Nitrogen and Sulfur

N and S have a varied and significant environmental chemistry, related in both cases with the broad range of oxidation states feasible. Biological nitrogen fixation converts atmospheric N₂ into organic compounds required through life. A high proportion is recycled under the biosphere, although a number of microorganisms convert it into nitrate (nitrification) and others decrease nitrate to N₂ (dinitrification), both processes being employed to get metabolic energy. So denitrification recycles N back under the atmosphere. The main human perturbations to the cycle arrive from the make use of nitrate fertilizers (that can lead to not desirable concentrations of in drinking water) and high temperature burning of fossil fuels, that create NO and NO₂. These gases are air pollutants, nearby since they are toxic and join in in photochemical processes which generate other noxious compounds and on a wider scale since they oxidize to nitric acid, that contributes to acid rain.

The atmospheric and biological redox chemistry of sulfur is also difficult. The major natural inputs to the atmosphere arrive from the biological decay (mainly H₂S) and emissions of dimethyl sulfide (CH₃)₂S through marine organisms, along with volcanic emissions (mainly SO₂). These natural sources are now goes beyond through the emission of SO₂ from burning sulfurcontaining fossil fuels. Several atmospheric sulfur compounds oxidize quickly to sulfuric acid, that is the main component of acid rain. The contrast of N and S is interesting, because the entire atmospheric inputs of the two elements are identical in magnitude (1-2×10¹¹ kg per year). The oxidation and removal of sulfur compounds is very much fast than for the extremely stable N₂ molecule and that's why the atmospheric concentrations are extremely diverse (approximately 1 p.p.b. for sulfur compounds, 78% for N₂).