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Environmental fate & pathways

Phototransformation in air

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Description of key information

In the atmosphere, Cl2 will degrade during daylight, with half-lives ranging from minutes to several hours, depending on latitude, season, and time of day.

Key value for chemical safety assessment

Additional information

The main reaction of molecular chlorine emitted to the atmosphere is photolysis, which generates atomic chlorine:

Cl2  + hv => 2Cl°

Photolysis occurs only during daylight hours. The estimated half-life for this process is approximately 2-4 hours, which is consistent with an atmospheric lifetime of less than 0.001 year (Doc IIIA, Section A7.3.2/02). The chlorine atoms formed by that process can then react with other species present in the atmosphere.

Reaction of atomic chlorine with saturated hydrocarbons:

Cl° + R-H => HCl + 

HCl formed by this process can be washed out by rain water or react with hydroxyl radicals OH° to regenerate atomic chlorine:

HCl + OH° => H2O + Cl°

Reaction of atomic chlorine with unsaturated hydrocarbons:

Atomic chlorine can also react with unsaturated organic compounds by addition to unsaturated sites. This reaction can lead to the formation of chloro-organic compounds in the atmosphere.

Reaction of atomic chlorine with ozone

Atomic chlorine can also react with ozone, as shown below:

Cl° + O3 => ClO + O2

ClO + HO2° => HOCl + O2

HOCl can be photolysed to regenerate Cl°.

HOCl + hv => HO + Cl°

These reactions indicate a participation of the catalytic cycle of ozone destruction. Overall, there is competition between the reaction of chlorine atoms with hydrocarbons and ozone.

Troposphere ozone formation potential

Atomic chlorine is a very reactive species and much more reactive than molecular chlorine. It reacts with non-methane hydrocarbons (NMHC) and some of the reaction products can form peroxy radicals which will contribute to the formation of ozone. Atomic chlorine can also substitute hydrogen atoms from aldehydes to form hydrochloric acid. In the case of acetaldehydes, the acetyl radical can then add oxygen and nitrogen to form peroxy acetyl nitrate (PAN). Peroxy radicals and PAN are two principal marker species of photochemical smog and add to the formation of ozone. On the other side, chlorine can also react with ozone, see reaction. However, the net effect of chlorine on the ozone concentration is still under discussion.