The size of atmospheric nuclei: Some deductions from measurements of the number of charged and uncharged nuclei at kew observatory

The first object of this paper is to set out formulae to be used in determining the size of condensation nuclei from observation of the concentration in the atmosphere of charged and uncharged nuclei and observation of the numbers of small ions of either sign. It is well known that ratios of the combination coefficients which specify the rates of combination of small ions with charged or uncharged nuclei can be deduced from such observations. In the present analysis the following additional principles are assumed. (1) The combination coefficients are related to w , the mobility of small ions by Whipple's hypothetical equation η 12 - η 10 = 4π ew , and (2) combination between small ions and uncharged nuclei are produced solely by collisions due to the random Brownian movements of the small ions. On the assumption that a small ion is equal in mass to eleven molecules of water it is deduced from Scrase's observations at Kew Observatory that so long as the relative humidity is less than 80 per cent the radius of a nucleus is approximately 3 × 10 -6 cm. With higher humidities the radii are larger, at 82 per cent 4.2 × 10 -6 cm. and at 89 per cent 4.5 × 10 -6 cm. In the last section of the paper Köhler's theory of the growth of nuclei in an atmosphere in which humidity increases is applied, and on the assumption that the hygroscopic nuclei at Kew are of diluted sulphuric acid, it is found that in summer the mass of the acid in a nucleus is about 18 × 10 -17 g., the corresponding mass in winter being 12 × 10 -17 g. If the predominant nuclei are of common salt or of nitrous acid these estimates must be halved to give the mass of salt or of the latter acid in a nucleus.