Exploring the Crucial Role of Atmospheric Carbonyl Compounds in Regional Ozone heavy Pollution: Insights from Intensive Field Observations and Observation-based modelling in the Chengdu Plain Urban Agglomeration, China

Abstract. Gaseous carbonyl compounds serve as crucial precursors and intermediates in atmospheric photochemical reactions, significantly contributing to ambient ozone formation. To investigate the impact of gaseous carbonyls on regional ozone pollution, simultaneous field observations and observation-based modelling of ambient carbonyls were conducted at nine sites within the Chengdu Plain Urban Agglomeration (CPUA), China during August 4–18, 2019, when three episodes of regional heavy ozone pollution occurred across eight cities within CPUA. Throughout the study, the total mixing ratios of 15 carbonyls ranged from 10.70 to 35.18 ppbv, in which formaldehyde (48.1 %), acetone (19.9 %), and acetaldehyde (17.5 %) were most abundant within the CPUA. Ambient levels of carbonyls and ozone showed some positive correlations in space (especially pronounced around Chengdu in both northern and southern directions) and in diurnal variations with higher concentrations of carbonyls during ozone pollution episodes. Photochemical reactivity analysis emphasized the significant contributions of carbonyls, especially formaldehyde and acetaldehyde, to ozone formation. The ozone formation sensitivity for sites experiencing severe ozone pollution were classified as VOCs-limited regime, while others were categorized as transitional regime. Local primary emissions, mutual air transportation among cities within the CPUA and photochemical secondary processes were recognized to contribute significantly to the production or the contamination of carbonyls in ambient air, with alkenes and alkanes being important secondary precursors of carbonyls. This study highlights the pivotal role of carbonyls in heavy ozone pollution within the CPUA, China, providing valuable scientific insights to guide the development of effective countermeasures for regional ozone pollution control in the future.