An acid is a molecule that can donate a proton or accept an electron in a chemical reaction.
Air pollutants are carried by wind and clouds. There are no borders in the atmosphere.
Green lichens are more prevalent in areas with short distances, while white/gray lichens tend to increase in areas with intermediate distances. On the other hand, yellow lichens are only found in a very small area.
This shows that the types of lichens that live in different places vary, reflecting differences in environmental conditions.