An electrodynamic balance (EDB) for extraterrestrial cloud formation studies

Abstract

Ice clouds scatter and absorb solar radiation, affecting atmospheric and surface temperatures (Gettelman et al., 2012). On Mars, where ice contained in clouds makes up a large portion of total atmospheric water vapor, ice clouds also alter the planetary water budget (Maltagliati et al., 2011; Rafkin et al., 2013). Thus, it is important for climate models to be able to accurately predict the conditions under which ice clouds can form. Typical Martian temperatures at cloud-formation height range from -150-200 K (Trainer, Toon, & Tolbert, 2009). Heterogeneous deposition nucleation is thought to be the dominant freezing mechanism on Mars due to the abundance of mineral dust to serve as ice nuclei (IN) (Mdittanen et al., 2005). The parameters for such nucleation are not well characterized at such low temperatures (Trainer et al., 2009). Previous experimental studies have investigated the relative humidity required for deposition nucleation within the Martian temperature range. However, most studies took place on bulk aerosol samples, did not use mineral dusts analogous to Martian dust, or were constrained by particle lifetime and temperature limits. In this project, we re-purpose a single-particle instrument and set it up to perform experiments for more precise ice nucleation data under Martian atmospheric conditions. We use an electrodynamic balance (EDB) to levitate individual particles with diameters around 10 pm. We calculate the size of the particle and changes in size based on the holding voltages. The system can be cooled to 200 K in its current configuration, and the relative humidity and atmospheric constituents can be controlled by adding gas. To test the EDB, we perform validation experiments. We investigate deliquescence and efflorescence on salts at room temperature and 0 'C. We modify the cooling system, thermocouples, and relative humidity sensors and begin freezing experiments with Arizona Test Dust (ATD) and with Mojave Mars Simulant (MMS) dust. We investigate water uptake on MMS particles and find it to be non-hygroscopic but wettable, uptaking monolayers of water between 65-95% relative humidity. From 200 K to 220 K, MMS does not nucleate up to 115% RHice, suggesting that higher supersaturations are needed for ice clouds to form; some Martian cloud modelers should revisit the critical supersaturation parameterization. Future work will improve the EDB and use it to examine phase functions and light scattering.