is not necessarily equal to any particular condition in the Northern hemisphere. Stanford Research Institute Menlo Park California 94025, U.S.A.

WILLIAMVIEZEE R. T. H. C~LLIS

REFERENCES MCCLATCHEYet al. (1970) Optical Properties of the Atmosphere, Environmental Research Papers, No. 331, Air Force Cambridge Research Laboratories, 22 September. BULLRICHK. (1964) Scattered radiation in the atmosphere and the natural aerosol. Adv. in Geophys. 10,99.

AUTHORS’

REPLY

IN COMMBNTINO on the comparison of our average turbidity profile with the results of ELTE~MANet al. (1969), Viezce and Collis have raised two points. Firstly, it has been suggested that the computations of optical turbidity are particularly sensitive to the exclusion of B”(h) in the transmission correction term (p. 183). We believe that Viezee and Collis have misinterpreted our paper with regard to this point. Our transmission corrections do in fact include the effects of aerosol scattering. It should be noted that, as correctly stated in the paper, the quantity dropped from the transmission correction term is /3,(h), the value of ,‘I_,corresponding to the lower limit of the integral, whereas the altitude variable in the transmission correction term is actually h’. Thus, in the stepwise method used to correct the scattering profile over successive height intervals, h,, to (heal - Ah), heal to (h=., - 2Ah), etc. values of /3,(h,., - Ah), ,9&z,., - 2Ah) are successively evaluated. However, the method used consists of initially dropping the quantity BJh,., - nAh) from the transmission correction term at that stage of the computation where flA(hcal - nAh) is to be calculated. All other terms, gl(&., - Ah). . . to B&r,., - (n -1) Ah) are included. The value of j34(h,.l - nAh) is thus calculated, and used in the next step of the computation. The second point concerns the magnitude of the aerosol phase function for backscatter. The authors concur with Viezee and Collis that there is some uncertainty as to the exact value of the aerosol phase function for backscatter, P_,(T), and regret the omission of a fuller discussion in the paper. On examination of both experimental and theoretical evidence, it appears that for the moist aerosols generally observed over Adelaide, the value of P,,(T) is closer to O-25 than to 0.44. Indeed, the experimental results of REIXXRand Sraom~~ (1946) upon which the calculations of PA(B) by ELTERMAN et al. (1%9) are based, and which is subsequently used by them to determine the optical turbidity, give rise to a value of P”(n) slightly less than @2. * D. Y. GAMBLINGand K. BARTUSEK(1972) Atmospheric Environment 6,181-190. 869

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