Published online 24 June 2009
Hydroxyl (OH) radicals are 'cleaners' of the atmosphere: they react with air pollutants, such as carbon monoxide and volatile organic compounds, to produce hydroperoxy (HO2) and organic peroxy (RO2) radicals in the troposphere. The reaction of HO2 or RO2 with nitrogen monoxide (NO) regenerates the OH radicals, but also produces ozone — a major component of photochemical smog.
Now, an international team led by Yuanhang Zhang at Peking University in Beijing, China, and Franz Rohrer at the Institute for Chemistry and Dynamics of the Geosphere in Jülich, Germany, has found evidence of an alternative regeneration pathway. They measured the concentrations and turnover rates of OH and HO2 radicals in the air of Pearl River Delta, and found particularly high OH concentrations during some parts of the day.
The researchers performed 'box-model' calculations to simulate the production and loss rates of OH and HO2 radicals. The box model produced an accurate description of HO2 concentrations for the entire day, but could match the behaviour of OH concentrations only in the early morning when NO concentrations were high. For most of the day, OH concentrations measured three to five times higher than the calculated predictions.
The findings suggest that an additional mechanism might be at play in OH radical regeneration. The researchers have not yet elucidated the exact mechanism, but it most likely does not involve the formation of ozone — thus offering a better way to clean up the atmosphere.