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RAPID COMMUNICATIONS This section was established to reduce the lead time for the pub- lication of Letters containing new, significant material in rapidly advancing areas of optics judged compelling in their timeliness. The author of such a Letter should have his manuscript reviewed by an OSA Fellow who has similar technical interests and is not a member of the author's institution. The Letter should then be submitted to the Editor, accompanied by a LETTER OF ENDORSE- Extension of atmospheric light scattering measurements into the uv region Jost Heintzenberg and Georg Witt University of Stockholm, Meteorological Institute, Fack, S- 106 91 Stockholm, Sweden. Received 24 February 1979. Sponsored by K. Bullrich, Johannes Gutenberg- Universität. 0003-6935/79/091281-03$00.50/0. © 1979 Optical Society of America. The scattering properties of individual particles depend on the ratio of their size to the wavelength (size parameter). In general, the functions describing this dependency strongly increase with increasing size parameter. They reach maxi- mum values where the circumference of the scattering particle is roughly equal to the wavelength, allowing a resonance. In the atmosphere, where the aerosol is distributed over many sizes, the scattering functions are to be weighted by an ap- propriate number size distribution function. This determines the observable light scattering properties of the atmospheric aerosol. Generally atmospheric size distributions exhibit distinct maxima with a steep concentration decrease toward larger particle sizes. Because of this the integrated scattering properties of atmospheric aerosols are dominated by particles of a certain size range. This dominant size range is shifted with the light scattering wavelength. Therefore atmospheric scattering investigations can be directed toward the study of predetermined size ranges by merely selecting the appropriate wavelength. The aim of aerosol light scattering measurements in the atmosphere is to provide either the input to an inversion procedure for obtaining a plausible size distribution function or a direct interpretation in terms of the integral aerosol properties' total number, surface, volume, or mass. Unam- biguous interpretation of light scattering studies depends not only on the measured scattering parameters, but also on the choice of the appropriate wavelength. In our present Communication we concentrate on scattering studies based on measurements with integrating nephelom- eters. The principle and techniques of the integrating nephelometer have been described elsewhere. 1,2 Basically this instrument measures a quantity closely related to the total volume scattering coefficient on an absolute scale. The in- strument uses continuum light sources in conjunction with interference filters, currently confined to the 400-800-nm interval. Thus the information content of available nephe- lometer measurements is dominated by particles within a restricted size range. On the other hand, experimental studies have shown good correlation of nephelometer measurements with the total mass of suspended submicrometer particles and even with particulate sulfate in the air. 3 The latter is a con- MENT FROM THE OSA FELLOW (who in effect has served as the referee and whose sponsorship will be indicated in the published Letter), A COMMITMENT FROM THE AUTHOR'S INSTITUTION TO PAY THE PUBLICATION CHARGES, and the signed COPYRIGHT TRANSFER ' AGREEMENT. The Letter will be published without further ref- ereeing. The latest Directory of OSA Members, includ- ing Fellows, was published in the Spring 78 issue of Optics News. sequence of the fact that the volume of sulfate-carrying aerosols is concentrated in the optically most effective size range. 3 However, cases exist where measurements in the visible do not encompass that part of the aerosol population dominating such integral aerosol parameters as total surface or volume. A better overlay of scattering and surface or volume vs size is obtained if the measuring wavelength is shifted toward the uv. This is verified by our model calculations simulating nephelometric measurements. The integrands of the scattering coefficients in different spectral bands have been calculated, i.e., the function where [dn(r)]/d log(r)] is the number size distribution as taken from the literature, 4 8 and Q s is the Mie scattering ef- ficiency factor depending on r, λ, and the complex refractive index m. No dispersion of m was taken into account. The values for m in the visible have been taken from the literature. S(λ) is the relative spectral sensitivities to simulate mea- surements with 50-nm bandwidth filters at the given center wavelengths. In Figs. 1-3 the differential scattering coefficients f (r) have been plotted for wavelengths in the visible and the uv together with surface and volume aerosol size distributions covering a wide range of atmospheric aerosols. To show quantitatively what fractions of the three modes of the aerosol size distri- bution are seen by the different spectral channels of a neph- elometer the radii which cut off the lower and the upper 2.5% of the different integral aerosol properties are listed in Table I. As an example 95% of the volume of the total urban aerosol lies between 0.042-µm and 7.4-µm radius, whereas 95% of the nephelometer signal at 550 nm is caused by particles between 0.086 µm and 0.64 µm. As can be seen in Fig. 1 nephelometer measurements at 400-nm wavelength are well suited to de- termine in situ the total urban aerosol volume or the total suspended particulate sulfate. For the stratospheric aerosol layer a two-wavelength inte- grating nephelometer operating at 240 nm and 550 nm could measure the total surface and the total volume of the sus- pended particles (see Fig. 2). In the vicinity of fine particle sources, however, nephelometer measurements in the visible region are not suitable for the determination of the total suspended volume. The measurements of Whitby et al. 8 with a commercial nephelometer confirm our results. Figure 3 shows that a nephelometer operating in the uv can monitor the total particle volume in this case. Alongside the model calculations we explored the possi- bilities of adapting the integrating nephelometer to uv wavelengths. The nephelometer used in these experiments is described in Ref. 2. The opal glass diffuser of that instru- ment was replaced by two Suprasil disks ground on both sides. 1 May 1979 / Vol. 18, No. 9 / APPLIED OPTICS 1281
