Acrylaldehyde

Phototransformation in air

Description of key information

The stability of acrolein in the atmosphere is limited by the rapid gas-phase reactions with the hydroxyl radical and ozone. The calculated half-life of acrolein for the reaction with the OH-radical in the troposphere (*OH concentration 5 x 105 molecules/cm³, 24 hours) is less than one day and is is in accordance with the half-life values derived from experiments. Unlike to the US ATSDR Report and the EU RAR the WHO Report stated the calculated atmospheric half-life of acrolein, based on rate constants for hydroxyl radical reaction, to be between 3.4 and 33.7 h. Other degradation routes, such as the reaction with nitrate radical (night-time; half-life: 16 days) and ozone (half-life: 59 days) as well as photolysis (daytime; half-life: 10 days in the lower troposphere and less than 5 days in the upper troposphere), are considered to be less significant. Based on the short estimated half-lives, acrolein is not a candidate for long-range atmospheric transport.

Additional information

1. European Union Risk Assessment Report of Acrolein (EU, 2001)

Photo-oxidation in air: The reaction with hydroxyl radicals (*OH) is described as the major degradation route of acrolein in the troposphere, whereby acrolein can react both as olefin and an aldehyde. The reaction as an aldehyde is faster than the reaction as an olefin. Degradation products of these reactions are formaldehyde, carbon dioxide, glyoxal, carbon monoxide, glycolaldehyde, ketene and acryloylperoxinitrate (dependent on the formation rate of NO2-molecules). The calculated half-life of acrolein for the reaction with the OH-radical in the troposphere (*OH concentration 5 x 10⁵ molecules/cm³ and 24 hours) is less than one day. The calculated half-life is in correspondence with the half-life values derived from experiments. Other degradation routes of acrolein in the air are the reactions with ozone, with nitrate radical and O (3P) (atomic oxygen in the electronic ground state). The reaction with nitrate radicals gains importance primarily at night when the concentration of the OH radicals decreases and no photolysis occurs. The reaction with ozone is secondary, but nevertheless still plays a substantial role in the degradation of acrolein. Degradation products for the reaction with ozone are formaldehyde, glyoxylic acid, formic acid and glyoxal.

Photolysis: Photolysis competes with photo-oxidation, but plays a lesser role in the degradation of acrolein in the troposphere. Irradiation of acrolein in synthetic air with UV-light results mainly in the formation of carbon monoxide and ethane. Other organic products, e.g. formaldehyde, carbon dioxide and small amounts of hydrogen and methane, were detected as well. Photolysis is low at normal atmospheric pressure, but increases at lower atmospheric pressure. The half-life of photolysis is 10 days in the lower troposphere and less than 5 days in the upper troposphere.

2. Agreement with further international Reports and Studies published after finalisation of the EU Risk Assessment Report 2001

US ATSDR (2007):

Photo-oxidation in air: The dominant removal process for acrolein in ambient air is predicted to be the reaction with photochemically generated hydroxyl radicals in the troposphere. The atmospheric half-life for acrolein is estimated to be 15-20 hours, based on experimentally determined hydroxyl radical reaction rate constants ranging  between 1.9 x 10^-11 and 2,53 x 10^-11cm³/molecule x sec at 25-26°C and an average ambient hydroxyl radical concentration of 5 x 10⁵ molecules/cm³. Acrolein reacts with hydroxyl radicals as both as olefin and as aldehyde. Products of this reaction include carbon monoxide, formaldehyde, glyoxal, and glycolaldehyde. In the presence of nitrogen oxides, products include peroxynitrate, nitric acid, glycidaldehyde, malonaldehyde, and 3-hydroxypropanaldehyde.

Experimental data indicate that reaction of acrolein with ozone (k=2.8 x 10^-19 cm³/molecules x sec at 25°C; half-life, 59 days) or nitrate radicals (k=5.9 +/-2.8 x 10^-16cm³/molecules x sec at 25°C; half-life, 16 days) in the troposphere would be too slow to be environmentally significant.

Photolysis: Direct photolysis in the ambient atmosphere occurs but is expected to be of minor importance. Gardner et al. reported that the quantum yields for irradiation of acrolein at low air pressures were 0.0066 at 313 nm and 0.0044 at 334 nm. The authors used a computer analysis of their photodissociation data to estimate the half-life of acrolein to be 10 days in the lower troposphere and < 5 days in the upper troposphere. The ultraviolet (UV) spectrum of acrolein in hexane shows moderate absorption of UV light in the environmentally significant range (wavelengths >290 nm), suggesting that acrolein might undergo photolysis in natural waters; however, hydration of acrolein destroys the chromophores that absorb UV light, and the equilibrium appears to be far on the side of the hydration product. Thus, the potential for direct photolysis of acrolein in natural waters is probably slight.

WHO (2002): Acrolein emitted to air reacts primarily with photochemically generated hydroxyl radicals in the troposphere . Minor processes include direct photolysis, reaction with nitrate radicals, and reaction with ozone. The calculated atmospheric half-life of acrolein, based on rate constants for hydroxyl radical reaction, is between 3.4 and 33.7 h. The overall reactivity-based half-life of acrolein in air is less than 10 h. Based on these short estimated half-lives, acrolein is not a candidate for long-range atmospheric transport.

3. Substantial disagreements in comparison to further international Reports to European Union Risk Assessment Report 2001

None

4. Additional aspects in further international Reports

None

5. Additional information in new Studies, not included in the European Union Risk Assessment Report 2001 or further cited international reports

None

6. Conclusions

The stability of acrolein in the atmosphere is limited by the rapid gas-phase reactions with the hydroxyl radical and ozone. The calculated half-life of acrolein for the reaction with the OH-radical in the troposphere (*OH concentration 5 x 105 molecules/cm³, 24 hours) is less than one day and is is in accordance with the half-life values derived from experiments. Unlike to the US ATSDR Report and the EU RAR the WHO Report stated the calculated atmospheric half-life of acrolein, based on rate constants for hydroxyl radical reaction, to be between 3.4 and 33.7 h. Other degradation routes, such as the reaction with nitrate radical (night-time; half-life: 16 days) and ozone (half-life: 59 days) as well as photolysis (daytime; half-life: 10 days in the lower troposphere and less than 5 days in the upper troposphere), are considered to be less significant. Based on the short estimated half-lives, acrolein is not a candidate for long-range atmospheric transport.