Freezing rain is a common meteorological calamity in many parts of the world, including tropical locations, throughout the winter and early spring. It forms in the air as supercooled water (below 0 °C) that freezes as soon as it contacts cold surfaces. Freezing rain is most common in hilly parts of southern China. Researchers from the Chinese Academy of Sciences’ Institute of Atmospheric Physics have discovered that air pollution can cause freezing rain in the northern hemisphere.Their research was published in the journal Atmospheric Environment.
Raindrops remain supercooled in the environment below 0 °C due to the nature of freezing rain. Previous research has linked the absence of ice nuclei to the existence of supercooled droplets in the atmosphere. This isn’t the case, because ice nuclei stay in supercooled drops after snow melts, forming these drops in even higher and warmer temperatures.
Increased soluble molecules assist the drops stay supercooled, according to laboratory research, and this chemical effect, known as freezing point depression, is also replicated by numerical models. Field measurements, on the other hand, show that the effect of freezing point depression in precipitation does not exist.
Drs. Li Xingyu and Pan Yuepeng of IAP have been measuring the physical and chemical composition of freezing rain at Mount Heng, which is located in south-central China at 27°18′6′′N, 112°41′5′′E, since 2015. The sampling site is 1265.9 metres above the top of the atmospheric boundary layer, with temperatures near 0 degrees Celsius in the winter, encouraging the formation of supercooled fog and freezing rain.
This sampling location has also documented China’s record icing thickness. The first seven years of observations revealed that the freezing rain at Mount Heng was acidic (pH 5) and had larger amounts of soluble ions and organics than rain.
The team also collaborated with climatologists from Canadian and Chinese universities and research institutes to investigate the processes of freezing rain generation and future trends. They discovered that as the concentrations of soluble ions (particularly ammonium nitrate) rise, the ambient temperatures of freezing rainfall.
Prof. Ronald Stewart of the University of Manitoba in Canada, a coauthor of the paper, states, “This work gave the first chemical view on freezing rain, a physical phenomenon in conventional belief.” “Higher ionic concentrations may lower the temperature required to freeze droplets, allowing freezing rain to last longer.”
“With global warming, the influencing region of the freezing rain would shift northward, and the effect of freezing point depression generated by air pollution in northern China may be increased,” said another research coauthor, Dr. Zhu Xiaying of the National Climate Center.