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Environmental fate & pathways

Phototransformation in air

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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. 

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Additional information

Summary concerning photodegradation of Formaldehyde as given in WHO (2002) and Environment Canada (2001).

“Formaldehyde emitted to air primarily reacts with photochemically generated hydroxyl (OH) radicals in the troposphere or undergoes direct photolysis. Minor fate processes include reactions with nitrate (NO3) radicals, hydroperoxyl (HO2) radicals, hydrogen peroxide (H2O2), ozone (O3) and chlorine (Cl2).

Reaction with the hydroxyl radical is considered to be the most important photooxidation process, based on the rate constants and the concentrations of the reactants. Factors influencing the atmospheric lifetime of formaldehyde, such as time of day, intensity of sunlight, temperature, etc., are mainly those affecting the availability of hydroxyl and nitrate radicals. The atmospheric halflife of formaldehyde, based on hydroxyl radical reaction rate constants, is calculated to be between 7.1 and 71.3 hours (Atkinson, 1985; Atkinson et al., 1990). Products that can be formed from hydroxyl radical reaction include water (H2O), formic acid (HCOOH), carbon monoxide (CO) and the hydroperoxyl/ formaldehyde adduct (HCO3) (Atkinson et al., 1990).

Photolysis can take two pathways. The dominant pathway produces stable molecular hydrogen and carbon monoxide. The other pathway produces the formyl (HCO) radical and a hydrogen atom, which react quickly with oxygen to form the hydroperoxyl radical and carbon monoxide. Under many conditions, the radicals from formaldehyde photolysis are the most important net source of smog generation. When the rates of these reactions are combined with estimates of actinic radiance, the estimated halflife of formaldehyde due to photolysis is 1.6 hours in the lower troposphere at a solar zenith angle of 40°. A half-life of 6 hours was measured based on simulated sunlight (Lowe et al., 1980).

Overall half-lives for formaldehyde in air can vary considerably under different conditions. Estimations for atmospheric residence time in several cities ranged from 0.3 hours under conditions typical of a rainy winter night to 250 hours under conditions typical of a clear summer night (assuming no reaction with hydroperoxyl radicals). During the daytime, under clear-sky conditions, the residence time is determined primarily by its reaction with the hydroxyl radical. Photolysis accounted for only 2–5% of the removal. Given the generally short daytime residence times for formaldehyde, there is limited potential for long-range transport of this compound. However, in cases where organic precursors are transported long distances, secondary formation of formaldehyde may occur far from the actual anthropogenic sources of the precursors."

Summary concerning photodegradation of Formaldehyde as given in OECD (2004) SIDS Formaldehyde:

"In the atmosphere, formaldehyde is expected to be indirectly photodegraded by reaction with OH-radicals, with a half life of 1.71 d (Atkinson 1992). Direct photolysis is also a relevant removal process for formaldehyde in air. A half-life of 4.1 hours was measured (Gardner et al. 1984)."