(17). For most types of fogs and precipitation, it is a good approximation to assume that the particles consist of pure water. For clouds, which consist of nuclei surrounded by condensed water, this assumption may not be valid. In the Mie scattering regime, scattering, absorption, and extinction coefficients and differential scattering cross sections are calculated using the code hsphr (18). The code is restricted to spherical particles but does have provisions for heterogeneous compositions in which a spherical nucleus of radius «0 and complex index of refraction n0 is surrounded by a second material having a concentric radius a and complex index of refraction n. Theoretical treatments of the Mie problem are well documented in the literature and will not be repeated here. The method adopted here is the classical numerical treatment given by van de Hulst (19). The code was checked against the concentric sphere calculations of Kerker et al. (20) with excellent numerical agreement. The visibility or, more precisely, meteorological range as used in this study is defined by Koschmieder's relation, where ^sc is the aerosol scattering coefficient at 0.55 /rm, chosen because the peak sensitivity of the human eye occurs at this wavelength. The use of /3SC instead of /3ex (extinction coefficient) implies that the absorption coefficient (/3„) is small enough to neglect at visual wavelengths, a good assumption except for polluted air. From the foregoing relation, it is evident that the transmittance for a path length equal to Rm is 0.02. Haze. The atmospheric transmission efficiency for hazy conditions was calculated using representative aerosol models selected from the work of Shettle and Fenn (17). Results of calculations performed for hazy conditions (Rm = 5 km) are shown in Fig. 1. The aerosol models employed for each atmospheric layer are given in the figure insert. These curves show little fine structure as would be expected, since molecular absorption has been neglected. We can conclude that selection of a laser wavelength shorter than about 2 ptm is undesirable for propagation through haze. Furthermore, Rayleigh (molecular) scattering becomes significant at shorter wavelengths, scaling as X-4, and visible lasers would suffer attenuation due to this mechanism as well as because of haze aerosol extinction. The transmission efficiency as a function of altitude for propagation at a zenith angle of 50° under clear (Rm = 23 km) and hazy (Rm = 5 km) conditions is shown in Fig. 2. Clearly, receptor siting at elevations h > 1 km is desirable to partially mitigate the effects of haze. Thus, siting in basin or valley areas subject to weather inversions is undesirable, especially if the site is subject to urban pollution. Fog. We have taken the Mie calculations of Pinnick et al. (21,22) for four liquid water contents W (g/m3) of fog and replotted the extinction data as functions of wavelength. The fog particle-size measurements judged to be reliable were chosen to represent a wide range of conditions ranging from maritime and continental advection fogs (23, 24, 25) to inland radiation fogs (25, 26, 27, 28). These calculations are shown in Figs. 3 and 4. Error bars, if used, simply denoted the range of calculated /3ex for the various size distributions given by the respective authors. These figures show that laser operation at a wavelength around 11 /rm may be effective in partially mitigating the effects of light fog as has been confirmed experimentally (29,30). The minimum scatter in these data around 11 pun is in conformance
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