Light Scattering in Inhomogeneous Nano- and Microdroplets

Scattering and radiative transfer in turbid media such as aerosols is based on the interaction between the electromagnetic irradiation and aerosol particles, which is mainly determined by microphysical properties of the suspended matter and the wavelength of the irradiation. Until few years ago due to theoretical and computational limitations the assumption of homogeneity of particles was widely accepted, although it was known that most of atmospheric, biological or technical aerosols consist of inhomogeneous particles, such as e.g. droplets with inclusions. For that reason a substantial interest develops recently in the field of light scattering from such objects, since they can be viewed as representative of atmospheric aerosols, as well as aerosols found in various indoor and outdoor environments, where e.g. bioorganisms survive for extended periods of time within liquid coating. A real time detection and measurement of such aerosols is of major interest. In this research project we will tackle on one hand impact of non-homogeneity of aerosol particles on optical properties of single particles. On the other hand we will investigate the influence of inclusions in microdroplets on scattering and radiative transfer in turbid media containing an ensemble of such particles. The experimental evidence is still incomplete. We plan to investigate experimentally and theoretically the optical characteristics of single droplets and aerosols in the size range from about 0.1 - 5 m containing well defined inclusions starting from nanometer sizes and spanning over about 2 decades in size. The research will be based on single optical particle spectrometry, as well as on nephelometric, i.e. particle ensemble measurement using an expansion cloud chamber technique. The results of this project will provide a better understanding and insight into scattering characteristics of turbid media consisting of non-homogeneous aerosol particles. Moreover, we will establish a firm experimental evidence for verification of various theoretical models in this field of research. The results obtained will also contribute to a further development of instrumental efforts for identification and analysis of atmospheric, biotechnological, or industrial aerosols.

Aerosol particles in the atmosphere play an important role for the global climate, the radiative forcing effects and the Earth`s radiation budget. They affect the local, regional and planetary optical properties of atmosphere and hence influence the radiation transfer, light propagation and the visual clarity. The interaction of an incoming radiation with aerosol particles and in consequence the light scattering is determined the particle`s micro-physical quantity, its complex refractive index and its size. For that reason a substantial interest developed recently in the field of light scattering from such objects, since they can be viewed as representative of atmospheric aerosols, as well as aerosols found in various indoor and outdoor environments, where e.g. bio-organisms survive for extended periods of time within liquid coating. A real time detection and measurement of such aerosols is of major interest. In this research project we dealt on one hand with the problem of measurement of complex refractive index of particles and an impact of non-homogeneity of aerosol particles on optical properties of single particles. On the other hand we investigated the influence of inclusions in microdroplets on scattering and radiative transfer in an ensemble of such particles. The experimental evidence which was now provided helps to verify theoretical models and support development of an appropriate instrumentation. Our single particle spectrometry approach proposed here involves the use an innovative dual wavelength method for simultaneous determination of particle size and complex refractive index. For this a priori ill-posed problem we found from model calculations based on Mie theory a suitable optical arrangement allowing to retrieve the aerosol size and its complex refractive index evaluating the simultaneously four light fluxes scattered from a particle The single-valued determination of these quantities is now possible even in the sub-micrometer particle size range. This achievement is of importance for measurement of atmospheric aerosols but it is also a very interesting instrumental contribution to metrology of ambient particulate matter. We are positive that results of this project provide a better understanding and insight into light propagation through atmosphere and to determination of scattering characteristics of media containing non-homogeneous aerosol particles. Moreover, we established a firm experimental evidence allowing verification of various theoretical models in this field of research. The results obtained also contribute to a further development of instrumental efforts for better identification and analysis of atmospheric, biotechnological, or industrial aerosols.