Impact of reversible and irreversible gas-particle partitioning of atmospheric carbonyl compounds on oxalic acid formation and δ¹ ³C distribution in Northwest China.

The gas-particle partitioning of carbonyl compounds through reversible and irreversible pathways constitutes a critical route for secondary organic aerosol formation. This study simultaneously observed distribution of formaldehyde (FA), acetaldehyde (AA), glyoxal (GLY), and methylglyoxal (MGLY) in both gas and particle phases, as well as concentration and stable carbon isotope (δ¹³C) of oxalic acid. The results show that in the reversible partitioning process, the carbonyl compounds during clean period are mainly partitioned into organic phase, and the salt-in effect exhibits kinetic limitations. However, they are mainly partitioned into aerosol liquid water during haze periods with lower partitioning coefficients, and there is a significant salt-in effect. The carbonyls present different irreversible partitioning pathways to contribute oxalic acid formation in haze periods. In Haze I (O3: 24 ± 11 ppb), the carbonyl compounds are mainly partitioned into aqueous phase and reacted with water to form polymers or undergoing self-polymerization. The large multifunctional compounds decomposed to yield oxalic acid, leading to progressive enrichment of δ¹ ³C. In Haze II (O3: 41 ± 13 ppb), the elevated atmospheric oxidation capacity promoted aqueous-phase oxidation of the carbonyls to generate intermediate products and ultimately forming oxalic acid. Under these conditions, oxalic acid-C exhibited a depleted δ¹ ³C signal.