Removal of pollutants from the air, or atmospheric deposition, is a natural cleaning mechanism. However, the removed toxic matters cannot just disappear from the Earth.

Atmospheric deposition is an important source of heavy metals in the soil but it tends to be neglected and can't easily be perceived. The paucity of measurements also makes it difficult to track what happens to heavy metals when they fall from the air to the soil.

"The concentrations of fine particles and sulfate aerosols were reduced significantly during 2013 to 2017, after the Clean Air Act was implemented. "Will the Clean Air Act also affect the atmospheric deposition? If yes, what implication can we get?" said Dr. PAN Yuepeng from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences.

Dr. PAN and his group tracked the atmospheric depositions of heavy metals at an agricultural site of rural Beijing. They found that the deposition flux of heavy metals that exist entirely in fine particles declined significantly compared to those that exist in coarse particulate form, indicating that the Clean Air Act was effective, with beneficial effect in reducing ambient heavy metals from anthropogenic emissions.

Their study was published in Atmosphere on Feb. 22.

Field sampling for atmospheric deposition (Image by PAN Yuepeng)

PAN also noticed that while the bulk deposition flux of heavy metals declined substantially compared to the records ten years ago, the current deposition flux of lead was still higher than that reported in Europe.

In addition, PAN and his team found that the annual bulk deposition flux of heavy metals tended to decrease during 2017-2020, coinciding with the annual variations of particulate matter. They attributed this decline to the fluctuations of the emissions from their major sources rather than to precipitation controlling the deposition processes.

"This beneficial 'side effect' of Clear Air Act is totally unintended, but from this we can learn that to further reduce the airborne heavy metals in the North China Plain, future control measures should pay more attention to soil/dust, biomass burning, coal combustion and industrial emissions," PAN suggested, "because these sources contributed to 90% of chemical components in atmospheric depositions."