Iodine

Phototransformation in air

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

phototransformation in air

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: The data in the JPL Publications are peer-reviewed by NASA Panel for Data Evaluation and the scientific community and commonly used for stratospheric modelling. Therefore these data can be considered as reliable.

Qualifier:

no guideline available

Principles of method if other than guideline:

no information available, but there is no guideline for gas phase chemistry available

GLP compliance:

not specified

Reaction with:

ozone

Rate constant:

0 cm³ molecule-1 d-1

Reaction with:

OH radicals

Rate constant:

0 cm³ molecule-1 d-1

Validity criteria fulfilled:

not applicable

Remarks:

no standard guideline available

Conclusions:

As the data are peer-reviewed by the NASA Panel for Data Evaluation and applied for atmospheric modelling by the scientific community, they can be considered as reliable for the assessment of the environmental fate.
Degradation rate constant:
1.04E-7 cm³ molecule-1 d-1 for reaction with: ozone;
1.6E-5 cm³ molecule-1 d-1 for reaction with: OH radicals

Reference 2

Endpoint:

phototransformation in air

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline available

Principles of method if other than guideline:

The reactions IO + NO2 + M -> IONO2 + M, IO + IO -> Products , and I + O3 -> IO + O2 were studied by using a photochemical modulation technique with detection of IO by absorption spectroscopy.

GLP compliance:

not specified

Light source:

other: fluorescent photolysis lamps

Details on light source:

The lamps were powered by a square-wave-modulated 250-V dc supply. IO radicals were produced in chemical systems involving either photolysis of NO2 using fluorescent lamps emitting at 350 ± 50 nm (Philip TW 40/08, ”Blacklamps”) in the presence of I2, or photolysis of I2, using 570 ± 70 nm radiation (Thorn EMI 40 W Gold) in the presence of O3.

Details on test conditions:

The experiments were conducted in gas mixtures flowing through a silica reaction vessel 120 cm in length and 3.0 cm internal diameter. The vessel was surrounded by a jacket through which a thermostated solution of 33% ethylene glycol in water was circulated, enabling regulation of the vessel temperature. Six 40-W fluorescent photolysis lamps, 120 cm in length, were radially mounted around the cylindrical reaction vessel. The concentrations of the precursor species were measured by conventional absorption spectroscopy in the reaction vessel, using either a tungsten halogen lamp (e.g., for I2 at 500 nm, σ = 2.14E-18 cm2) or a deuterium lamp source (e.g., for O3 at 254 nm, σ= 1.11E-17 cm2) with monochromator (0.75 m spex), photomultiplier (EM1 9781 B), and chart recorder.
IO was monitored at 426.9-nm absorption (tungsten-halogen lamp). The modulated absorption signals due to IO, resulting from intermittent photolysis of the precursor molecules, were detected by using a custom-built multichannel analyser incorporating an internally preprogrammed microcomputer (Harwell MOUSE type 6161) controlled externally by a CBM PET 3032.

I2 was generated by a flow of N2 (high purity, 6400 mL/min) over iodine crystals (BDH, Analar Grade). The vessel containing the crystals was immersed in a water bath. O3 was generated by passing O2 (B.O.C., Breathing Grade, < 150 mL/min) through a silent discharge ozonator into the vessel. Typical concentrations were 5E+14 molecule cm-3 of O3 and 5 E+13 molecule cm-3 of I2. I2 and O3 were mixed on the low-pressure side of the inlet needle valve because appreciable thermal reaction occurs at high partial pressures of O3 and I2. When NO2 was added, the reaction between O3 and NO2 was avoided in the same manner. NO2/N2 mixtures (Air Products, 900 ppm NO2 in high purity N2) were taken from a cylinder and metered through a calibrated rotameter. Concentrations in the vessel were varied between 5 E+12 and 1 E+14 molecule cm-3. NO2 concentrations were determined from the dilution in the manifold and the ratio of the pressure in the manifold to that in the vessel.

I2+hv(λ=500 nm)->I+I: k= 1.05E-2 s-1

 

I + O3 -> IO + O2: k1= 9.6 E-13 cm3 molecule-1 s-1, with an uncertainty of approximately ±30%.

 

IO + IO -> Products: kII= 1.25E-11cm3 molecule-1 s-1

Validity criteria fulfilled:

not applicable

Conclusions:

I + O3 -> IO + O2: k1= 9.6 E-13 cm3 molecule-1 s-1, with an uncertainty of approximately ±30%.
 
IO + IO -> Products: kII= 1.25E-11cm3 molecule-1 s-1

Executive summary:

A photochemical modulation technique was employed. IO radicals were monitored in absorption at 426.9 nm where the absorption cross section at this wavelength was determined to be (2.2±0.5) E-17 cm2 molecule-1. The reaction IO + NO2 + M -> IONO2 + M, studied between 35 and 404 torr at 277 K, was found to be in the falloff region between second- and third-order kinetics for this pressure range. The limiting rate constants were ko = (4.3±2.0) E-31 cm6 molecule-2 s-l and k∞= [0.8 ~2.2] E-11 cm3 molecule-1 s-1. The self-reaction IO + IO -> products was investigated between 10 and 100 torr at 303 K and between 25 and 404 torr at 277 K. The reaction appeared to proceed by at least two pathways, one pressure dependent and the other pressure independent. The pressure dependence of the overall rate coefficient is described by (5.6±0.8) E-30 X [M] + (8.0±2.7) E-l2 cm3 molecule s-l at 277 K. The results also allowed an estimate of (9.6±3.0) E-13 cm3 molecule-1 s-l for the rate coefficient of the reaction I + O3 - > IO + O2 at 26 torr and 303 K.

Reference 3

Endpoint:

phototransformation in air

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

The photolysis rate of molecular iodine was directly determined in the laboratory by an optical absorption technique (Saiz-Lopez et al., 2004).

GLP compliance:

not specified

The photolysis rate of molecular iodine was directly determined in the laboratory by an optical absorption technique (Saiz-Lopez et al., 2004) in order to confirm the rates calculated from the absorption cross-section, measured or modelled actinic flux and reported quantum yield. With the light intensity at one solar constant in these experiments, the observed first-order destruction rate of I2, corresponding to its photolysis, was 0.14 s−1.

Validity criteria fulfilled:

not applicable

Conclusions:

The photolysis rate of molecular iodine was 0.14 s−1.

Executive summary:

The photolysis rate of molecular iodine was directly determined in the laboratory by an optical absorption technique (Saiz-Lopez et al., 2004) in order to confirm the rates calculated from the absorption cross-section, measured or modelled actinic flux and reported quantum yield. With the light intensity at one solar constant in these experiments, the observed first-order destruction rate of I2, corresponding to its photolysis, was 0.14 s−1.

Reference 4

Endpoint:

phototransformation in air

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

As this publication describes the direct photolysis of molecular iodine in air, there is no standardised test guideline to compare with. However, the study follows scientific principles, is otherwise well documented and is therefore considered to meet the criteria of Klimisch score 2 - reliable with restrictions.

Qualifier:

no guideline available

Principles of method if other than guideline:

Measurement of photo-dissociation constant of gaseous iodine under controlled laboratory conditions by using a flow-through system. As light source an artificial sun light simulator was used. Photolysis was monitored by absorption of the 546 nm emission line of a low pressure Hg lamp. Recombination of iodine atoms was anticipated by addition of an excess of ozone to the gas flow.

GLP compliance:

no

Light source:

other: Oriel Instruments Model 91191 "solar simulator"

Light spectrum: wavelength in nm:

> 350 - <= 750

Details on light source:

- Emission wavelength spectrum: 350 - 750 nm
- Filters used and their purpose: airmass optical filter to simulate sunlight, natural density filters to attenuate light intensity (checked by Radiometer (Ramsden Scientific Instruments 550)
- Light intensity at sample and area irradiated: condenser lens was used to ensure illumination of complete reaction cell
- Relative light intensity based on intensity of sunlight: 1350 Wm-2 (ca. 1 solar constant)
- Duration of light/darkness: average residence time in reaction cell: 12.5 +/- 2.5 s

Reference substance:

no

Parameter:

max lambda

Value:

533 nm

Remarks on result:

other: absorption cross section = (4.24±0.50)E-18 cm^2/molecule

DT50:

0.14 min

Test condition:

natural solar irradiance

Transformation products:

not measured

Each experiment was seperated in three phases: a) only iodine in chamber, b) iodine + ozone (dark reaction) and c) photolysis reaction. For each phase the concentration of the iodine was determined. Potential adverse effects of particle extinction were considered and additional measurements for the dark and light reaction at three other wavelengths of the mercury lamp were performed, where the absorption by iodine and ozone is negligible. In both cases particle extinction at 546 nm was found to be significant (30 -35 % of the total measured absorption for the dark reaction, and 80 -90 % for photolysis). Quintuplicate experiments at five different irradiances in order to analyse the variation of the photolysis rate with the light intensity were performed.

The mean photo-dissociation rate constant for an irradiance of 1350 Wm^-2 (1 solar constant) was determined with J(I₂)= 0.14±0.04 s^-1 and a linear fit with a slope of (1.10±0.07) E-4 s^-1 W^-1 m² could be observed. Based on this observations a photo-dissociation constant for a irradiance of 1100 Wm^-2 with J(I₂)= 0.12±0.03 s^-1could be calculated. A computation value based on the SOLAR2000 model gives a value of J(I₂)= 0.012±0.015 s^-1 which is in very good agreement to the results of this study.

Validity criteria fulfilled:

not applicable

Remarks:

not applicable as there is no test guideline available

Conclusions:

There is no standard test guideline available to compare with this publication, the test is considered to follow scientific principles and is well documented. The determined halflife in the experiment is in good agreement with the theoretical results. Thus, it can be considered as adequate for the environmental fate assessment.

Executive summary:

Molecular iodine in the lower atmosphere is rapidly photolysed (lifetime less than 10 s for an overhead sun) due to the strong absorption of visible wavelength. Thus, photolysis can be considered as the major loss process for atmospheric iodine during daytime leading to formation of different iodine oxides or soluble organic iodine aerosol.

Description of key information

After evaporation iodine will be rapidly degraded either by photolysis or reaction with free atmospheric radicals (e.g. ozone, OH etc.) and enter into the natural geochemical cycle of iodine.

Key value for chemical safety assessment

Half-life in air:

0.14 min

Degradation rate constant with OH radicals:

0 cm³ molecule-1 d-1

Additional information

Saiz-Lopez determined the absolute cross-section as well as the photolysis rate of iodine in the gas phase in a laboratory set-up.

Quintuplicate experiments at five different irradiances in order to analyse the variation of the photolysis rate with the light intensity were performed. The mean photo-dissociation rate constant for an irradiance of 1350 Wm^-2(1 solar constant) was determined with J(I₂)= 0.14±0.04 s^-1and a linear fit with a slope of (1.10±0.07) E-4 s^-1W^-1m²could be observed. The findings of the study are in good agreement to estimations of an existing computational model on solar irradiance and photo-dissociation rates.

Since the study was conducted in accordance with generally accepted scientific principles, is well documented and the absence of a standard test guideline for photolysis in air, this study is considered to be reliable and adequate for the environmental risk assessment.

The presented degradation rate constants for the reactions of iodine with hydroxyl radicals and ozone support the very short lifetime of iodine (DeMore, 1997). As these peer-reviewed data are used for atmospheric modelling they can also be considered as reliable for the assessment of the atmospheric fate of iodine.

Based on the natural occurrence of iodine in all environmental compartments and the rapid degradation of gaseous iodine by photolysis and reaction with free radicals in the atmosphere it can be assumed that iodine does not play a relevant role in the destruction of the stratospheric ozone layer. Thus, it is not classified as hazardous to the ozone layer.