A. Majerowicz

Condensational growth at large vapour concentration: Limits of applicability of the mason equation

Condensational aerosol particle growth is important in many different areas including aerosol dynamics, atmospheric physics, aerosol measurement, reactor safety and chemical accidents. Growth or evaporation of aerosol particles is determined by simultaneous mass and energy fluxes in the surrounding gas. These fluxes are frequently determined by the well-known MaxweUian expressions (see Fuchs, 1959). In order to account for first order effects Wagner (1982) has introduced approximate corrrection factors to these Maxwellian fluxes. More accurate expressions have recently been presented by Seinfeld (1986) and Barrett and Clement (1988). For drop growth calculations the actual drop temperature has to be determined. For comparatively small temperature gradients approximate analytical solutions of the corresponding energy balance equation can be derived. Based on the Maxwellian fluxes Mason (1957, 1971) has obtained widely used growth equations. Improved analytical solutions were presented by Wagner (1982) and Barrett and Clement (1988). For more accurate drop growth calculations numerical solutions of the energy balance equation are required (Wagner, 1982; Barrett and Clement, 1988). In this paper we are presenting a somewhat improved expression for the continuum mass flux towards the drop surface and a new analytical solution of the energy balance equation, which differs slightly from the corresponding solution of Barrett and Clement (1988). Furthermore several analytical and numerical drop growth calculations are quantitatively compared for the condensation of water, methanol and n-butanol vapours at various supersaturations.

Kinetics of particle growthin supersaturated binary vapor mixtures

Abstract Growth rates of droplets in the unary vapor systems water and n-propanol and in the binary vapor mixture water - n-propanol have been determined experimentally. The size range of the growing droplets covers the transition as well as the continuum range. The experimental data are compared to steady state droplet growth calculations assuming a uniform droplet composition. Good agreement between experimental and theoretical values can be observed when the respective sticking probabilities and the thermal accomodation coefficient are chosen to be unity.

Multiple scattering in aerosols: different theoretical approaches and comparison with experimental data

Multiple scattering effects in droplet aerosols were investigated theoretically using three different numerical approaches and experimentally by means of a specially developed measuring system. The system allows quantitative determination of laser light extinction and scattering. Measurements were performed in scale model clouds formed by growing water droplets. This was accomplished in an expansion cloud chamber under well-controlled laboratory conditions. In many cases however, the investigated models and the experimental evidence showed substantial discrepancies depending on parameters and ranges of variables within which a model was used. The work is in progress and preliminary experimental results are discussed and compared with computations. Based on the experimental conditions chosen in this study, an attempt was made to explain the accuracy of the applied models to describe multiple scattering effects in aerosols.

Direct optical aerosol concentration determination in multiply scattering media

Abstract Aerosol number concentrations based on quantitative determination of transmittance of a narrow laser beam were measured in growing droplet aerosols under multiple scattering conditions. A correction term to the Beer-Lambert attenuation law accounting for multiple scattering effects was computed by means of Monte-Carlo simulations. Numerical procedure was applied to find the value of the multiple scattering correction term for a satisfying the corrected attenuation law for a given measurement and hence yielding the actual concentration. The results were compared with aerosol electrometer measurements and show that the described method is suitable for accurate concentration determination in the presence of multiple scattering.

Measurement and modeling of multiple scattering in droplet aerosols

Abstract Two measuring systems using lasers with wavelengths of 633 nm and 670 nm anddifferent measuring geometries were used to investigate the same droplet cloud yielding two independent particle concentration measurements. Measurements were performed in such a way that multiple scattering was significant only in one of the measuring systems allowing the determination of the magnitude of multiple scattering effects in a given droplet aerosol. Numerical computations of multiple scattering were performed using a solution of the radiative transfer equation in the small angle approximation. Experimental results are in good agreement with the theoretical data for optically thin aerosols. For larger optical depths the experimental results indicate less attenuation then predicted by the above mentioned theoretical description, however good agreement of this data was found with an expression for radiation transmission through a cloud of scatterers based on Monte Carlo simulations.

Experiments on the sticking probability for water molecules during condensation of water vapor in air

Condensational particle growth has been investigated by many authors using various measuring principles and observation techniques (see (i) for review). While diffusion chambers are restricted to the study of slow growth processes, expansion chambers have been applied to investigate fast particle growth at high saturation ratios. Comparison of experimental and theoretical growth rates allows a determination of the sticking probability for the condensing molecules. For the case of water molecules considerable disagreement can be observed in the literature different authors reporting values for the sticking probability varying over a range from about 0.01 to i. This conflict has not yet been resolved. In the present paper we report measurements of water drop growth in air in an expansion chamber covering the entire range of saturation ratios from about 106 to 360 %. The results are compared with numerical calculations accounting for nonlinear effects and the sticking probability for water molecules is estimated.