The PH value of acid rain is less than _ _ _ _, which is mainly caused by _ _ _ _.

The analysis shows that acid rain is caused by a large number of fossil fuels such as coal, oil and natural gas with high sulfur content. After combustion, sulfur oxides or nitrogen oxides undergo complex chemical reactions in the atmosphere to form sulfuric acid or nitric acid aerosols, or are captured and absorbed by clouds, rain, snow and fog, and fall to the ground to become acid rain. Rain with pH less than 5.6 is considered as acid rain. The pH value of normal rain is neutral or slightly less than 7, slightly acidic. Generally, normal rain fluctuates around 7 and can be greater than 7. Some people think that it is caused by alkaline gas, but it should actually contain alkaline particles, such as the influence of sand blowing weather. Because the air contains carbon dioxide, the normal rainfall range should be 5.6-7, but it can be greater than 7 in areas with serious sand blowing.

So the answer is: 5.6; A large number of acidic substances were artificially discharged into the atmosphere.