Multiple scattering enhancement of transmitted radiation in water droplet aerosols

Abstract

A new expansion cloud chamber was constucted to investigate the contribution of multiple scattering to the transmittance of visible laser light through a water droplet aerosol. The droplets generated in the expansion cloud chamber had precisely known diameters in the range of about 1–3 μm and a very narrow size distribution. The investigated aerosol had optical depths up to about seven. The multiple scattering effects were determined from the light transmittance measurements by simultaneously using two optical systems for the same droplet cloud, of which one (the reference system) was negligibly affected by multiple scattering. The experimental data were compared with numerical simulations. Modelling was performed using a solution of the radiative transfer equation in the small angle approximation, as well as Monte Carlo simulations of beam propagation in turbid media. It was found that Monte Carlo results were in good agreement with the experimental data, whereas the small angle approximation considerably underestimated the multiple scattering contributions to the transmitted light for our experimental conditions. Furthermore, a technique for accurate particle concentration determination from transmittance measurement affected by multiple scattering is presented. It shows very good agreement with conventional aerosol concentration measurement.