Using Depolarization to Quantify Ice Nucleating Particle Concentrations: A New Method

Abstract

We have developed a new method to determine ice nucleating particle (INP) concentrations observed by a Continuous Flow Diffusion Chamber (CFDC) under a wide range of operating conditions. In this study, we manipulate differences in particle optical properties detected by the Cloud and Aerosol Spectrometer with POLarization (CASPOL) to differentiate ice crystals, droplets, and aerosols. The depolarization signal from the CASPOL instrument is used to determine the occurrence of water droplet breakthrough (WDBT) conditions in the CFDC, under which the traditional analysis method fails. To overcome the challenge of WDBT, we design a new analysis method using depolarization ratio that can extend the range of operating conditions of the CFDC. The method agrees reasonably well with the traditional method under non-WDBT conditions with a mean percent error of ± 32.1 %. Additionally, a comparison with the Colorado State University (CSU) CFDC is used to show that the new analysis method can be used reliably during WDBT conditions. Due to the high detection limit of the CASPOL, the new method is only suitable for laboratory use when high INP concentrations (> 50,000 L^-1.) can be generated and not for ambient measurements where the INP concentration is ≤ 100 L^-1.