In atmospheric chemistry, NOx is shorthand for nitric oxide (NO) and nitrogen dioxide (NO2), the nitrogen oxides that are most relevant for air pollution. These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone.
NOx gases are usually produced from the reaction between nitrogen and oxygen during combustion of fuels, such as hydrocarbons, in air; especially at high temperatures, such as in car engines. In areas of high motor vehicle traffic, such as in large cities, the nitrogen oxides emitted can be a significant source of air pollution. NOx gases are also produced naturally by lightning.
NOx does not include nitrous oxide (N2O), a fairly inert oxide of nitrogen that contributes less severely to air pollution, notwithstanding its involvement in ozone depletion and high global warming potential.
NOy is the class of compounds comprising NOx and the NOz compounds produced from the oxidation of NOx which include nitric acid (HNO3), nitrous acid (HONO), dinitrogen pentoxide (N2O5), peroxyacetyl nitrate (PAN), alkyl nitrates (RONO2), peroxyalkyl nitrates (ROONO2), the nitrate radical (NO3), and peroxynitric acid (HNO4).
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Formation and reactions
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 NOx refers to the total concentration of NO and NO2 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 NO to NO2 is determined by the intensity of sunshine (which converts NO2 to NO) and the concentration of ozone (which reacts with NO to again form NO2).
In other words, the concentration of ozone in the atmosphere is determined by the ratio of these two species.
The symbol
M
{\displaystyle {\ce {M}}}
represents a "third body", a molecular species that is required to carry away energy from the exothermic reaction 2. Equation 4 relates the concentrations of NOx and ozone, and is known as the Leighton relationship.
The time
τ
{\displaystyle \tau }
that is needed to reach a steady state among NOx and ozone is dominated by reaction (3), which reverses reactions (1)+(2):
for mixing ratio of NO, [NO] = 10 part per billion (ppb), the time constant is 40 minutes; for [NO] = 1 ppb, 4 minutes.
Formation of smog
When NOx 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.
Formation of nitric acid and acid rain
NO2 is further oxidized in the gas phase during daytime by reaction with OH
NO2 + OH (+M) → HNO3 (+M),
where M denotes a third molecule required to stabilize the addition product. Nitric acid (HNO3) is highly soluble in liquid water in aerosol particles or cloud drops.
NO2 also reacts with ozone to form nitrate radical
NO2 + O3 → NO3 + O2.
During the daytime, NO3 is quickly photolyzed back to NO2, but at night it can react with a second NO2 to form dinitrogen pentoxide.
NO2 + NO3 (+M) → N2O5 (+M).
N2O5 reacts rapidly with liquid water (in aerosol particles or cloud drops, but not in the gas phase) to form HNO3,
N2O5 + H2O(liq) → 2 HNO3(aq)
These are thought to be the principal pathways for formation of nitric acid in the atmosphere. This nitric acid contributes to acid rain or may deposit to soil, where it makes nitrate, which is of use to growing plants. The aqueous phase reaction
2 NO2 + H2O → HNO2 + HNO3
is too slow to be of any significance in the atmosphere.
