Impact of formaldehyde on ozone formation in Central China: Important role of biogenic emission in forest region

Formaldehyde (HCHO) is an important source for driving tropospheric ozone (O₃) formation. This study investigated the combined effects of anthropogenic and biogenic emission on O₃ formation in the Guanzhong Basin (GZB), Central China, providing useful information into the mechanisms of O₃ formation due to the interaction between anthropogenic and biogenic volatile organic compounds (VOCs). A severe O₃ pollution episode in summer of 2017 was simulated using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to examine the impacts of ambient HCHO on ground-level O₃. Results showed secondary HCHO dominated ambient levels, peaking in the afternoon (up to 86 %), while primary emissions contributed 14 % on average. This enhanced O₃ production by 7.7 % during the morning rush hour and 24.3 % in the afternoon. In addition, HCHO concentration peaked before that of O₃, suggesting it plays significant role in O₃ formation. Biogenic emission oxidation contributed 3.1 μg m⁻³ (53.1 %) of HCHO and 5.2 pptv (40.1 %) of hydroperoxyl radicals ([rad]HO₂) in average urban areas, where the downwind regions of the forests had high nitrogen oxides (NO_x) levels and favorable conditions for O₃ production (17.3 μg m⁻³, 20.5 %). In forested regions, sustained isoprene oxidation led to elevated oxidized VOCs including HCHO and acetaldehyde downwind, which practiced further photolysis of O₃ formation with anthropogenic NO_x in urban areas. Sensitivity experiments recommend controlling industrial and traffic NO_x emissions, with regional joint prevention and regulation, which are essential to reduce O₃ pollution.