Thus acid rain can fall far from the source of pollution. When mist or fog droplets condense they will remove pollutants from the air and can become more strongly acid than acid rain. Even snow can be acid. Gases and particles, not dissolved in water, with a low pH can also be deposited directly onto soil, grass and leaves. Not much is known about this process, and it is particularly difficult to study.
Acid rain can increase the acidity of soil, water and shallow groundwater.
Acid rain has been linked with the death of trees in Europe and North America. In spite of a great deal of research, no one yet knows exactly how acid rain harms forests. Most of the forests of Europe consist of huge areas of one tree species. This encourages the spread of plant pests and diseases. It seems likely that acid rain weakens the trees, perhaps helped by other pollutants such as ozone, and then leaves the trees open to attack by disease.