Contributions of local emissions and regional background to summertime ozone in central China

Source contributions and regional transport of maximum daily average 8-h (MDA8) O₃ during a high O₃ month (June 2019) in Henan province in central China are explored using a source-oriented Community Multiscale Air Quality (CMAQ) model. The monthly average MDA8 O₃ exceeds ∼70 ppb in more than half of the areas and shows a clear spatial gradient, with lower O₃ concentrations in the southwest and higher in the northeast. Significant contributions of anthropogenic emissions to monthly average MDA8 O₃ concentrations of more than 20 ppb are predicted in the provincial capital Zhengzhou, mostly due to emissions from the transportation sector (∼50%) and in the areas in the north and northeast regions where industrial and power generation-related emissions are high. Biogenic emissions in the region only contribute to approximately 1-3 ppb of monthly average MDA8 O₃. In industrial areas north of the province, their contributions reach 5-7 ppb. Two CMAQ-based O₃-NO_x-VOCs sensitivity assessments (the local O₃ sensitivity ratios based on the direct decoupled method and the production ratio of H₂O₂ to HNO₃) and the satellite HCHO to NO₂ column density ratio consistently show that most of the areas in Henan are in NO_x-limited regime. In contrast, the high O₃ concentration areas in the north and at the city centers are in the VOC-limited or transition regimes. The results from this study suggest that although reducing NO_x emissions to reduce O₃ pollution in the region is desired in most areas, VOC reductions must be applied to urban and industrial regions. Source apportionment simulations with and without Henan anthropogenic emissions show that the benefit of local anthropogenic NO_x reduction might be lower than expected from the source apportionment results because the contributions of Henan background O₃ increase in response to the reduced local anthropogenic emissions due to less NO titration. Thus, collaborative O₃ controls in neighboring provinces are needed to reduce O₃ pollution problems in Henan effectively.