Modelling of peroxyacetyl (PA) radicals in contrasting environments: precursors, reaction pathways and their contributions to peroxyacetyl nitrate (PAN) and ozone (O3) formation

Peroxyacetyl (PA) radicals are one of the critical intermediates in the atmospheric oxidation and combustion of VOCs, dominating the secondary formation of peroxyacetyl nitrate (PAN) and also promoting ozone (O₃) formation. This study simulated the formation pathways of PA radicals from their precursors and evaluated their contributions to PAN and O₃ formation in urban, rural, and mountainous background areas in southern China with the application of an observation-based photochemical box model with master chemical mechanism (PBM-MCM).

It was found that PA formation at the mountainous background site was dependent on both volatile organic compounds (VOCs) and NO_x precursors (transition regime). In contrast to dominated acetaldehyde oxidation in previous urban and rural research, PA formation at the mountainous background site was primarily formed by methylglyoxal (38%), acetaldehyde (28%), other radicals (20%), and other oxygenated VOCs (13%).

Methylglyoxal oxidation was the major contributor to PA formation in urban and rural areas in southern China due to the relatively high levels of isoprene and aromatics in this region. Except for biogenic sources, xylene isomers and C₄-C₅ alkenes, largely from vehicle exhaust and solvent usage, were identified as the predominant contributors to PA formation in this region.

At last, PAN was net formed through the reaction of PA and NO₂ in most urban, rural, and mountainous background areas in southern China, while it was thermal decomposed in coastal urban areas in the cold season due to the high level of PAN from regional transport, which released NO₂ and PA radicals and promoted the local O₃ formation. Overall, this study deepens our understanding of PA radical chemistry and provides valuable insights into secondary PAN and O₃ formation control in contrasting environments.