Despite their crucial roles in health and climate concerns, the gas-particle partitioning of carbonyl compounds is poorly characterized in the ambient atmosphere. In this study, we investigate their partitioning by simultaneously measuring six carbonyl compounds (formaldehyde, acetaldehyde, acetone, propionaldehyde, glyoxal, and methylglyoxal) in the gas and particle phase at an urban site in Beijing. The field-derived partitioning coefficients ( K_p^f) are in the range of 10^-5-10^-3 m³ μg^-1, and the corresponding effective Henry's law coefficients ( K_H^f) should be 10⁷-10⁹ M atm^-1. The Pankow's absorptive partitioning theory and Henry's law both significantly underestimate concentrations of particle-phase carbonyl compounds (10⁵-10⁶ times and >10³ times, respectively). The observed "salting-in" effects only partially explain the enhanced partitioning to particles, which is approximately 1 order of magnitude. The measured K_p^f values are higher at low relative humidity, and the overall effective vapor pressure of these carbonyl species are lower than their hydrates, indicating that carbonyl oligomers potentially formed in highly concentrated particle phase. The reaction kinetics of oligomer formation should be included if applying Henry's law to low-to-moderate relative humidity, and the high partitioning coefficients observed need to be proved by further field and laboratory studies. These findings provide deeper insights into the formation of carbonyl secondary organic aerosols in the ambient atmosphere.

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