Nearly 80% of the air we breathe is nitrogen gas (N₂), and is only directly usable as a biological nitrogen source by a certain group of bacteria. Atmospheric N₂ is converted to nitrate (NO₃^-), a biologically important form, by lightning. Volatile organic nitrogen compounds are released to the atmosphere during plant decay. Industrial emissions and fossil fuel combustion contribute gaseous nitrous oxides and nitrate as nitric acid (one component of "acid rain") from sources sometimes hundreds of miles distant. Atmospheric nitrogen is delivered to sea and land in rainfall as dissolved compounds ("wet" deposition) and as adsorbed compounds on dust particles and leaves ("dry" deposition). Dry deposition, or air pollution, contributes large amounts of nitrogen to watersheds. In some areas the amount of nitrogen in dry deposition is as large as that in wet deposition or precipitation. Coastal fogs are another source of nitrogen. Where the watershed covers a much larger area than does the estuary (as is the case locally), atmospheric deposition onto the land surface rather than the estuary itself is more important. Atmospheric deposition can comprise half of the total input of nitrogen to southern New England watersheds. However, only about a tenth of this nitrogen can be expected to arrive at the receiving estuary in groundwater, due in part to uptake and storage by vegetation.