Inorganic composition and occult deposition of frost collected under severe polluted area in winter in the North China Plain

Abstract

Frost as a kind of deposition plays an important role in the removal of atmospheric compounds. However, studies concerning frost in the atmospheric environment were rare although chemical composition in frost samples might be affected by the surrounding atmospheric environment. In this study, a total of 35 frost samples were collected by means of a homemade glass-plate frost condenser under severe polluted condition in the North China Plain (NCP) from Dec. 4, 2018 to Mar. 2, 2019. The pH values and water-soluble ions (WSI) were conducted. The extremely high concentrations of WSI were found, which reflected the severe pollution significantly affecting the level of chemical composition in frost. The major ions were Ca2+, SO42− and HCO3− with averaged concentrations of 1242, 1143, 1076 μeq L−1, respectively. These ions were at least one order of magnitude higher than the previous frost studies. HCO3− was one of the most abundant components in frost. Its high proportion contributed to the ionic balance and led to the alkaline characteristic of frost. SO42− had the almost doubled ratio in frost compared with the concurrent PM2.5 samples. Different from the huge diversity of chemical components in PM2.5, the frost had similar ratio of WSI under great variety of PM2.5 concentrations. It proved that PM2.5 had less effect on the ratio of WSI in frost. Nutrient ions of NH4+, NO3− and K+ accounted for 13.9%, 5.4% and 1.6% of the total averaged concentrations, respectively. On average, per square meter soil would receive 563 μg nitrogen and 123 μg potassium nutrient during a frost night. High occult deposition flux of ions indicated the strong scavenging effect from the frost event. In addition, the occult deposition flux of SO42− was comparable to the dry deposition flux, further emphasizing frost process as a non-negligible atmospheric removal pathway of SO42−.