Wet deposition and scavenging ratio of air pollutants during an extreme rainstorm in the North China Plain

Atmospheric wet deposition plays an important role in the supply of nutrients and toxic substances to terrestrial and aquatic environments. Although long-term（e.g. annual, multi-year） wet deposition is recorded well, pronounced and short-term changes in precipitation chemistry are less well investigated. In the present study, the precipitation chemistry and scavenging ratio of air pollutants were observed during an extreme torrential rain event（325.6 mm at the observation site） that occurred over 19–21 July 2016 in the North China Plain（NCP）. The scavenging ratio of particles showed a similar spatial distribution to that of the precipitation amount in the NCP, indicating the efficient removal of particulate matter due to the large amount and precipitation intensity of the storm. In addition, the scavenging ratio of water soluble ions was larger than that of organics and gaseous pollutants such as SO_2 and NO_2, likely due to their differences in water solubility.Consequently, raindrops incorporated more aerosol sulfate than gaseous compounds. Due to the heavy precipitation amount, almost all species in rainwater during this storm showed their lowest concentration but the highest flux compared with other rain events, indicating an important role played by this storm in terms of the substances received by the terrestrial and marine ecosystems of the region. However, the contribution of this storm to the annual chemical flux was lower than that of precipitation amount, indicating that the atmospheric compounds were scavenged below-cloud first and were then diluted by the cloud/rainwater. Future studies are needed in the context of the occurrence of extreme rainfall events in the NCP from the perspective of climate variability.

大气湿沉降是陆地和水体环境中营养成分和有毒物质的重要来源。以往湿沉降研究较多关注年尺度或年际变化,极端降水事件鲜见报道。通过实验观测,发现2016年7月19–21日华北极端暴雨(北京观测站325.6 mm)有效清除了区域大气颗粒物,但不同化学成分被湿清除的效率存在差异(无机离子比有机物和气态污染物更容易被湿清除)。此次极端降水铵盐和硝酸盐的湿沉降通量占北京全年的22%和4%,相当于青藏高原一年的氮沉降。考虑到气候背景下极端暴雨出现的频次,极端降水的生态环境影响需要更精细化的评估。