Pollution characteristics, sources, and photochemical roles of ambient carbonyl compounds in summer of Beijing, China.

Ambient carbonyls are important precursors of radicals and ground-level ozone (O). In this study, sources, precursors, and impacts on radicals and O of carbonyls were investigated based on online observations of volatile organic compounds (VOCs) at an urban site in Beijing during June 2021. Carbonyls accounted for 36% and 42% of mixing ratios and OH reactivity for total measured VOCs, respectively. Formaldehyde was the most abundant carbonyl, with the mean level of 4.13 ± 2.28 ppb. Source apportionment results based on the multi linear regression (MLR) method suggested that secondary production contributed 41%, 25%, 36%, and 30% of formaldehyde, acetaldehyde, propanal, and acetone, respectively. Key precursors of carbonyls were then identified based on the calculation of their production rates. It was found that alkenes contributed 59%-80% of aldehydes production. Impacts of carbonyls on HO radicals (OH and HO) and O production were explored using a box model based on observations (OBM). Photolysis of HONO, formaldehyde, and O were the dominant primary sources of HO radicals during daytime of O pollution days, with average relative contributions of 52%, 28%, and 19% to the total primary production rate of HO, respectively. Aldehydes accounted for 32% (20% from formaldehyde) of average HO removal rates. The relative incremental reactivity (RIR) values of NO determined by the OBM were negative, suggesting that the O-VOCs-NO sensitivity was in the VOCs-limited regime. Using the observed concentrations of carbonyls as constraints of OBM, the absolute values of RIR for NO tended to increase but those for anthropogenic VOCs tended to decrease. Formaldehyde showed the largest RIR value for anthropogenic VOCs during O pollution days. These findings indicated the important impacts of carbonyls on O production and O-VOCs-NO sensitivity.