OLALA First Results: Depolarization Ratios of Fine Mode Aerosol Particles

Mineral dust is a major component of atmospheric aerosol loading and typically shows high depolarization ratios because of its irregular particle shape. The depolarization ratio measurements make mineral dust easier to distinguish from other aerosols in lidar observations. However, the complex and diverse morphology of dust particles is difficult to represent accurately in scattering models used for lidar retrievals, which introduces uncertainties in the derived microphysical properties. To address these limitations a new scattering laboratory has been established under the course of the project OLALA (Optical Lab for Lidar Applications). The goal is to perform controlled measurements that will help to better constrain scattering models and improve aerosol property retrievals from lidar observations. Using this experimental setup, an extensive dataset of backscattering properties of size resolved mineral dust particles will be obtained at three standard lidar wavelengths: 355 nm, 532 nm, and 1064 nm.

At present, the laboratory setup is fully operational at a single wavelength of 532 nm. Our optical setup uses a continuous wave laser at 532 nm as the light source and a 50:50 beam splitter to acquire the exact backscattering geometry (180±0.2°). In the receiver section, a polarizing beam splitter cube separates the parallel and perpendicular component of backscattered light and directs them towards their respective detection channels. Our aerosol chamber is a vertically oriented 1 m long tube with an inner diameter of 1.5 cm. The total particle concentration entering the aerosol chamber is monitored with a condensation particle counter (CPC) and at the exit of the aerosol tube an optical particle sizer (OPS) measures the concentration and size distribution of aerosol particles. A differential mobility analyzer is used to select particles of a defined mobility diameter, producing monodisperse aerosols.

We have performed initial measurements with different types of aerosols to demonstrate the performance and potential of our setup. For spherical particles we used polystyrene latex particles of 1000 nm diameter and ammonium sulfate particles of 250 nm diameter and observed very low depolarization ratios of less than 2%. For non-spherical particles we used sodium chloride, Arizona Test Dust and German soil dust at four different sizes of 250 nm, 450 nm, 650 nm and 800 nm in diameter. We observed an increasing trend in the depolarization ratio with an increase in particle size for non-spherical fine mode aerosol samples.

After the successful implementation of the 532 nm setup, we are now focused on extending the optical setup by incorporating 1064 nm and 355 nm wavelengths. Once the triple wavelength setup is operational, we plan to perform measurements with natural mineral dust samples collected from different deserts around the globe.