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 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 crystallisation by acting as nuclei centre (NC), the supercooling level decreases of 6°C in presence of ATD; (ii) ice crystallisation is affected by the citric acid mass fraction: for concentrations over 50 wt % crystallisation does not systematically appear during the cooling phase, but during the warming phase, which highlights an amorphous organisation; (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 mono-site with continuous framework for low concentrations to multiple sites with multi-branching 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.

The formation of liquid cloud droplets from aerosol particles in the Earth atmosphere is still under debate particularly because of the difficulties to quantify the importance of bulk and surface effects in these processes. Recently, single-particle techniques have been developed to access experimental key parameters at the scale of individual particles. Environmental scanning electron microscopy (ESEM) has the advantage to provide in situ monitoring of the water uptake of individual microscopic particles deposited on solid substrates. In this work, ESEM was used to compare droplet growth on pure ammonium sulfate (NH4)2SO4 and mixed sodium dodecyl sulfate/ammonium sulfate (SDS/(NH4)2SO4) particles and to explore the role of experimental parameters, such as the hydrophobic–hydrophilic character of the substrate, on this growth. With hydrophilic substrates, the growth on pure salt particles was strongly anisotropic, but this anisotropy was suppressed by the presence of SDS. With hydrophobic substrates, it is the wetting behavior of the liquid droplet that is impacted by the presence of SDS. The wetting behavior of the pure (NH4)2SO4 solution on a hydrophobic surface shows a step-by-step mechanism that can be attributed to successive pinning–depinning phenomena at the triple-phase line frontier. Unlike the pure (NH4)2SO4 solution, the mixed SDS/(NH4)2SO4 solution did not show such a mechanism. Therefore, the hydrophobic–hydrophilic character of the substrate plays an important role in the stability and dynamics of the liquid droplets’ nucleation by water vapor condensation. In particular, hydrophilic substrates are not suited for the investigation of the hygroscopic properties (deliquescence relative humidity (DRH) and hygroscopic growth factor (GF)) of particles. Using hydrophobic substrates, data show that the DRH of (NH4)2SO4 particles is measured within 3% accuracy on the RH and their GF could indicate a size-dependent effect in the micrometer range. The presence of SDS does not seem to modify the DRH and GF of (NH4)2SO4 particles. This study shows that the water uptake on deposited particles is a complex process but, once carefully taken into account, ESEM is a suitable technique to study them.