Phase transformations of liquid drops containing mineral dust and organic compound (citric acid)

We investigated the impact of organic matter and mineral particles on atmospheric ice freezing to better understand the mechanisms of nucleation and crystal growth. Differential scanning calorimetry (DSC) and optical cryomicroscopy have been used to investigate liquid-solid transitions of citric acid aqueous solutions in the presence of 1 wt % of Arizona Test Dust (ATD). These two methods have led to complementary results: DSC enables the quantification of the latent heat and phase transitions, while optical cryomicroscopy allows the in situ observation of freezing phenomena and ice growth rates. Different trends have been observed: (i) ATD facilitates the crystallization by acting as nuclei center (NC), the supercooling level decreases around 6 degrees C in the presence of ATD; (ii) ice crystallization is affected by the citric acid mass fraction: for concentrations over 50 wt %, crystallization does not systematically appear during the cooling phase but during the warming phase, which highlights an amorphous organization; (iii) there is a threshold of citric acid concentration (around 30 wt %) beyond which the ice growth rate significantly decreases. In this work, we have shown that the crystal morphology is linked to the citric acid concentration and evolves from one nucleation monosite with continuous framework for low concentrations to multiple sites with multibranching pattern at high concentrations. A mathematical model is proposed as a first step in the modeling of ice crystal growth in a binary system (water/citric acid) containing mineral particles (ATD), based on the correlation between the ice growth rate and the inverse of the liquid viscosity. These results will be useful for chemical engineering applications and biotechnology, for example, freeze-drying of formulations as well as for better understanding the formation of ice clouds and atmospheric aerosol.