The hydrosphere consists of all water within the Earth, on the Earth's surface, and water vapor in the Earth's atmosphere. This covers a lot of widely varied territory. The most common gases dissolved in natural water are nitrogen, oxygen, and carbon dioxide. Nitrogen (N2) is found in water simply because it is the most abundant gas in the atmosphere (80%). Oxygen (O2) is the second most abundant gas in the atmosphere (19%). All other gases are trace gases in comparison. But carbon dioxide (CO2) is a bit of a special case since it dissolves in water to form carbonic acid. This is why most natural water is slightly acidic since carbonic acid is a weak acid. Other gasses can be present in water in specific areas as well. For example, sulfur dioxide (SO2) can be present in ocean water near underwater volcanic activity or in water vapor near industrial smokestacks.