Determination of aerosol droplet size and concentration from simple transmittance measurements

A method is described, and equations are developed for the determination of aerosol droplet size and concentration based upon simple transmittance readings. Data and experimental results are presented for water, but the method is applicable to water solutions or other materials as well for which complex indices of refraction are known at specific wavelengths in the visible or the ir. An equation is given for transmittance at 0.63 microm (He:Ne laser) which allows the interrelationship between water cloud or fog droplet size and concentration to be determined. A second equation permits specific values of concentration and droplet size to be determined knowing transmittance values at one or more ir wavelengths located in regions of spectral absorption by the liquid comprising the droplets. For water, data are given for ir wavelengths of 8.5 microm, 10.5 microm, and 12.57 microm, and specific determinations of droplet size and concentration are tabulated based upon these observation wavelengths. In addition, a composite of wavelengths in the 8-13-microm window is presented so that aerosol determinations can be made simply by comparing transmittances at 0.63 microm to those taken broadband at 8-13 microm by devices operating in the latter region without wavelength-scanning capability. For water drops, the method produces results of about +/-10% precision. The method is based upon the constancy of the product of extinction coefficient and droplet diameter in Mie calculations for water at 0.63 microm and is applicable for droplet sizes of from 6 microm to at least 30 microm. Determinations for droplets as small as 2 microm can be made at +/-30% precision.