Ambient carbonyls were continuously observed in the field during a heavy ozone pollution episode in Chengdu,China from August 4 to August 19,2019,and the pollution character-istics,atmospheric photochemical reactivity,human health risk,and sources of carbonyls were analyzed.Fifteen carbonyls were quantified with average total mixing ratios of 20.38 ppbv Formaldehyde (9.86 ppbv),acetone (4.41 ppbv),and acetaldehyde (3.57 ppbv) were the three most abundant carbonyls.During the heavy ozone pollution episode,the concentra-tion of carbonyls was found to be higher on pollution days than on the clean days,and rela-tively higher in the daytime,especially at noon on the pollution days.This was influenced by the intensity of photochemical reactions and precipitation.The “weekend effect” with the concentration of carbonyls was higher on the weekends than on the weekdays was pointed out.Formaldehyde,acetaldehyde and hexaldehyde were the dominant oxidative species during the observation.The carcinogenic and non-carcinogenic risk values of formaldehyde and acetaldehyde were higher on pollution days than on clean days,and these values were higher compared with those of other cities in China and abroad.Long-term exposure to these compounds should therefore be avoided.Diagnostic ratios and correlation analysis together with backward trajectory analysis showed that primary emission and secondary formation accounted 66％-76％ and 24％-34％ of carbonyls in Chengdu,respectively,with primary emission being the main sources of carbonyls,and carbonyls from the surrounding cities and emission from natural sources also had a significant contribution to the carbonyls in Chengdu.