From the earliest geological periods, the Earth's atmosphere has contained sulfur compounds over a range of oxidation states and the occasional sulfur polymer. These have ranged from reactive organosulfur gases to stable sulfates as particles. The current atmosphere is oxidizing, so there is a tendency for atmospheric sulfur compounds to oxidize toward sulfate as an ultimate product. Sources of atmospheric sulfur include large fluxes as sulfate from the oceans associated with sea-salt particles. Additionally, there are emissions of more reduced sulfur compounds from volcanoes and biological sources, which derive largely from the oceans. Human activities, most notably the combustion of fossil fuels, especially coal, make anthropogenic contributions of SO2 that are larger than that from natural sources such as volcanoes. Coal burnt in cities leads to high SO2 concentrations that have been destructive to both human health, the urban fabric and at longer distances, ecosystems, which are affected by acid rain. Some industries can release more reduced compounds such as H2S or organosulfides. The OH• radical is important in oxidizing sulfur compounds in the atmosphere, along with some oxidation in rain droplets or on the surface of particles. Sulfur in the stratosphere arises from explosive volcanism or more continuously through the transfer of OCS across the tropopause. Such transfers contribute to the sulfate that plays a role in depletion of ozone in the upper atmosphere and affects the radiative balance of the Earth.