Retention During Freezing of Raindrops

Understanding the interaction between freezing processes and the vertical movement of trace gases into the upper atmosphere during intense convection is essential for analyzing the distribution of aerosol precursors and their impact on the climate. We conducted experimental studies in a cold room with freely suspended raindrops (2 mm diameter) using an acoustic levitation setup. For the first time, we examined how freezing affects the retention of organic species, using silver iodide as the ice nucleating agent. Through quantitative chemical analysis, we calculated the retention coefficient, which represents the proportion of a chemical species that remains in the ice phase relative to its concentration prior to freezing. We measured the retention coefficients of nitric acid, formic acid, acetic acid, and 2-nitrophenol as individual compounds, as well as in binary and complex mixtures. Our findings indicate that physical properties have a greater influence on overall retention than chemical properties in the case of the larger raindrops we studied. Therefore, nearly all substances are completely retained during the freezing process in rain-sized drops, even those with low Henry’s law constants. An ice shell forms within 4.8 milliseconds after freezing begins, and this ice shell formation is the key factor preventing the expulsion of dissolved substances from the drop.