"The field study has exposed that state-of-the-art models of atmospheric chemistry do not provide an adequate description of the tropospheric oxidation efficiency in the chemical environment of the Pearl River Delta," Franz Rohrer of Jülich Research Centre, Germany, told environmentalresearchweb. "The observed abundance of atmospheric OH radicals is much larger than can be explained by photochemical theory at conditions of high loads of volatile organic compounds (VOCs) and relatively small NO concentrations. As a result, the degradation of atmospheric trace gases and pollutants is significantly faster than expected, but apparently with less formation of tropospheric ozone than previously thought."
Rohrer and colleagues from Peking University, the Research Center for Environmental Changes in Taipei, Shanghai Jiaotong University, all in China, and Ibaraki University and the University of Tokyo, Japan, believe that this indicates the existence of a pathway for regenerating OH in the absence of NO.
Traditionally it's thought that hydroxyl radicals are formed by the photolysis (splitting in sunlight) of ozone, nitrous acid and hydrogen peroxide. The radicals then react with pollutants such as carbon monoxide and volatile organic compounds to form hydroperoxyl (HO2); in the case of volatile organic compounds the hydroperoxyl is formed via a subreaction with NO. Hydroxyl radicals are regenerated by reaction of the hydroperoxyl with NO, a step that also creates NO2, which splits in sunshine to produce ozone.
This study, however, indicates that the hydroxyl regeneration step is taking place independently of NO, with less associated production of ozone. The exact mechanism for this reaction isn't yet clear.
The study took place about 60 km northwest of Guangzhou City and formed part of the Chinese Program of Regional Integrated Experiments on Air Quality over the Pearl River Delta of China (PRIDE-PRD). According to Rohrer, this project aims to study processing of gaseous pollutants and aerosols in the region, to investigate impacts on regional and global atmospheric composition and to develop strategies for air pollution control in China and other countries.
"The Pearl River Delta represents one of the major industrialized regions in Asia and includes megacities like Guangzhou and Hong Kong," he said. "Owing to the fast growing economy and urbanization, air pollution has greatly increased in this region during the last decade."
Now the team plans to investigate the chemistry responsible for the unexplained high OH concentrations in the Pearl River Delta using dedicated experiments in the SAPHIR large outdoor atmosphere simulation chamber at the Jülich Research Centre. The researchers will also carry out further field campaigns "in order to improve the general applicability of atmospheric chemistry models for predictions of air quality and climate".
The researchers reported their work in Sciencexpress.