Retention during freezing of raindrops – Part 1 : Investigation of single and binary mixtures of nitric, formic, and acetic acids and 2-nitrophenol

Abstract

The influence of freezing processes and vertical transport of trace gases into the upper atmosphere during deep convection is critical to understanding the distribution of aerosol precursors and their climate effects. We conducted experimental studies inside a walk-in cold room for freely levitating raindrops (drop diameter: 2 mm) using an acoustic levitator apparatus. We investigated the effect of freezing raindrops on the retention of organic species for the first time with silver iodide as the ice-nucleating agent. Quantitative chemical analysis determined the retention coefficient, which is defined as the fraction of a chemical species remaining in the ice phase compared to their initial liquid-phase concentrations. We measured the retention coefficients of nitric acid, formic acid, acetic acid, and 2-nitrophenol as single components. Furthermore, we determined the retention coefficients of these substances as binary mixtures. Our results show the dominance of physical aspects such as drop size and ice shell formation over their chemical counterparts with regard to overall retention for the investigated large drops. Thus, for rain-sized drops, almost everything is fully retained during the freezing process, i.e., retention coefficients close to 1, even for species with low effective Henry's law constants (H ∗ < 10−4). An ice shell is formed within 4.8 ms around the drops just after the freezing is initiated. This ice shell formation was found to be the controlling factor for the overall retention of the investigated species, which inhibited any further expulsion of dissolved substances from the drop.