Formaldehyde

Phototransformation in air

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

phototransformation in air

Type of information:

calculation (if not (Q)SAR)

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

accepted calculation method

Remarks:

Estimation according to generally accepted methods. Thereby, only peer reviewed and generally accepted compilation of data were used (secondary citation).

Qualifier:

no guideline available

Principles of method if other than guideline:

Best fit of reviewed data from several authors.

GLP compliance:

no

Specific details on test material used for the study:

For estimation, 100% purity was assumed

Estimation method (if used):

The rate constant of formaldehyde for the photochemical oxidative reaction with OH radicals was fitted by Atkinson (1994) using reviewed data from several authors.
The degradation half-life for OH radicals was calculated by the applicant using the formula τ1/2 = ln(2)/(kOH·[OH]). The calculation is based on 0.5·10exp6 OH radicals per cm3 for a 24-hours-day according to the TGD (EC 2003, part II chapter 3, 2.3.6.3, p. 51).

Reference substance:

no

Test performance:

The OH rate constant of formaldehyde was fitted using reviewed data from several authors. The OH rate constant was evaluated from an Arrhenius plot. Also a critical review of NO3 rate constant was given. In addition, rate constants for ozone and nitrate were given in AOPWIN v1.91.
Half-life was calculated by the applicant using the formula τ1/2 = ln(2)/(kOH·[OH]) and the parameter given in the TGD (EC 2003, part II, chapter 3, 2.3.6.3, p. 51).

DT50:

1.7 d

The formation of degradation products was not tested in the study. However, degradation products were given by secondary citations.

Rate constants:

OH radicals: 9.37 · 10^-12 cm³ molecule^-1 s^-1 with estimated overall uncertainty at 298 K of 25%.

Ozone: 2.1 · 10^-24 cm³ molecule^-1s^-1 (AOPWIN v1.91)

NO3 radicals: 5.8 · 10^-16cm³ molecule^-1 s^-1 (Atkinson 1997, AOPWIN v1.91)

Following transformation products were given:

HCHO + •OH -> H2O + •HCO

Validity criteria fulfilled:

yes

Conclusions:

The results suggest that formaldehyde is rapidly degraded in air by photo-oxidative processes. Significant amounts of formaldehyde are not likely to persist in air.

Executive summary:

The relevant indirect degradation pathway is with OH radicals; degradation by nitrate and ozone is negligible.

The specific degradation rate constant with OH radicals was fitted to be 9.37 · 10^-12 cm³ molecule^-1s^-1 with estimated overall uncertainty at 298 K of 25%.

The rate constants for ozone was 2.1 · 10^-24cm³ molecule^-1 s^-1 and for NO₃radicals 5.8 · 10^-16 cm³ molecule^-1 s^-1.

Phototransformation of formaldehyde has been estimated according to generally accepted methods. The estimation was considered to be valid.

Furthermore, the Henry's law constant is relatively low (Bettterton, 1988). Therefore, formaldehyde is not expected to volatilize to air from water surfaces in significant quantities and the amount which reaches the air compartment will be washed out by rain.

Reference 2

Endpoint:

phototransformation in air

Type of information:

calculation (if not (Q)SAR)

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

accepted calculation method

Remarks:

Estimation according to generally accepted methods. Thereby, only peer reviewed and generally accepted compilation of data were used (secondary citation).

Qualifier:

no guideline available

Principles of method if other than guideline:

UV spectrum and quantum yield measurements. Computer simulations of photooxidation.

GLP compliance:

no

Remarks:

not applicable

Specific details on test material used for the study:

For estimation, 100% purity was assumed
Test material form: gas

Estimation method (if used):

The total photodegradation of formaldehyde by hydroxyl radicals and photolysis was estimated by computer simulation (Gardner et al., 1984). These were compared with photolysis (Horowitz & Calvert, 1978).
The formation of degradation products was not tested in the study. However, degradation products were given by secondary citations.

Reference substance:

no

DT50:

1.7 d

Test condition:

direct photolysis

Transformation products:

yes

No.:

#4

No.:

#3

No.:

#2

No.:

#1

Absorption cross-sections and quantum yields for formaldehyde photolysis over the wave-length region 240 - 360 nm (T = 285 K) are tabulated in the publications.

Decomposition rate at 40 °solar zenith angle: 4.7 · 10-⁵s^-1 (corresponding to 2.82 · 10^-3 min^-1 (Gardner et al. 1984)

Photodecomposition rate at 40 °solar zenith angle: 1.71 · 10^-3 min-1 (Horowitz and Calvert 1978)

Therefore, photodecomposition is 1.5 times higher than by OH radicals (1.71 · 10^-3 min^-1 /(2.82 · 10^-3 min-1 - 1.71 · 10^-3 min^-1)).

 

The following transformation products were given by secondary citation in Atkinson et al. (1997):

(1)     HCHO + h υ -> H (#1)+ •HCO (#4)
a further product of •HCO in air is CO + •HO2

(2)     HCHO + h υ -> H2 (#2)+ CO (#3)

The second reaction is two times higher than the first reaction. Therefore, the main transformation products are hydrogen and carbon monoxide (Lelieveld 1990).

 

Validity criteria fulfilled:

yes

Conclusions:

The results suggest that the test item is rapidly degraded in air by photo-oxidative processes. Significant amounts of the test item are not likely to persist in air.

Executive summary:

The results show the degradation by photolysis is 1.5 times higher than by OH radicals. Therefore, for a conservative estimation the degradation by direct photolysis was set equal to the degradation by OH radicals (cf. indirect photolysis). The numerical half-life is given below. The main transformation products are hydrogen and carbon monoxide.

Direct photolysis of formaldehyde has been estimated according to generally accepted methods. Peer reviewed generally accepted compilation of data were used (secondary citation). The estimation was considered to be valid.

Furthermore, the Henry's law constant is relatively low (cf. 5.4.2). Therefore, the test item is not expected to volatilise to air from water surfaces in significant quantities and the amount which reaches the air compartment will be washed out by rain.

Description of key information

Reaction with the hydroxyl radical is considered to be the most important photooxidation process. The atmospheric halflife of formaldehyde, based on hydroxyl radical reaction rate constants, is calculated to be between 7.1 and 71.3 hours.

Additional information

Direct and indirect photolysis of the test item were estimated via accepted calculations based on literature data and secondary citation.

Direct photolysis of the test item has been estimated according to generally accepted methods. Peer reviewed generally accepted compilation of data were used (secondary citation). The estimation was considered to be valid.

The results show the degradation by photolysis is 1.5 times higher than by OH radicals. Therefore, for a conservative estimation the degradation by direct photolysis was set equal to the degradation by OH radicals. The numerical half-life was calculated as 1.7 d. The main transformation products are hydrogen and carbon monoxide (Atkinson et al., 1997).

Indirect photolysis occurs mainly via reaction with OH radicals; degradation by nitrate and ozone is negligible. The specific degradation rate constant with OH radicals was fitted to be 9.37 · 10^-12 cm³ molecule^-1s^-1 with estimated overall uncertainty at 298 K of 25%. The rate constants for ozone was 2.1 · 10^-24cm³ molecule^-1 s^-1 and for NO₃radicals 5.8 · 10^-16 cm³ molecule^-1 s^-1.

Furthermore, the Henry's law constant is relatively low (cf. Betterton, 1988). Therefore, the test item is not expected to volatilize to air from water surfaces in significant quantities and the amount which reaches the air compartment will be washed out by rain. The results suggest that the test item is rapidly degraded in air by photo-oxidative processes. Significant amounts of the test item are not likely to persist in air.