<Tr> <Td_colspan="2"> Infobox references </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> </Tr> <P> Carbon dioxide (chemical formula CO) is a colorless gas with a density about 50% higher than that of dry air . Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms . It occurs naturally in Earth's atmosphere as a trace gas at a concentration of about 0.04 percent (400 ppm) by volume . Natural sources include volcanoes, hot springs and geysers, and it is freed from carbonate rocks by dissolution in water and acids . Because carbon dioxide is soluble in water, it occurs naturally in groundwater, rivers and lakes, ice caps, glaciers and seawater . It is present in deposits of petroleum and natural gas . Carbon dioxide is odorless at normally encountered concentrations, however at high concentrations it has a sharp and acidic odor . </P> <P> As the source of available carbon in the carbon cycle, atmospheric carbon dioxide is the primary carbon source for life on Earth and its concentration in Earth's pre-industrial atmosphere since late in the Precambrian has been regulated by photosynthetic organisms and geological phenomena . Plants, algae and cyanobacteria use light energy to photosynthesize carbohydrate from carbon dioxide and water, with oxygen produced as a waste product . </P>

What is the standard notation for carbon dioxide
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