Investigation of long-term variation of atmospheric oxidation in background areas of China

The atmospheric oxidizing capacity (AOC) is closely related to the self-cleaning ability of the atmosphere in which the air pollutants were removed through reacting with oxidations such as OH radical. The level of OH radical is a dominant indicator to the AOC in clean regions characterized as low levels of NO_x which is another factor that influences AOC. Due to a lack of VOCs-related mechanisms in model simulation and high cost of the direct observations of OH radical, the long-term trend of OH radical in China is still unclear, especially under the circumstance of significant reduction of Chinese emissions in recent years. In this study, three methods based on a proxy gas CH₃CCl₃ from 5 regional background stations in China have been developed to investigate the long-term variation of OH radical in China. The concentration of OH radical in the background area of China is approximately (0.8±0.1)*10⁶ molecular/cm³, lower than the results in other background regions of the world. This could be explained by the larger depletion of OH radical in China due to the higher concentrations of polluted gases (i.e., NO_x, CO and CH₄). The different methods showed almost consistent results for the long-term trends of OH radical in China. From 2006 to 2017, the annual averaged OH concentration showed a slow downward but insignificant trend with the anomalous annual changes ranging from -0.1% to 0.15%. However, significant inter-annual fluctuations were also detected concurrently with a period about two years. This is consistent with the 2-3 years Quasi-biennial Oscillation (QBO) in the long-term variation of surface O₃ concentrations. These results provide the new insights into the annual variation of OH radical in China, which could help improve our understanding of the long-term characteristics of atmospheric oxidation in background areas of China.