Spatial variation of sources and photochemistry of formaldehyde in Wuhan, Central China

In August 2016, ambient formaldehyde (HCHO) and related parameters were measured simultaneously at a suburban site (Wujiashan, WJS), an urban site (Ziyang, ZY) and a roadside site (Hankou, HK) in Wuhan, Central China. The mixing ratios of HCHO at WJS (2.1 ± 0.2 ppbv) were comparable (p > 0.05) to those at ZY (2.2 ± 0.4 ppbv) and lower (p < 0.01) than those at HK (3.4 ± 0.5 ppbv). Secondary formation made the greatest contributions to ambient HCHO at WJS (61.9 ± 5.9%) and ZY (69.3 ± 7.3%), while vehicle exhaust (i.e., diesel and CNG exhaust, gasoline exhaust) (47.2 ± 10.4%) was the dominant HCHO source at HK. Vehicle exhaust was also the dominant primary source of HCHO at WJS and ZY, accounting for 16.0 ± 4.0%, and 21.9 ± 8.0%, respectively. Biogenic emissions made a non-negligible contribution at suburban site WJS (9.8 ± 3.2%). The precursors of the secondary HCHO were investigated using a photochemical box model. Alkenes and aromatics were found to be the most important contributors to secondary HCHO. The impact of observed HCHO on the production of hydroperoxy radical (HO₂) and ozone (O₃) were also assessed. The photochemical reactions of HCHO (i.e., the photolysis and OH-initiated oxidation of HCHO) accounted for 10.5 ± 0.6%, 10.6 ± 0.8% and 14.8 ± 0.9% to the formation of HO₂ at WJS, ZY and HK, respectively, and reducing HCHO led to a considerable decrease in O₃ formation. This study demonstrates the important roles of vehicle exhaust and secondary formation in the sources of HCHO, which have significant photochemical relevance. Attention should be paid to HCHO to control the photochemical pollution in Wuhan effectively.