Expansion cloud chamber observations on the nucleation and growth of sulfuric acid—water droplets

Abstract

Attempts to observe critical supersaturations in mixtures of sulfuric acid and water vapors, using an expansion cloud chamber, have yielded uncertain results. A careful theoretical analysis of the situation reveals the problems that can arise because of long relaxation times to the steady state, long droplet growth times, and depletion of nucleating vapor at high rates of nucleation. Growth times are dominated by the time required to reach the so-called “activation size,” larger than the normal critical size, when the system is supersaturated with respect to water. The time to reach the activation size may be considerable. All of these times may exceed the sensitive time of the expansion cloud chamber. This paper contains a detailed analysis of these effects and a description and interpretation of new expansion chamber measurements in light of this analysis. Our measurements show effects consistent with the analysis. One concludes that the measurement of critical supersaturations in H 2OH 2SO 4 vapor mixtures are difficult if not impossible using either expansion cloud chambers or nozzles.