nitrogen oxide

Compounds composed of only nitrogen and oxygen

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Nitrogen oxides refer to compounds composed of only nitrogen and oxygen, including many compounds, such as Nitrous oxide （N two O）、 nitric oxide （NO）、 Nitrogen dioxide （NO two ）、 Nitrogen trioxide （N two O three ）、 Nitrogen tetroxide （N two O four ）And Nitrogen pentoxide （N two O five ）Etc. Nitrogen monoxide removal^[1] Except for nitrogen dioxide, other nitrogen oxides are unstable, and turn into nitrogen dioxide and nitric oxide when exposed to light, moisture or heat, and nitric oxide turns into nitrogen dioxide again. Therefore, the occupational environment is exposed to several gas mixtures, often called nitrous fume (gas), mainly nitric oxide and nitrogen dioxide, and nitrogen dioxide is the main gas. Nitrogen oxides have different degrees of toxicity.

Chinese name: nitrogen oxide
Foreign name: nitrogen oxides
Water solubility: Soluble in water except nitric oxide

Appearance: Under normal temperature, nitrogen pentoxide is white solid, nitrogen trioxide and nitrogen dioxide are reddish brown gas, under low temperature, nitrogen trioxide is blue liquid or solid, and the rest is colorless
Hazard description: It has different degrees of toxicity

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Main types

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Common nitrogen oxides are nitric oxide (NO, colorless) Nitrogen dioxide （NO two , reddish brown) Nitrous oxide （N two O）、 Nitrogen pentoxide （N two O five ）Among them, except for nitrogen pentoxide, which is solid in normal state, other nitrogen oxides are gaseous in normal state. Nitrogen oxides (NO) as air pollutants x ）Usually NO and NO two 。

Nitrogen oxides (NO x ）There are many kinds, including nitrous oxide (N two O）、 Nitric oxide (NO), nitrogen dioxide (NO two ）Nitrogen trioxide (N two O three ）Nitrogen tetroxide (N two O four ）And nitrous pentoxide (N two O five ）In addition, there are nitric oxide dimer (N ₂ O ₂), azide nitrosyl (N four O）、 Nitrogen trioxide (NO ∨), but mainly NO and NO two They are common atmospheric pollutants. in addition Trinitramine N(NO two ) three It is also a compound only composed of nitrogen and oxygen elements, but not an oxide in the strict sense.

N two O three And N two O five Both are acid oxides, N two O three The corresponding acid of is Nitrite （HNO two ），N two O three Is an anhydride of nitrous acid; N two O five The corresponding acid of is nitric acid, N two O five It is an anhydride of nitric acid. NO、N two O、N two O four And NO two None of them are acid oxides.

NO naturally emitted x , mainly from the decomposition of organic matter in soil and ocean, belonging to the natural nitrogen cycle process. Most of the NO emitted by human activities comes from the combustion process of fossil fuels, such as automobiles, aircraft, internal combustion engines and industrial kilns; It also comes from the process of producing and using nitric acid, such as nitrogen fertilizer plants, organic intermediate plants, nonferrous and ferrous metal smelters, etc. It was estimated in the early 1980s that the world's annual emissions of NO to the atmosphere due to human activities x About 53 million tons. NO x It is extremely harmful to the environment. It is not only one of the main substances that form acid rain, but also an important substance that forms photochemical smog in the atmosphere and consumes O three An important factor of.

Under high temperature combustion conditions, NO x It mainly exists in the form of NO, and the initial emission of NO x NO accounts for about 95%. However, NO in the atmosphere is very easy to react with oxygen in the air to generate NO two , so NO in the atmosphere x Generally NO two Exists in the form of. NO and NO in air two Balance is achieved through photochemical reaction and mutual transformation. When the temperature is high or there is cloud, NO two It further interacts with water molecules to form the second important acid in acid rain, nitric acid (HNO three ）。 In the presence of catalyst, if appropriate meteorological conditions are added, N0 two The rate of conversion to nitric acid is accelerated. Especially when NO two And SO two When they exist simultaneously, they can catalyze each other and form nitric acid faster.

In addition, NO x It can also increase the concentration of exhaust gas due to the gradual accumulation of exhaust gas emitted by aircraft in the stratosphere. NO x And O in the stratosphere three React to generate NO and O two , NO and O three Further reaction generates NO two And O two To break O three Balance, make O three Concentration decreases, resulting in O three Layer depletion.

Physical and chemical properties

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Nitrogen pentoxide

Except for nitrogen pentoxide, which is a solid, the rest are gases. Molecular formula NO x 。 Among them, nitrogen tetroxide is a dimer of nitrogen dioxide, which is often mixed with nitrogen dioxide to form an equilibrium mixture. The mixture of nitric oxide and nitrogen dioxide is also called nitrous oxide gas (smoke). They are slightly soluble in water, and the aqueous solution is acidic to varying degrees. Nitric oxide and nitrogen dioxide are decomposed into nitric acid and nitrogen oxide in water. Only when the temperature of nitrous oxide is above 300 ℃, can it be strongly oxidized. The rest have different degrees of oxidation, especially nitrous pentoxide, which decomposes to give off oxygen and nitrous oxide above - 10 ℃. Nitrogen oxides are non combustible substances, but they can support combustion, such as nitrous oxide (N two O）、 Nitrogen dioxide and nitrogen pentoxide can cause explosion in case of high temperature or combustible materials.

Invasion route

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Inhalation is mainly through respiratory tract.

toxicology

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Nitrous oxide in nitrogen oxides（ Laughing gas ）As Inhalation anesthetic , not industrial poisons. In addition to nitrogen dioxide, the others can produce nitrogen dioxide when exposed to light, humidity or heat, mainly due to the toxic effect of nitrogen dioxide, which mainly damages the deep respiratory tract. Nitric oxide can also be caused by combining with hemoglobin Methemoglobinemia 。 The MLC of nitrogen dioxide inhaled by human for one minute is 200ppm.

clinical manifestation

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Acute poisoning: when inhaling gas, there may be no obvious symptoms or eye and upper respiratory tract irritation symptoms, such as throat discomfort, dry cough, etc. Delayed pulmonary edema and adult respiratory distress syndrome often occur after a 6-7 hour incubation period. Pneumothorax and mediastinal emphysema may be complicated. About 2 weeks after the regression of pulmonary edema, there appeared delayed constrictive bronchiolitis, which led to cough, progressive chest tightness, respiratory distress and cyanosis. A few patients had no obvious poisoning symptoms after inhaling gas, but the above lesions occurred 2 weeks later. Blood gas analysis showed that arterial partial pressure of oxygen decreased. Chest X-ray film showed pulmonary edema or miliary shadows in both lungs. High concentration of nitric oxide in nitrate gas may cause methemoglobinemia.

Treatment: After acute poisoning, leave the site to fresh air. Inhale oxygen immediately. Close contacts were observed for 24 to 72 hours. Timely observe the chest X-ray changes and blood gas analysis. Symptomatic and supportive treatment. Actively prevent and treat pulmonary edema, and give reasonable oxygen therapy; Keep the respiratory tract unobstructed, use bronchospasmolytic agents, give defoamers such as Xiaofengjing when pulmonary edema occurs, and perform tracheotomy and mechanical ventilation when necessary; Early, moderate and short-term application of glucocorticoid. If dexamethasone can be given 10~60mg/day in batches according to the severity of the disease, the dose will be reduced after the disease improves. Large dose application generally does not exceed 3~5 days. In order to prevent obstructive bronchiolitis in severe cases, the time of small dose application can be extended as appropriate; Limit liquid intake in the short term. Rational use of antibiotics. Dehydrating agents and morphine should be used with caution. The dosage of cardiotonic should be reduced. In case of methemoglobinemia, 5~10mL of 1% methylene blue can be slowly injected intravenously. Treat symptomatically.

Air pollution

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Nitrogen oxides (NO x ）There are many kinds of air pollution, mainly caused by nitric oxide (NO) and nitrogen dioxide (NO two ）Therefore, nitrogen oxides in environmental science generally refer to the general term of the two.

Globally, the nitrogen oxides in the air mainly come from natural sources, but most of the nitrogen oxides in the urban atmosphere come from fuel combustion, that is, anthropogenic sources, such as mobile sources such as automobiles, and fixed sources such as industrial kilns.

According to calculation, the amount of nitrogen oxides produced by the combustion of various fuels is:

1 ton of natural gas, 6.35 kg

1 ton of oil, 9.1-12.3 kg

1 ton of coal, 8-9 kg

However, the concentration of nitrogen oxides in the exhaust of automobiles fueled with gasoline and diesel is quite high. In the non heating period, more than half of the nitrogen oxides in Beijing come from vehicle emissions.

The combination of nitrogen oxides and water in the air will eventually transform into nitric acid and nitrate, which will be removed from the air with precipitation and dust fall. Nitric acid is one of the causes of acid rain; It and other pollutants can produce photochemical smog pollution under certain conditions.

Beijing starts from the prevention of vehicle exhaust pollution, and the prevention measures include mandatory installation of external purifiers; Strictly control new car pollution; Promote the use of clean fuels and so on.

Nitrogen oxide pollution in indoor air mainly comes from outdoor air pollution.

In the national "12th Five Year Environmental Protection Plan", nitrogen oxide will become the pollutant with total amount control after sulfur dioxide. The reduction of total amount control mainly comes from flue gas denitration of power plants, flue gas denitration of coal-fired boilers, vehicle exhaust treatment, etc. The strict control of nitrogen oxides indicates that China has taken a new step from simply controlling sulfur dioxide in acid rain to comprehensively controlling acid gas emissions.

nature

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It mainly includes nitric oxide, nitrogen dioxide and nitric acid fog, mainly nitrogen dioxide. Nitric oxide is a colorless, odorless and inactive gas, which can be oxidized into nitrogen dioxide. Nitrogen dioxide is a brownish red gas with pungent odor.

Nitrogen pentoxide is the anhydride of nitric acid, which combines with water to form nitric acid.

Nitrogen trioxide is unstable, and it is the intermediate product of gaseous decomposition of nitrogen pentoxide. It exists for a short time.

harm

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Nitrogen oxides can stimulate the lungs, making people more difficult to resist respiratory diseases such as colds. People with respiratory problems, such as asthma patients, will be more vulnerable to nitrogen dioxide. For children, nitrogen oxides may cause damage to lung development. The research indicates that long-term inhalation of nitrogen oxides may lead to changes in lung structure, but the nitrogen oxide content and inhalation time that lead to such consequences are still uncertain.

Nitrogen oxides, mainly nitric oxide and nitrogen dioxide, are an important reason for the formation of photochemical smog and acid rain. The toxic smog formed by the reaction of nitrogen oxides and hydrocarbons in automobile exhaust with ultraviolet radiation is called photochemical smog. Photochemical smoke has a special smell, which can stimulate eyes, harm plants and reduce atmospheric visibility. In addition, nitric acid and nitrite generated by the reaction of nitrogen oxides with water in the air are components of acid rain. Nitrogen oxides in the atmosphere mainly come from the burning of fossil fuels and the burning of plants, as well as the transformation of nitrogen compounds in farmland soil and animal excreta.

processing method

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In industry, reducing agents (ammonia, urea, alkanes, etc.) are mainly used to neutralize nitrogen oxides by chemical reaction with nitrogen oxides. The processes mainly include selective catalytic reduction (SCR) and selective non catalytic reduction (SNCR). Ammonia reacts with nitrogen oxides to generate nitrogen gas and water, so as to achieve pollution-free emissions. It is mainly used in heating, power supply and other industries. However, in the ship industry and other industries, the implementation of nitrogen oxide control is more difficult (mainly because it is difficult to manufacture ammonia and it is dangerous to carry ammonia tanks), but there are also some application achievements.^[2]

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