Chemical characteristics of freezing rain observed at Mount Heng in southern China

Freezing rain usually occurs when snow falls into an above-freezing layer aloft where it melts before reaching the sub-freezing surface as supercooled water. Although the occurrence of freezing rain is uncommon, it can produce extremely costly impacts. To date, field chemical observations of supercooled drops that form freezing rain are lacking. In this study, we collected 101 precipitation (freezing rain/rain/snow) samples on Mt. Heng in southern China for 7 years in the winters between 2015 and 2021. The winter precipitation collected at Mt. Heng was mostly acidic with pH values lower than 5. Acidity, contributed from NO3− and SO42−, was neutralized by NH4+ and Ca2+, indicating the additional contribution from organic acids. The major ions were NH4+, Ca2+, SO42− and NO3−, with concentrations of 135.7, 86.3, 72.5 and 61.8 μeq L−1, respectively. In general, the total ionic concentrations of freezing rain were, in comparison with mountain-based observations elsewhere, lower than those in cloud water but higher than those in rain. The higher concentrations of major ions in freezing rain would depress the freezing point of supercooled water drops and act to reduce the likelihood of freezing with effects increasing with concentration. This process would be countered by the presence of ice nuclei that is acting to freeze supercooled drops. These opposing processes complicate the understanding of the onset of freezing and consequently the occurrence of freezing rain. This study highlighted that additional model simulations and laboratory experiments are needed to validate the relationship between air pollution and freezing rain.