The Ground-based Fog and Aerosol Spectrometer – Technical description and first applications

Abstract. High-precision measurements of cloud microphysical properties, such as particle size and phase, remain challenging. The most common method for in-situ single-particle measurements is via light scattering, where size is inferred from the scattered-light intensity using Mie-Lorenz theory. Here, we present the newly developed instrument, the Ground-based Fog and Aerosol Spectrometer (GFAS). Besides particle size and number concentration obtained from forward-scattering measurements, the GFAS also measures the change in polarization of backward scattered light, which allows for investigating particle morphology and composition. Within this work, we present a new methodology to calibrate detectors of light scattering spectrometers using a poly-disperse spray of pure water coupled with theoretical Mie-Lorenz modeling. Furthermore, we show the importance of proper calibration procedures to derive reasonable and interpretable data with respect to particle size and degree of linear polarization. In particular, we show that the new autocalibration method that uses a spray of pure water droplets leads to lower uncertainties compared to the standard glass bead calibration and is also usable for the polarized backward scattering detectors. Finally, we evaluate the GFAS using laboratory and field measurements. Under laboratory conditions, polarization measurements distinguish water droplets from nonspherical dust. Under ambient conditions, however, this distinction becomes considerably more challenging. For the in-situ observations, we find that cloud particle optical properties are distinctly different compared to those of pure water measured under laboratory conditions, indicating a potentially elevated refractive index in cloud droplets compared to pure water.