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Nitrogen oxides

DESIGNATIONS

CAS No.:
Registry name:
Nitrogen oxides
Chemical name: Nitrogen oxides
Synonyms, Trade names: NOx, N-oxides, Nitrous gases
Chemical name (German):
Stickstoffoxide, Stickoxide, Nitrose Gase
Chemical name (French):
Oxydes d'azote
Appearance:
brownish yellow - reddish brown gases depending on temperature and concentration
Note: "Nitrogen oxides" is a collective name of compounds of nitrogen with oxygen (often abbreviated NOx). Mainly nitrogen monoxide (NO) and nitrogen dioxide (NO2) are relevant to environmental impacts. Other oxides such as N2O, N2O3 and N2O5 are of minor importance in this respect.

CAS No. 10102-43-9 10102-44-0
Chemical name: Nitrogen monoxide Nitrogen dioxide
Synonyms, Trade names: Nitrogen oxide, Nitrogen(II)oxide Nitrogen peroxide, Nitrogen(IV)oxide
Chemical name (German): Stickstoffmonoxid Stickstoffdioxid
Chemical name (French): Oxyde d'azote Bioxyde d'azote
Appearance: colourless and odourless gas reddish brown gas with penetrating, acidic odour


BASIC CHEMICAL AND PHYSICAL DATA

Empirical formula: NO NO2
Rel. molecular mass: 30.01 g 46.01 g
Density: 1.34 g/l at 0C 1.45 g/cm3
Relative gas density: 1.04  
Boiling point: -152C 21C
Melting point: -164C -11C
Vapour pressure:   960 hPa
Solvolysis/solubility: in water: 73.4 ml/l at 0C  
Conversion: 1 ppm = 1.247 mg/m3

1 mg/m3 = 0.8702 ppm

1 ppm = 1.91 mg/m3

1 mg/m3 = 0.52 ppm

Note:
NO2 is in a temperature-dependent equilibrium with its dimer N2O4. Below 0C, all NO2 molecules have dimerised; at higher temperatures, the equilibrium is shifted towards NO2. Above 150C, NO2 begins to dissociate forming NO and O2. This reaction goes to completion at about 650C.

ORIGIN AND USE

Origin/derivation:
NOx are major air pollutants. They are produced in all combustion processes. In 1982, total emissions in Germany were about 3 Mio t. The major part of the emissions comes from motor vehicle exhausts (50%), power plants (30%) and industry (15%). Additionally, considerable amounts are produced by soil bacteria (denitrification) [RMPP, 1985].

Use:
Nitrous gases (NO/NO2) are used in the production of nitric acid (oxidation of NH3) and sulphuric acid (lead chamber process). Additionally, NO is used in nitrosation processes, and NO2 (N2O4) is used as an oxidising agent and in the manufacture of explosives.

Toxicity

Humans: LCLo 200 ppm, inhalation (1 min), (NO2) acc. UBA, 1986
TCLo 90 ppm, inhalation (40 min), (NO2) acc. UBA, 1986
Mammals:
Rat: LC50 88 ppm, inhalation (4 h), (NO2) acc. UBA, 1986
LC50 8.8 ppm, inhalation (4 h), (NO2) acc. HORN, 1989
Mouse: LCLo 250 ppm, inhalation (30 min), (NO2) acc. UBA, 1986
Rabbit: LC50 315 ppm, inhalation (15 min), (NO2) acc. UBA, 1986
Dog: LCLo 123 mg/m3, inhalation, (NO2) acc. UBA, 1986
Guinea pig: LC50 30 ppm, inhalation (1 h), (NO2) acc. UBA, 1986
Hamster: LC50 36 ppm, inhalation (48 h), (NO2) acc. UBA, 1986
Monkey: MCL 44 ppm (6 h), (NO2) acc. HORN, 1989
Aquatic organisms:
Mosquito fish: TLm 72 ppm (96 h, freshwater), (NO2) acc. UBA, 1986
Cockle: LC50 330-1,000 ppm (48 h, saltwater), (NO2) acc. UBA, 1986

Characteristic effects:

Humans/mammals: Nitrogen monoxide is oxidised forming nitrogen dioxide when it comes into contact with air. Thus, poisoning by nitrous gases is mainly due to nitrogen dioxide. Nitrogen dioxide is highly toxic and irritates both the skin and the mucous membranes. Dilutions of between 0.2 and 0.5 g/m3 can be inhaled without any adverse effects over a longer period (UBA, 1986). Nitrogen dioxide penetrates the alveoli. The formation of nitrous/nitric acid in the pulmonary tissue damages the capillary walls causing oedema after a latent period of 2-24 hours. Typical symptoms of acute poisoning are burning and running eyes, cough, dyspnoea and finally death.

Plants: Different species of plants exhibit considerable divergence in terms of resistance. All nitrous gases turn the edges of leaves brown or brownish black and cause blotches. Plant cells start to shrink and protoplasms detach themselves from the cell wall. This process ultimately results in the damaged parts of the cell drying out.

ENVIRONMENTAL BEHAVIOUR

Air:
90% of nitrogen oxide emissions come from furnaces and combustion engines. Thus, nitrogen monoxide is predominant in the vicinity of the source, whereas some 80% is transformed into nitrogen dioxide following long-distance transportation. Nitrogen oxides play an important role in the formation of ozone in the low atmospheric layer. Nitrogen dioxide is decomposed by sunlight into nitrogen monoxide and atomic oxygen which reacts immediately with atmospheric oxygen molecules, forming ozone. This equilibrium reaction depends on the NO2/NO ratio and on the intensity of the sunlight. Especially in summer and at high traffic volumes, this ratio is increased by atmospheric reactions of volatile hydrocarbons from automobile exhaust fumes resulting in a strong increase of the ozone concentration. Nitrogen oxides are washed out from the atmosphere by precipitation as nitrous or nitric acid, respectively.

Water:
Nitrogen oxides are only slightly soluble in water but they form nitrous or nitric acid when they come into contact with water. In Germany, nitrogen dioxide is listed in water hazard class 1.

Soil:
The adverse effects in soil result from its acidification which may cause nutrient relocation and elution depending on the soil type.

ENVIRONMENTAL STANDARDS

Medium/
acceptor
Sector Country/
organ.
Status Value Cat. Remarks Source
Nitrogen dioxide
Air:   CDN

(L)

0.06-0.1 mg/m3

  Annual average acc. BUB, 1986
  CDN

(L)

0.2 mg/m3

  24 h acc. BUB, 1986
  CDN

(L)

0.4 mg/m3

  1 h acc. BUB, 1986
  CH

(L)

0.03 mg/m3

  Annual average acc. BUB, 1986
  CH

(L)

0.08 mg/m3

  24 h acc. BUB, 1986
  D

L

0.2 mg/m3

MIK 30 min acc. UBA, 1986
  D

L

0.1 mg/m3

MIK 24 h acc. UBA, 1986
  D

L

0.05 mg/m3

MIK 1 a acc. UBA, 1986
  D

L

0.1 mg/m3

IW1   acc. TA-Luft, 1986
  D

L

0.3 mg/m3

IW2   acc. TA-Luft, 1986
  D

G

0.2 mg/m3

  1/2 h, VDI acc. BUB, 1986
  D

G

0.1 mg/m3

  24 h, VDI acc. BUB, 1986
  E

L

0.4 mg/m3

  1/2 h acc. MEINL et al., 1985
  E

L

0.1 mg/m3

  Annual average acc. MEINL et al., 1985
  E

L

0.565 mg/m3

  Smog alarm level I acc. MEINL et al., 1985
  E

L

0.75 mg/m3

  Smog alarm level II acc. MEINL et al., 1985
  E

L

1 mg/m3

  Smog alarm level III acc. MEINL et al., 1985
  EC

(L)

0.2 mg/m3

  98% percentile, year acc. LAU-BW, 1989
  EC

(L)

0.05 mg/m3

  50% percentile, year acc. MEINL et al., 1985
  F

(L)

0.2 mg/m3

  24 h, 95% percentile acc. MEINL et al., 1985
  GR

L

0.2 mg/m3

  1 h, smog warning acc. MEINL et al., 1985
  GR

L

0.5 mg/m3

  1 h, smog alarm stage I acc. MEINL et al., 1985
  GR

L

0.7 mg/m3

  1 h, smog alarm stage II acc. MEINL et al., 1985
  I

G

0.2 mg/m3

  1 h acc. MEINL et al., 1985
  J

(L)

0.074-0.112 mg/m3

  24 h acc. BUB, 1986
  NL

(L)

0.15 mg/m3

  24 h acc. BUB, 1986
  NL

G

0.095 mg/m3

  4 h acc. BUB, 1986
  NL

(L)

0.3 mg/m3

  1 h acc. BUB, 1986
  SF

(L)

0.15 mg/m3

  24 h acc. OECD, 1988
  SF

(L)

0.3 mg/m3

  1 h acc. OECD, 1988
  USA

(L)

0.1 mg/m3

  Annual average acc. BUB, 1986
  WHO

G

0.03 mg/m3

  Annual average acc. BUB, 1986
  WHO

G

0.095 mg/m3

  4 h acc. BUB, 1986
  WHO

G

0.4 mg/m3

  1 h, human beings acc. LAU-BW, 1989
  WHO

G

0.15 mg/m3

  24 h, human beings acc. LAU-BW, 1989
  WHO

G

0.095 mg/m3

  4 h, vegetation acc. LAU-BW, 1989
  WHO

G

0.03 mg/m3

  24 h, vegetation acc. LAU-BW, 1989
Emiss. D

L

500 mg/m3

  mass flow > 5 g/h2) acc. TA Luft, 1986
Workp D

L

9 mg/m3

MAK   DFG, 1989
Workp SU

(L)

2.mg/m3

    acc. SORBE, 1989
Workp USA

(L)

10 mg/m3

STEL   ACGIH, 1986
Nitrogen monoxide
Air:   CDN

(L)

0.2 mg/m3

  Long-time value acc. OECD, 1986
  CH

G

0.2 mg/m3

  Annual average acc. MEINL et al., 1985
  CH

G

0.6 mg/m3

  30 min, 95% percentile acc. MEINL et al., 1985
  D

L

1 mg/m3

  30 min acc. UBA, 1986
  D

L

0.5 mg/m3

  24 h acc. UBA, 1986
  D

L

0.1 mg/m3

MIK 1 a acc. UBA, 1986
  D

L

0.2 mg/m3

IW1 TA-Luft acc. UBA, 1986
  D

L

0.6 mg/m3

IW2 TA-Luft acc. UBA, 1986
  D

(L)

0.5 mg/m3

  24 h, VDI-R. 2310 acc. LAU-BW, 1989
  D

(L)

1 mg/m3

  30 min, VDI-R. 2310 acc. LAU-BW, 1989
  J

(L)

0.075-0.1 mg/m3

  Long-time value acc. acc. OECD, 1986
  YU

(L)

0.085 mg/m3

  Long-time value acc. OECD, 1986
  YU

(L)

0.085 mg/m3

  Short-time value acc. OECD, 1986
Workp USA

(L)

30 mg/m3

TWA   ACGIH, 1986
Workp USA

(L)

45 mg/m3

STEL   ACGIH, 1986


Comparison/reference values

Medium/origin Country Value Source
Air
NO3 radical, at night   350 ppt acc. UBA, 1988
NO3 on particles

S

0.5-3 mg/m3 (nitrogen) acc. UBA, 1987
PAN1), afternoons

USA

40 ppb acc. UBA, 1988
PAN1)

S

0.1-2 mg/m3 (nitrogen) acc. UBA, 1987
HNO2, motorway intersections

USA

8 ppb acc. UBA, 1988
HNO2

S

0.1-0.3 mg/m3 (nitrogen) acc. UBA, 1987
HNO3

S

0.5-3 mg/m3 (nitrogen) acc. UBA, 1987

Note:
All values for Sweden relate to rural areas in southern Sweden.
1) pan = peroxide, acetyl nitro
2) no and NO2, stated as NO2

Assessment/comments

As nitrogen oxides and their related products are highly toxic to humans and hazardous to the environment, their emission should be reduced as far as possible e.g. by using catalysts in automobiles.


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