<P> It is estimated that transportation fuels cause 54% of the anthropogenic (i.e. human - caused) NO . The major source of NO production from nitrogen - bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NO during combustion . During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N, or NO . Fuel NO can contribute as much as 50% of total emissions when combusting oil and as much as 80% when combusting coal . </P> <P> Although the complete mechanism is not fully understood, there are two primary paths of formation . The first involves the oxidation of volatile nitrogen species during the initial stages of combustion . During the release and before the oxidation of the volatiles, nitrogen reacts to form several intermediaries which are then oxidized into NO . If the volatiles evolve into a reducing atmosphere, the nitrogen evolved can readily be made to form nitrogen gas, rather than NO . The second path involves the combustion of nitrogen contained in the char matrix during the combustion of the char portion of the fuels . This reaction occurs much more slowly than the volatile phase . Only around 20% of the char nitrogen is ultimately emitted as NO, since much of the NO that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon . </P> <P> This third source is attributed to the reaction of atmospheric nitrogen, N, with radicals such as C, CH, and CH fragments derived from fuel, where this cannot be explained by either the aforementioned thermal or fuel processes . Occurring in the earliest stage of combustion, this results in the formation of fixed species of nitrogen such as NH (nitrogen monohydride), HCN (hydrogen cyanide), H CN (dihydrogen cyanide) and CN (cyano radical) which can oxidize to NO . In fuels that contain nitrogen, the incidence of prompt NO is especially minimal and it is generally only of interest for the most exacting emission targets . </P> <P> NO reacts with ammonia, moisture, and other compounds to form nitric acid vapor and related particles . Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases . Inhalation of such particles may cause or worsen respiratory diseases, such as emphysema or bronchitis, or may also aggravate existing heart disease . </P>

What happens when nitrogen dioxide reacts with water