Abstract
A theoretical analysis of transient and steady state supersaturation distribution inside a thermal diffusion type cloud nuclei counter is presented for simulation of cloud in the laboratory and also for the measurement of cloud nuclei spectra. Solutions of one-dimensional partial differential equations of water vapour diffusion and heat conduction under appropriate initial and boundary conditions are used in this analysis. The height of the chamber is taken as 2.5 cm in which top and bottom plate temperatures are 35 and 30°C respectively. The lower surface temperature corresponds to a typical average environmental temperature of Poona. Different cases of sample entry are discussed. Analysis shows that transient super-saturation exceeding the steady state value arises if the incoming sample is colder than the hot top plate temperature and saturated at that temperature. Transient subsaturation is created initially inside the diffusion chamber when the sample is saturated at the top plate temperature. The chamber passes gradually from a state of transient subsaturation to steady state supersaturation when the incoming air sample is maintained at either top plate or bottom plate temperature and having certain relative humidity. Operation of the chamber under the above non-steady state conditions can introduce large unpredictable errors into the measurement of cloud nuclei spectra, particularly when the incoming air sample is saturated and colder than the top-plate temperature.
Published Version
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