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 (O-3) formation. This study investigated the combined effects of anthropogenic and biogenic emission on O-3 formation in the Guanzhong Basin (GZB), Central China, providing useful information into the mechanisms of O-3 formation due to the interaction between anthropogenic and biogenic volatile organic compounds (VOCs). A severe O-3 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 O-3. Results showed secondary HCHO dominated ambient levels, peaking in the afternoon (up to 86 %), while primary emissions contributed 14 % on average. This enhanced O-3 production by 7.7 % during the morning rush hour and 24.3 % in the afternoon. In addition, HCHO concentration peaked before that of O-3, suggesting it plays significant role in O-3 formation. Biogenic emission oxidation contributed 3.1 mu g m(-3) (53.1 %) of HCHO and 5.2 pptv (40.1 %) of hydroperoxyl radicals (center dot HO2) in average urban areas, where the downwind regions of the forests had high nitrogen oxides (NOx) levels and favorable conditions for O-3 production (17.3 mu g 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 O-3 formation with anthropogenic NOx in urban areas. Sensitivity experiments recommend controlling industrial and traffic NOx emissions, with regional joint prevention and regulation, which are essential to reduce O-3 pollution.