Photochemical impacts of atmospheric carbonyl compounds on ozone formation and radical chemistry in urban Shanghai.

Carbonyl compounds have been recognized as important precursors for photochemical ozone (O3) and secondary organic aerosols; however, their quantitative contributions to urban photochemistry remain poorly constrained due to limited field measurements and chemical speciation. This study measured 47 atmospheric carbonyl compounds at an urban site in Shanghai during the summer of 2021 using a newly developed ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS) method. The total carbonyl concentrations varied from 1.85 ppbv to 13.4 ppbv, with an average of 5.68 ppbv, dominated by aliphatic mono-carbonyls (92 %). Among all measured VOC groups, carbonyls contributed most to ozone formation potential (OFP, 48.3 %) and OH reactivity (40.6 %), surpassing the contributions from aromatics, alkenes and alkanes. An observation-based box model was employed to investigate the in-situ photochemistry of carbonyls and their impact on O3 and radical production. Relative incremental reactivity (RIR) analysis suggested a VOC-limited O3 formation regime in urban Shanghai, with carbonyls exhibiting much higher RIR values than alkenes and aromatic hydrocarbons. Carbonyls photolysis was identified as the primary pathway for ROx radical production (ROx=OH+HO2+RO2), with formaldehyde being the main contributor (70.1 %) for HO2 radicals. Notably, this study revealed for the first time the dominant role (57.4 %) of 2,3-butanedione (diacetyl) in RO2 radical production in an urban environment, emphasizing the need for more comprehensive measurements of di-carbonyls. The findings underscore the critical role of carbonyl compounds in O3 formation and radical chemistry and highlight the importance of targeting various carbonyls to effectively mitigate photochemical air pollution in urban areas.