Acid rain is caused by the release of the gases SO2 (sulphur
dioxide) and NOX (nitrous oxides). The main sources of NOX
emissions are vehicles and fuel combustion.
Sulphur dioxide reacts with water vapour and sunlight to form
sulphuric acid. Likewise NOX form nitric acid in the air. These
reactions takes hours, or even days, during which polluted air may
move hundreds of kilometers. 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. It is possible that even more acidity is deposited in this
way than by rain. Not much is known about this process, and it is
particularly difficult to study.