Chlorine atoms (Cl) are more reactive in the atmosphere than other oxidants. In recent years, researchers have seen increased concentrations of CI precursors in inland areas. The atmospheric oxidation reaction caused by Cl is becoming more and more important.

Methylacrolein (MACR) is a key intermediate in atmospheric oxidation of biogenic isoprene. The oxidation and degradation of MACR play an essential role in the formation of atmospheric ozone and secondary organic aerosols.

A team of researchers led by Prof. ZHANG Weijun from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS), recently investigated Cl-initiated oxidation reactions of MACR under nitrogen oxides (NOx)-free conditions, by using a home-made photoionization time-of-flight mass spectrometer complemented with theoretical calculations.

In this study, the researchers used a microwave discharge flow tube reactor to investigate the oxidation reaction of Cl + MACR. Key species such as intermediate radicals and products during the oxidation process were detected online and confirmed in photoionization mass spectra.

The results showed that the reaction of MACR with Cl atoms could generate the C₄H₅O and C₄H₆OCl radicals via hydrogen abstraction and the addition of Cl atom to the C=C double bond, respectively.

"This is the first time that the transient C₄H₅O and C₄H₆OCl radicals are experimentally detected here," said LIN Xiaoxiao, first author of the study.

The C₄H₅O and C₄H₆OCl radicals could react with oxygen to produce the corresponding peroxy radicals C₄H₅OO₂ and C₄H₆OClO₂. Under low NOx conditions, these peroxy radicals would perform bimolecular reactions with themselves and the HO₂ radicals.

Combined with theoretical calculation, the specific products obtained can be identified in the photoionization mass spectrometry.

This work elucidates the chemical mechanisms of Cl-initiated oxidation of MACR, which is helpful to understand the chemical behavior of MACR in the atmosphere.

Fig. 1. Photoionization mass spectra acquired (a) without O₂ and (b) adding O₂ into the fast flow tube. (Image by LIN Xiaoxiao) 

Fig. 2. Reaction mechanisms of Cl-initiated oxidation of MACR under NOx free conditions. (Image by LIN Xiaoxiao)