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Photochemistry Unearthed

Could dirt help clean the atmosphere? According to a recent study,1 humic acids in soil may use sunlight to produce chemicals that can break down pollutants in the troposphere–the lowest layers of the atmosphere.

Humic acids are a complex mixture of macromolecular organic compounds produced by the degradation of certain soils. The study claims that during the day, humic acids may be turning atmospheric nitrogen dioxide NO2 into nitrous acid HONO, a precursor of hydroxyl radicals. And it is these hydroxyl radicals that can break down pollutants in the troposphere.

HONO is produced when nitrogen dioxide, a major combustion by-product, reacts with water or organic compounds on the surface of particles. It was previously assumed that HONO was only produced at night due to it being rapidly broken down by sunlight. But quite recently, scientists discovered that HONO was also being produced during the day.

A team of researchers from CNRS, Switzerland, and Germany,2 found that when they exposed thin films of humic acid to nitrogen dioxide in the presence of light, HONO was produced at almost the same rate as nitrogen dioxide was being consumed. They observed that the rate at which the HONO was being produced could explain the high levels observed in the troposphere during the day.

Although hydroxyl radicals help clear the atmosphere of pollutants, it is not all good news: hydroxyl radicals also accelerate the production of secondary pollutants like ozone. This discovery will force us to reevaluate the mechanisms by which we analyze atmospheric pollution and the exact role played by HONO in the lower atmosphere.


Steven Perkins





Notes :

1. K. Stemmler et al., “Photosensitized reduction of nitrogen dioxide on humic acid as a source of nitrous acid,” Nature. 440: 195-198. 2006.
2. The Paul Scherrer Institute in Switzerland, the University of Wuppertal in Germany and the Chemical Applications for the Environment Laboratory (Laboratoire d'Application de la Chimie à l'Environnement, LACE) in Villeurbanne (CNRS / Université Lyon-I joint lab).

Contacts :

Christian George
LACE, Villeurbanne.


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