5. Although the measures of lockdown in some countries during the epidemic period have caused reduction in some air pollutants, the concentration of carbon dioxide (CO₂) in the atmosphere have still been rising. Now, the concentration of CO₂ is 420 ppm at I atm in the atmosphere. When the rainwater and the air are completely balanced, the rainwater will infiltrate into the layer of groundwater in the catchment area, where it contains calcium carbonate (CaCO₃).