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) formation. This study investigated the combined effects of anthropogenic and biogenic emission on O formation in the Guanzhong Basin (GZB), Central China, providing useful information into the mechanisms of O formation due to the interaction between anthropogenic and biogenic volatile organic compounds (VOCs). A severe O 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. Results showed secondary HCHO dominated ambient levels, peaking in the afternoon (up to 86 %), while primary emissions contributed 14 % on average. This enhanced O production by 7.7 % during the morning rush hour and 24.3 % in the afternoon. In addition, HCHO concentration peaked before that of O, suggesting it plays significant role in O formation. Biogenic emission oxidation contributed 3.1 μg m (53.1 %) of HCHO and 5.2 pptv (40.1 %) of hydroperoxyl radicals (HO) in average urban areas, where the downwind regions of the forests had high nitrogen oxides (NO) levels and favorable conditions for O production (17.3 μg m, 20.5 %). In forested regions, sustained isoprene oxidation led to elevated oxidized VOCs including HCHO and acetaldehyde downwind, which practiced further photolysis of O formation with anthropogenic NO in urban areas. Sensitivity experiments recommend controlling industrial and traffic NO emissions, with regional joint prevention and regulation, which are essential to reduce O pollution.