NOx

Key takeaways

NOx belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect NOx to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NOx from memory before moving on to harder problems.

Reference excerpt

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).

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.

Sources

Natural sources Nitric oxide is produced during thunderstorms due to the extreme heating and cooling within a lightning strike. 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 (15 lb) of nitrogen into chemically reactive NOx. With 1.4 billion lightning flashes per year, multiplied by 7 kilograms per lightning strike, they estimated the total amount of NOx produced by lightning per year is 8.6 million tonnes. However, NOx 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 NOx. Atmospheric constituents such as nitrogen oxides can be stratified vertically in the atmosphere. Ott noted that the lightning-produced NOx is typically found at altitudes greater than 5 km, while combustion and biogenic (soil) NOx are typically found near the sources at near surface elevation (where it can cause the most significant health effects).

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with NOx

Start with the simplest possible case. Write down what NOx claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to NOx before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about NOx ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of NOx

In research: NOx appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses NOx in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: NOx is common in secondary-school and first-year university syllabi. It links to neighbouring topics Airborne pollutants, Nitrogen oxides, Smog, so understanding it makes those chapters shorter.
In everyday life: Look for NOx outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study NOx in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NOx means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain NOx out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NOx in simple terms?

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.

Why does NOx matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study NOx?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on NOx.