pollution

[COLOR=plum][B][I]Most pollutants are emitted both by natural as well as by anthropogenic sources. Natural sources are not influenced by humans or by human-induced activities. Volcanoes are a good example of this type of source. Many emissions are biogenic, i.e., produced by living organisms, but these emissions are very often influenced by human’s activities. [/I][/B][/COLOR][URL="http://www.eoearth.org/article/Nitrous_oxide"][COLOR=plum][B][I]Nitrous oxide[/I][/B][/COLOR][/URL][COLOR=plum][B][I] (N2O) is a greenhouse gas that is for a large part emitted during nitrification and de-nitrification processes (the conversion of ammonium to nitrate and of nitrate to N2O and ammonium respectively), which take place in the soil. The largest N2O emissions are observed where [/I][/B][/COLOR][URL="http://www.eoearth.org/article/Nitrogen"][COLOR=plum][B][I]nitrogen[/I][/B][/COLOR][/URL][COLOR=plum][B][I]-containing fertilizer is applied in [/I][/B][/COLOR][URL="http://www.eoearth.org/article/Agriculture"][COLOR=plum][B][I]agriculture[/I][/B][/COLOR][/URL][COLOR=plum][B][I]. [/I][/B][/COLOR]
[COLOR=plum][B][I]The ratio between anthropogenic and natural emissions is very important, as only the anthropogenic portion can be influenced, e.g., by abatement measures. A good example is provided by abatement measures for [/I][/B][/COLOR][URL="http://www.eoearth.org/article/Smog"][COLOR=plum][B][I]photochemical smog[/I][/B][/COLOR][/URL][COLOR=plum][B][I], with [/I][/B][/COLOR][URL="http://www.eoearth.org/article/Ozone"][COLOR=plum][B][I]ozone[/I][/B][/COLOR][/URL][COLOR=plum][B][I] and peroxyacyl nitrates (PAN) as important secondary products. Photochemical smog is caused by nitrogen oxide (NOx) and volatile organic compound (VOC) emissions. Therefore, the U.S. Environmental Protection Agency (EPA) decided, some 20 years ago, to combat photochemical smog in the U.S. by stringent reduction of VOC emissions and to a lesser extent, reduction of NOx; however, this policy did not produce the desired results. Natural emissions, especially of terpenes and isoprenes, proved to be so large that sufficient VOC concentrations were available in the [/I][/B][/COLOR][URL="http://www.eoearth.org/article/Atmospheric_composition"][COLOR=plum][B][I]atmosphere[/I][/B][/COLOR][/URL][COLOR=plum][B][I] for oxidant production, despite efforts to limit anthropogenic sources[/I][/B][/COLOR]