Ozone

Phototransformation in air

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

phototransformation in air

Type of information:

experimental study

Adequacy of study:

other information

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Peer-reviewed, well-documented scientific publication.

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

Laboratory irradiation experiments were done with waters of chemical compositions that reflect the characteristics of atmospheric waters (droplets of clouds and fog).

GLP compliance:

not specified

Specific details on test material used for the study:

- Test material form: gas, in-situ generated
- The stock solution of O3 (O3 dissolved in water) was prepared as described previously (Hoigne and Bader, 1976).
- For the photochemical experiments, the concentration of O3 (up to 10 µM) was determined by the indigo method (Bader and Hoigne, 1981).

Estimation method (if used):

n/a

Light source:

sunlight

Details on light source:

Sunlight irradiations were performed using quartz tubes (diameter about 3 cm) positioned at about 30° from horizontal (Haag and Hoigne, 1985). When extended times of irradiations were required (e.g. in the absence of iron(III)-oxalate), the samples were immersed into a thermostated water bath. Sunlight intensity was measured with a pyranometer that responds only to light of 400-1000 nm, but was calibrated to give sunlight intensity over the range from 280 to 2800 nm. I = 910 W/m²

Details on test conditions:

The rate of depletion of O3 measured in the dark (blanks) gave an estimate for the rate of depletion of O3 preceding the illumination.
To measure the rate of direct photolysis of aqueous O3 by tropospheric sunlight, the dark decomposition of O3 and the succeeding chain reactions were minimized by applying double-distilled water and lowering the pH of the solution to 2.5 by adding phosphoric acid.

Duration:

180 min

Preliminary study:

n/a

Test performance:

n/a

Parameter:

not applicable

% Degr.:

ca. 60

Sampling time:

180 min

Transformation products:

yes

No.:

#1

Direct photolysis of aqueous ozone

Controls that were kept at dark lost less than about 0.5 µM or 5% of the O3 per hour. This corresponds approximately to 14 x 10-6 s-1 when expressed as an apparent first-order rate constant, k’_ΔO3(see Fig. 2). The samples that were exposed to sunlight exhibited a loss-rate that was about 4 times higher. The H2O2 found after all O3 was photolysed accounted for about 50 mole % of the depleted O3.

The experimental rate constant of the direct photolysis of less than about 60 x 10-6 s-1 (that included the dark reaction) proves that direct solar photolysis is too slow to significantly contribute to the loss of O3 in atmospheric waters when compared with the reaction of O3 with pollutants present in any atmospheric droplet and with the rate of indirect photoreactions that are sensitized by ubiquitous iron(III) compounds (see second experiment).

The formation of some H2O2 during solar irradiation of aqueous O3 at low pH can be interpreted as follows: direct photolytic reactions of O3 by absorption of radiation in the solar UV band is known to produce O(1D). In the liquid phase all of this (>95%) adds to H2O to produce H2O2 (Taube and Bray, 1940). Correspondingly, we have found in complementary experiments that upon UV irradiation at a wavelength of 255 nm and at pH 2.5, all O3 that was photolyzed produced H2O2 (Hoigne and Bader, to be published). In our present solar irradiations of long durations, we only found half of the depleted O3 as H2O2. The missing H2O2 cannot be due to photolysis of H2O2 by sunlight. This occurs only very slowly (DeMore et al., 1987). It is rather due to the primary and the secondary dark reactions that convert a further fraction of O3.

Validity criteria fulfilled:

not applicable

Conclusions:

The screening experiments by Jans and Hoigne (2000) suffice to show that aqueous-phase direct photolysis of ozone (in atmospheric water) is too slow to be of environmental interest.

Executive summary:

Laboratory irradiation experiments were done with waters of chemical compositions that reflect the characteristics of atmospheric waters (droplets of clouds and fog). In droplets of clouds and fog the transformation of ozone (O3) to secondary oxidants, such as hydroxyl radicals (OH•), is an important process. This is mainly accomplished by radical-type chain reactions (see endpoint record radical-type chain reactions) and to a much lesser extent by direct photolysis of ozone. The effect of direct photolysis of ozone was measured in this study.

The authors showed, the light absorption bands of aqueous O3 are not significantly shifted from those of gaseous O3. Therefore, the wavelength region of sunlight that is absorbed by aqueous O3 is already highly screened by stratospheric O3 and the direct photolysis of aqueous O3 by solar light becomes slow (time scale of hours) when compared with thermal chemical transformations.

This generally known information has been confirmed by the authors in the experiment summarised here.

Reference 2

Endpoint:

phototransformation in air

Type of information:

experimental study

Adequacy of study:

other information

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Peer-reviewed, well-documented scientific publication.

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

Laboratory irradiation experiments were done with waters of chemical compositions that reflect the characteristics of atmospheric waters (droplets of clouds and fog).

GLP compliance:

not specified

Specific details on test material used for the study:

- Test material form: gas, in-situ generated
- The stock solution of O3 (O3 dissolved in water) was prepared as described previously (Hoigne and Bader, 1976).
- For the photochemical experiments, the concentration of O3 (up to 10 µM) was determined by the indigo method (Bader and Hoigne, 1981).

Estimation method (if used):

n/a

Light source:

sunlight

Details on light source:

Sunlight irradiations were performed using quartz tubes (diameter about 3 cm) positioned at about 30° from horizontal (Haag and Hoigne, 1985). Sunlight intensity was measured with a pyranometer that responds only to light of 400-1000 nm, but was calibrated to give sunlight intensity over the range from 280 to 2800 nm. I = 520 W/m²

Details on test conditions:

The rate of depletion of O3 measured in the dark (blanks) gave an estimate for the rate of depletion of O3 preceding the illumination.
500 ml of the iron(III)-oxalate solution with 30 mM NaClO4, formaldehyde (or sodium formate) and sodium acetate was prepared in the dark. A known amount of the O3 stock solution was added. Just before irradiation, 20 ml samples were quickly distributed into quartz tubes fitted with stoppers. After varied durations of reaction, tubes were withdrawn and the O3 residual concentration analyzed.

Duration:

11 min

Reference substance:

no

Preliminary study:

n/a

Test performance:

n/a

Parameter:

not applicable

% Degr.:

> 95

Sampling time:

0.5 min

Transformation products:

not specified

Iron oxalate sensitized photo-reactions of ozone

Fig. 3 illustrates the experimentally measured influence of sunlight on the rate of depletion of O3 in a solution that contains 1 µM iron(III)-oxalate (i.e., 1 µM Fe(III) and an excess of oxalate) to act as a photoinitiator, and dissolved formaldehyde and acetate to control the radical-type chain reaction decomposing O3. Already at dark, the depletion rate of O3 was faster than in the foregoing experiments. This is due to the well-known thermal chain reactions that in the presence of chain promoters and at pH 5 proceed faster than in clean water at pH 2.5. When these solutions were exposed to light, the observed rate of loss of O3 was highly accelerated. It became much faster than in the absence of iron(III) and formaldehyde (see direct photolysis experiment). In this case, the observed rate of loss of O3 was zero order in [O3].

Validity criteria fulfilled:

not applicable

Conclusions:

The photolysis experiment with added iron-oxalate complexes, formaldehyde and acetate by Jans and Hoigne (2000) illustrates the significant transformation of aqueous atmospheric O3 by radical-type chain reactions. It shows that the presence of promotors and initiators highly accelerates the ozone transformation, in sunlight much more than in darkness. This study was selected because it offers sufficient illustration and a review of general knowledge of the sensitized photo-reactions of ozone in atmospheric water.

Executive summary:

In droplets of clouds and fog the transformation of ozone (O3) to secondary oxidants, such as hydroxyl radicals (OH•), is an important process.

This is mainly accomplished by radical-type chain reactions (this entry) and to a much lesser extent by direct photolysis of ozone (see endpoint record direct photolysis).

The observed rate of loss of O3 in iron oxalate sensitized photo-reactions of ozone was much faster than in the absence of iron(III) and formaldehyde. In this case, the observed rate of loss of O3 was zero order in [O3]. In less than 1 minute all ozone had reacted away. The role of iron-oxalate is representative for the role of other organic iron complexes which also occur in cloud waters and also act as photolytic sources of O2(-I) (cf. Zuo and Hoigne, 1994).

Jans and Hoigne (2000) stated the following background information in their publication:

In any type of water in which O3 is not consumed quickly by direct molecular reactions with solutes, O3 becomes significantly transformed into secondary oxidants such as O2(-I) and OH•. The sequences of reactions and the kinetics of the radical-type chain reactions that accelerate such transformations have been investigated for many decades. According to existing models, a radical-type chain reaction is initiated by any process that generates O2(-I). When O2(-I) reacts with O3, OH• is produced. Reactions of OH• with compounds such as formaldehyde, formate, methanol, any carbohydrate etc., then convert the very reactive and unselective OH• fast and at a high yield into highly selective O2(-I). At the pH of typical cloud waters it is then O2-• (pKa of HO2• = 4.8) that transforms further O3 into OH• or that reduces Cu(II) to Cu(I) that also reacts with O3 to reproduce OH•. This chain of reactions is however inhibited by compounds that scavenge OH• without converting a significant fraction of it into O2(-I). Atmospheric waters also contain some hydrogen peroxide (H2O2). Its dissociated form (HO2•) also reacts highly selectively with O3. However, due to the high pKa of H2O2 (pKa = 11.3), the reactions of HO2• are not relevant at the low pH values (pH(5) encountered in typical cloud and fog waters.

Reference 3

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: Ozone half-life measurements conducted by the US Environmental Protection Agency (EPA)

Principles of method if other than guideline:

No guideline reported. Review article reporting measurement results without any details.

GLP compliance:

no

Details on test conditions:

Measurement of ozone half-life in the ambient atmosphere. No further details reported.

Key result

DT50:

12 h

Test condition:

Measurement of ozone half-life in ambient atmosphere

Transformation products:

yes

Remarks:

Oxygen

Validity criteria fulfilled:

not applicable

Conclusions:

The half-life of ozone in ambient atmosphere is 12 hours.

Executive summary:

This literature survey reports the half-life of ozone in the ambient atmosphere as measured by the U.S. Environmental Protection Agency to be on the order of 12 hours.

Description of key information

Ozone decomposes in the planetary boundary layer with a half-life of max 12 hours. Ozone in atmospheric water (fog and cloud droplets) is continuously involved in complex radical-type chain reactions responsible for the photolytic transformation of ozone. In contrast, direct photolysis of ozone is too slow to be of environmental interest.

Key value for chemical safety assessment

Half-life in air:

12 h

Additional information

Ozone can be decomposed via complex radical chain reactions in the presence of sunlight (phototransformation). The phototransformation of ozone in ambient air is well studied, and humidity was found to play an important role. Jans and Hoigne (2000) performed laboratory experiments to study transformation of ozone into OH-radicals (OH•) in waters of chemical compositions that reflect the characteristics of atmospheric waters (droplets of clouds and fog). This transformation is mainly accomplished by sensitised photoreactions promoted by radical-type chain reactions (see Jans and Hoigne 2000_Radical-type chain reactions) and to a much lesser extent by direct photolysis of ozone (see Jans and Hoigne 2000_Direct photolysis). Aqueous-phase direct photolysis of ozone (in atmospheric water) is too slow to be of environmental interest (time scale of hours). The main reason is that the wavelength region of sunlight that is absorbed by aqueous tropospheric O₃ (and needed for direct photolysis) is already highly screened by stratospheric O₃. In comparison, thermal chemical transformations (sensitised photoreactions) are much faster.

There are many factors influencing the fate of ozone in the atmosphere, therefore it is hard to define a general half-life value for ground-level ozone in air. It is a well-known fact that ozone, especially under dry conditions, is much more stable in air than in water. The half-life of ozone in ambient air has been measured by the US EPA to be in the order of 12 hours (Rice and Browning, 1980). This value is often cited in the ozone literature and seems reliable and conservative enough to select as key value.