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Administrative data

phototransformation in air
Type of information:
experimental study
Adequacy of study:
key study
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Valid scientific study

Data source

Referenceopen allclose all

Reference Type:
The atmospheric chemistry of hydrogen cyanide (HCN)
Ciccerone RJ and Zellner R
Bibliographic source:
J Geophys Res 88: 10689-96
Reference Type:
review article or handbook

Materials and methods

Test guideline
no guideline available
Principles of method if other than guideline:
valid measurement and estimation method
GLP compliance:
not specified

Test material

Constituent 1
Chemical structure
Reference substance name:
Hydrogen cyanide
EC Number:
EC Name:
Hydrogen cyanide
Cas Number:
Molecular formula:

Results and discussion

Results with reference substance:
Cicerone and Zellner (1983) developed a numerically iterative model that integrated the total atmospheric flux of HCN from reaction with the hydroxyl radical, singlet oxygen and direct photolysis. Input parameters were taken from earlier work of the same and other authors. The effective rate constant for OH addition decreased with rising altitude (hydroxyl radical concentration): KOH ranged from0.022 to 0.011E–12 cm3/molecule/s in the troposphere (3.20 - 8.60E6 hydroxyl radical/cm3), and 0.0088 to 0.0000069E–12 cm3/molecule/s in the stratosphere (21.3 - 4.81E6 hydroxyl radical/cm3, maximum 32.0E6 hydroxyl radical/cm3). The rate constant for reaction with stratospheric singlet oxygen was set at 10–12 cm3/molecule/s, independent of pressure and temperature (altitude). HCN photoabsorption and dissociation were assumed to be similar to those of HCl, and parameterised from other work by the same authors. The resulting overall atmospheric lifetime for HCN was estimated to be 2.5 years, or between 1.3 and 5.0 years when hydroxyl radical concentrations were doubled or halved, respectively.

Any other information on results incl. tables

HCN reacts with naturally occurring hydroxyl radicals formed by sunlight through addition on the double bond, followed by rapid oxidation to CO and nitric oxide (NO). This photo-oxidation occurs both in the troposphere (0 - 8 km) and stratosphere (up to 80 km). Another path for HCN oxidation, dominant in the higher stratosphere (> 34 km), was reportedly the reaction with singlet oxygen. Direct photolysis of HCN was assumed to play a minor role at high altitudes (> 54 km). About 98% of HCN (from ground-level sources) was considered to remain in the stratosphere. In all, tropospheric oxidation by hydroxyl radical addition presents the major sink of HCN (and a source of NOx, see below). Reactions with other gases such as CO or O3 do not occur. Because of its low solubility and weak acidity, HCN was expected to have a very long atmospheric lifetime against rainout. Thus, it would take 34 years of average rainfall (1 m/y) to remove 200 ppt of gaseous HCN from the atmosphere if the rain were saturated with HCN at pH 4. There should be no significant diurnal variation in HCN concentrations, and there were "no obvious and major roles" for HCN in tropospheric chemistry. This has been the conventionally held view until the 1990's.

Applicant's summary and conclusion

Validity criteria fulfilled:
reviewed as valid
Most (98%) HCN from ground-level sources remains in the stratosphere. Tropospheric oxidation by hydroxyl radicals presents the main atmospheric reaction for HCN. An alternative theory is that the ocean is a major sink for atmospheric HCN concentrations.