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

Abstract

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 ROx (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 ROx; 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.