Atmospheric oxidation capacity and its contribution tosecondary pollutants formation

Abstract

<p indent="0mm">Atmospheric oxidation capacity (AOC) is defined as the oxidation reaction rates of primary pollutants in the atmospheric chemical process, which is characterized by the oxidants concentration or the reaction rates. Atmospheric oxidation plays an important role in the formation of secondary pollutants, and works as an important indicator for the study of secondary pollution. Since 2013, the Chinese Government implemented unprecedented strident regulations to improve air quality. Although the overall levels of fine particulate matter (PM_2.5) have dropped considerably throughout China, it remains unclear how secondary pollutants have responded to the emission reductions in view of complex gas-precursor relationship and gas-particle interactions. Previous studies also reported the positive correlation between the AOC level and secondary pollutant concentrations. The updated Community Multi-scale Air Quality (CMAQ) model with the SAPRC 11 photo-chemical mechanism as well as the observation data was applied in this study to investigate the relationship of the secondary pollutants (ozone and secondary aerosols) and their precursors with the major oxidants (HO₂, OH and NO₃ radicals) in years 2013 and 2020 in China. And the process analysis tool was implemented in the CMAQ model to determine the key oxidants budget, which could better understand the primary sources and sinks of these oxidants. This analysis was able to calculate the chemical reaction rates at each integration time step. Radical source reactions were generally photolysis reactions, and the sink reactions removed radicals through the formation of stable products. The anthropogenic emissions were from Multiresolution Emission Inventory for China (MEIC) database, and the biogenic emissions were calculated from the Model of Emissions of Gas and Aerosols from Nature (MEGAN). The meteorology inputs were generated from the Weather Research and Forecasting (WRF) model. The CMAQ model predictions agree well with the observations, with most of the model performance within the criteria. The results show that the AOC level keeps identical from 2013 to 2020 throughout China. In the North China Plain (NCP) and the Pearl River Delta (PRD), the slightly increasing trends of AOC are even reported, which may aggravate the secondary pollution. The enhanced AOC level in the NCP is mainly from the reduced PM_2.5 concentration, which is regarded as the important sink of AOC. In addition, the concentration of major oxidants also represents different regional characteristics. The highest concentrations of HO_{<italic>x</italic>} and NO₃ radical are reported in the Sichuan Basin (SCB) and NCP, respectively. The ozone photolysis and the formation of HNO₃ (OH+NO₂) are reported as the most important AOC primary sources and sinks, respectively. In the NCP, the HONO photolysis is found as an important AOC source, which is consistent with previous studies. The AOC level increases significantly during the high ozone episode (increasing rate up to 475%). Also, the high level of AOC is corresponding to the significant increase of the secondary organic aerosols, indicating its important role in the formation of secondary pollutants. In summer, the reduction of NO_{<italic>x</italic>} emissions can reduce the AOC level. Meanwhile, a more complex response between the NO_{<italic>x</italic>} emissions and AOC level is found in winter. More rational emission reduction ratio of NO_{<italic>x</italic>} and volatile organic compounds (VOCs) should be applied to better control the AOC in China. In the future, more effective AOC control strategies should be established to reduce the PM_2.5 and O₃ coordinately in the 14th Five-Year Plan of China.