User:Liweichao.vivian/sandbox Chemistry2018

= NOx =

' is defined as the sum of plus the ' compounds produced from the oxidation of  which includes nitric acid, nitrous acid (HONO), dinitrogen pentoxide (N2O5), peroxyacetyl nitrate (PAN), alkyl nitrates (RONO2), peroxyalkyl nitrates (ROONO2), the nitrate radical (NO3), and peroxynitric acid (HNO4).

Formation and reaction
Because of energy limitations, oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they undergo an endothermic reaction producing various oxides of nitrogen. Such temperatures arise inside an internal combustion engine or a power station boiler, during the combustion of a mixture of air and fuel, and naturally in a lightning flash.

In atmospheric chemistry, the term denotes the total concentration of NO and  since the conversion between these two species is rapid in the stratosphere and troposphere. During daylight hours, these concentrations together with that of ozone are in steady state, also known as photostationary state (PSS); the ratio of to NO is determined by the intensity of sunshine (which converts  to NO) and the concentration of ozone (which reacts with NO to again form ). In other words, the concentration of ozone in the atmosphere is determined by the ratio of these two species.

(1)  $$\lambda < 424 nm$$

(2)

(3)

$$\frac{[NO_2]}{[NO]}=\frac{k_3 [O_3]}{j_{NO_2}}$$

This relationship between NOx and ozone is also known as the Leighton relationship.

The time τ that is needed to reach a steady state among NOx and ozone is dominated by reaction (3), which reverses reactions (1)+(2):

$$\tau = \frac{1}{k_3 [NO]}$$

implying that when the mixing ratio of NO, [NO] = 1 ppb, τ ≈ 40 minutes.

Formation of smog
When and volatile organic compounds (VOCs) react in the presence of sunlight, they form photochemical smog, a significant form of air pollution. The presence of photochemical smog increases during the summer when the incident solar radiation is higher. The emitted hydrocarbons from industrial activities and transportation react with NOx quickly and increase the concentration of ozone and peroxide compounds, especially peroxyacetyl nitrate (PAN).

Children, people with lung diseases such as asthma, and people who work or exercise outside are particularly susceptible to adverse effects of smog such as damage to lung tissue and reduction in lung function.

Natural sources
Nitrogen oxides are produced during thunderstorms due to the extreme heating and cooling within a lightning stroke. This causes stable molecules such as N2 and O2 to convert into significant amounts of NO similar to the process that occurs during high temperature fuel combustion. NOx from lightning can become oxidized to produce nitric acid (HNO3), this can be precipitated out as acid rain or deposited onto particles in the air. Elevated production of NOx from lightning depends on the season and geographic location. The occurrence of lightning is more common over land near the equator in the inter-tropical convergence zone (ITCZ) during summer months. This area migrates slightly as seasons change. NOx production from lightning can be observed through satellite observations.

Scientists Ott et al. estimated that each flash of lightning on average in the several mid-latitude and subtropical thunderstorms studied turned 7 kg of nitrogen into chemically reactive. With 1.4 billion lightning flashes per year, multiplied by 7 kilograms per lightning strike, they estimated the total amount of produced by lightning per year is 8.6 million tonnes. However, emissions resulting from fossil fuel combustion are estimated at 28.5 million tonnes.

A recent discovery indicated that cosmic ray and solar flares can significantly influence the number of lightning strikes occurring on Earth. Therefore, space weather can be a major driving force of lightning-produced atmospheric. It should also be noted that atmospheric constituents such as nitrogen oxides can be stratified vertically in the atmosphere. Ott noted that the lightning-produced is typically found at altitudes greater than 5 km, while combustion and biogenic (soil)  are typically found near the sources at near surface elevation (where it can cause the most significant health effects).

Biogenic sources
Agricultural fertilization and the use of nitrogen fixing plants also contributes to atmospheric, by promoting nitrification and denitrification by microorganisms. The nitrification process transforms ammonia into nitrate. And the denitrification is basically the reverse process of nitrification. During the denitrification, nitrate is reduced to nitrite then NO then N2O and finally nitrogen. Through these processes, NOx is emitted to the atmosphere.

A recent study conducted by the University of California Davis, found that adding nitrogen fertilizer to soil in California is contributing 25 percent or more to state-wide NOx pollution levels. When nitrogen fertilizer is added to the soil, excess ammonium and nitrate not used by plants can be converted to NO by microorganism in the soil, which escapes into the air. NOx is a precursor for smog formation which is already a known issue for the state of California. In addition to contributing to smog, when nitrogen fertilizer is added to the soil and the excess is released in the form of NO, or leached as nitrate this can be a costly process for the farming industry.

Industrial sources (anthropogenic sources)
The three primary sources of in combustion processes  :


 * thermal
 * fuel
 * prompt

Thermal formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. Fuel tends to dominate during the combustion of fuels, such as coal, which have a significant nitrogen content, particularly when burned in combustors designed to minimize thermal. The contribution of prompt is normally considered negligible. A fourth source, called feed  is associated with the combustion of nitrogen present in the feed material of cement rotary kilns, at between 300 °C and 800 °C, where it is considered a minor contributor.

Thermal
Thermal refers to  formed through high temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primarily a function of temperature and the residence time of nitrogen at that temperature. At high temperatures, usually above 1600 °C (2900 °F), molecular nitrogen (N2) and oxygen (O2) in the combustion air disassociate into their atomic states and participate in a series of reactions.

The three principal reactions (the extended Zel'dovich mechanism) producing thermal are:


 * N2 + O NO + N
 * N + O2 NO + O
 * N + OHNO + H

All three reactions are reversible. Zeldovich was the first to suggest the importance of the first two reactions. The last reaction of atomic nitrogen with the hydroxyl radical, OH, was added by Lavoie, Heywood and Keck to the mechanism and makes a significant contribution to the formation of thermal.

Fuel
It is estimated that transportation fuels cause 54% of the anthropogenic (i.e. human-caused). The major source of production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to  during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2, or NO. Fuel can contribute as much as 50% of total emissions through the combustion oil and as much as 80% through the combustion of coal.

Although the complete mechanism is not fully understood, there are two primary pathways for 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. The second pathway 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, since much of the that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.

Prompt
This third source is attributed to the reaction of atmospheric nitrogen, N2, with radicals such as C, CH, and CH2 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), H2CN (dihydrogen cyanide) and •CN (cyano radical) which can oxidize to NO. In fuels that contain nitrogen, the incidence of prompt is especially minimal and it is generally only of interest for the most precise emission targets.

Health and environment effects
The direct effect of he emission of has positive contribution to the greenhouse effect. Instead of reacting with ozone in Reaction 3, NO can also react with HO2· and organic peroxyradicals (RO2·) and thus increase the concentration of ozone. Once the concentration of exceeds a certain level, atmospheric reactions result in net ozone formation. Since tropospheric ozone can absorb infrared radiation, this indirect effect of is intensifying global warming.

There are also other indirect effects of that can either increase or decrease the greenhouse effect. First of all, through the reaction of NO with HO2 radicals, •OH radicals are recycled, which oxidize methane molecules, meaning emissions can counter the effect of greenhouse gases. For instance, ship traffic emits a great amount of NOx which provides a source of NOx over the ocean. Then, photolysis of NO2 leads to the formation of ozone and the further formation of hydroxyl radicals (·OH) through ozone photolysis. Since the major sink of methane in the atmosphere is by reaction with •OH radicals, the NOx emissions from ship travel may lead to a net global cooling. However, in the atmosphere may undergo dry or wet deposition and return to land in the form of HNO3/NO3-. Through this way, the deposition leads to nitrogen fertilization and the subsequent formation of nitrous oxide (N2O) in soil, which is another greenhouse gas. In conclusion, considering several direct and indirect effects, emissions have a negative contribution to global warming.

in the atmosphere is removed through several pathways. During daytime, NO2 reacts with hydroxyl radicals (·OH) and forms nitric acid (HNO3), which can easily be removed by dry and wet deposition. Organic peroxyradicals (RO2·) can also react with NO and NO2 and result in the formation of organic nitrates. These are ultimately broken down to inorganic nitrate, which is a useful nutrient for plants. During nighttime, NO2 and NO can form nitrous acid (HONO) through surface-catalyzed reaction. Although the reaction is relatively slow, it is an important reaction in urban areas. In addition, the nitrate radical (NO3) is formed by the reaction between NO2 and ozone. At night, NO3 further reacts with NO2 and establishes a equilibrium reaction with dinitrogen pentoxide (N2O5). Via heterogeneous reaction, N2O5 reacts with water vapor or liquid water and forms nitric acid (HNO3). As mentioned above, nitric acid can be removed through wet and dry deposition and this results in the removal of from the atmosphere.