Significance of carbonyl compounds to photochemical ozone formation in a coastal city (Shantou) in eastern China

Highlights: • The significance of carbonyls to O₃ formation in urban Shantou was confirmed. • O₃ formation was VOC-limited, and specifically most sensitive to carbonyls. • Photolysis of carbonyls dominated the primary source of HO₂ and RO₂ radicals. • Carbonyls accounted for 37% of the peak net production rate of O₃. Carbonyl compounds are ubiquitous in the troposphere, yet their contributions to ambient ozone (O₃) formation have rarely been quantified in China. To better understand their roles in O₃ pollution, a field campaign was conducted at an urban site of Shantou, a coastal city in eastern China, during 7th–29th October 2019. Seven carbonyls were quantified (average ± standard deviation: 14.42 ± 3.05 ppbv), among which formaldehyde (4.12 ± 1.02 ppbv), acetaldehyde (1.57 ± 0.30 ppbv), acetone (7.55 ± 2.10 ppbv), and methyl ethyl ketone (0.94 ± 0.28 ppbv) were the most abundant species. Relative incremental reactivity (RIR) analysis indicated that O₃ formation in Shantou was VOC-limited, specifically most sensitive to carbonyls, and formaldehyde showed the largest RIR values in terms of individual species. Budgets of O₃ and RO_x (OH, HO₂, and RO₂) radicals were elucidated with a chemical box model. Carbonyls played a vital role in both the primary formation and recycling of the RO_x; more than 80% of the primary source of HO₂ and RO₂ came from photolysis of formaldehyde and other oxygenated VOCs. Zero-out sensitivity studies showed that the seven measured carbonyls accounted for 37% of the peak net O₃ production rate, mainly by affecting the concentrations of HO₂ and RO₂. These results highlight the significance of carbonyls, especially formaldehyde, to photochemical O₃ formation, and carbonyls should be paid more attention to mitigate the worsening O₃ pollution in China.