Studying the atmosphere includes studying the interactions between the atmosphere and living organisms. The composition of the Earth's atmosphere changes as result of natural processes such as volcano emissions, lightning and bombardment by solar particles from the Sun's corona. It has also been changed by human activity. Some of these changes are harmful to human health, crops and ecosystems. Examples of problems include acid rain, ozone depletion, photochemical smog, greenhouse gases and global warming. Atmospheric chemists study the causes of these problems. Atmospheric chemists offer theories about these problems, then test the theories and possible solutions. Atmospheric chemists also note the effects of changes in government policy.