A neglected pathway for the accretion products formation in the atmosphere.

Highly oxygenated organic molecules (HOM) formed by the autoxidation of α-pinene initiated by OH radicals play an important role in new particle formation. It is believed that the accretion products, ROOR´, formed by the self- and cross-reaction of peroxy radicals (RO + R'O reactions), have extremely low volatility and are more likely to participate in nucleation. However, the mechanism of ROOR´ formation has not been fully demonstrated by experiment or theoretical calculation. Herein, we propose a novel mechanism of RO reacting with α-pinene (RO + α-pinene reactions) that have much lower potential barriers and larger rate constants than the reaction of RO with R'O, which explains the ROOR´ formation found in the mass spectrometry experiments. The ROOR´ resulting from the reaction of RO with α-pinene can produce HOM dimers and trimers with a higher oxygen-to‑carbon (O/C) ratio through a autoxidation chain. We also demonstrated that the presence of NO and HO radical will reduce the RO concentration, but cannot completely inhibit the formation of HOM monomers and ROOR´. Even if one or both of RO radicals are acyl peroxy radicals (RC(O)O), the potential barriers of the reactions between RC(O)O and α-pinene (RC(O)O + α-pinene reactions) are lower than that of RO reacting with RC(O)O (RO + RC(O)O reactions) or RC(O)O self-reactions (RC(O)O + RC(O)O reactions). The current work revealed, for the first time, a mechanism of RO/RC(O)O reacting with α-pinene in the atmosphere, which provides new insight into the atmospheric chemistry of accretion products as SOA precursors.