The methods of forming, detecting, and measuring the size and concentration of liquid aerosols in the size range of 0.01 to 0.25 microns diameter☆

Abstract 1. ( a ) The objective of this research was to develop methods of producing, detecting, and measuring rapidly and easily the size and concentration of liquid aerosol droplets in a range (0.01-0.08 μ) smaller than those at present available. 2. ( b ) The new contribution to the method of measurement of particle size consists in growing quantitatively exceedingly small aerosol droplets to a size range where they can be measured by established optical methods. 3. ( c ) Aerosol droplets with radii less than 0.08 μ are too small to be measured by any of the optical methods available. The electron microscope is not always suitable on account of the liquid character of the droplets and their volatility. 4. ( d ) The sizes of homogeneous aerosol droplets following controlled growth are easily measured by a variety of optical methods developed previously in the Columbia University Laboratories (under NDRC-OSRD contracts in 1940-45), for a size range terminating at the approximate lower limit of 0.08-μ radius, depending upon the index of refraction and the precision desired. The polarization of the scattered light furnishes the most sensitive measure in this range of size. As size is increased, measurement naturally becomes less difficult and more reliable. The various optical methods applicable in these upper ranges make contact with those of the ordinary light microscope. 5. ( e ) The homogeneous aerosol generator of La Mer and Sinclair can be used for the production of exceedingly small particles; 99% H 2 SO 4 and dibutyl phthalate have proven to be suitable examples. The lower limit of size production appears to be the critical nucleus (radius ∼ 10 −7 cm. or 0.001 μ); the smallest radius measured by the present growth is about 0.01 μ, although smaller sizes of the order 0.006 μ are detectable and these sizes can be estimated crudely. 6. ( f ) Aerosols of 99% H 2 SO 4 when exposed to H 2 SO 4 solutions of higher water content absorb water, and grow to a size such that the aqueous tension of the droplet equals that of the master solution to which it is exposed. Vapor pressure equilibrium is attained exceedingly rapidly. 7. ( g ) Similar behavior is exhibited by dibutyl phthalate aerosol droplets when exposed to the vapors of volatile solvents such as toluene, xylene, etc. Theory indicates, and there is no evidence to the contrary, that all liquid aerosols when exposed to the vapors of a miscible and volatile solvent can be grown under controlled conditions. 8. ( h ) In the case of 99% H 2 SO 4 , measurements can be made following growth by a factor as large as 8-fold in radius (512-fold in volume or an increase in the intensity of light scattering of roughly 4000- to 250,000-fold, depending upon initial radius). The control of the growth is more reliable the smaller the growth. Five-fold is recommended. 9. ( i ) The number and radii of the grown sulfuric acid droplets were determined by the optical transmission method. The mass of H 2 SO 4 thus calculated was compared with the mass deposited upon a filter which was washed, and the pH of the wash liquid was determined with a glass electrode. Considering the minute amount of material collected, satisfactory agreement was secured. 10. ( j ) The range and reliability of the transmission method was improved by generating new functions of the type K α n , where K is the optical scattering area coefficient, and α = 2πr λ ; n was varied from 1 to 3. These derived functions give characteristic maxima and minima in more desirable ranges of droplet radii.