Acta Phys. -Chim. Sin. ›› 2015, Vol. 31 ›› Issue (12): 2259-2268.doi: 10.3866/PKU.WHXB201510152


The Atmospheric Oxidation Mechanism of o-Xylene Initiated by Hydroxyl Radicals

Shan-Shan. PAN1,Li-Ming. WANG1,2,*()   

  1. 1 School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China
    2 Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou 510006, P. R. China
  • Received:2015-08-05 Published:2015-12-04
  • Contact: Li-Ming. WANG
  • Supported by:
    the National Natural Science Foundation of China(21177041, 21477038);Public Welfare Project of Ministry ofEnvironmental Protection of China(201409019)


The atmospheric oxidation mechanism of o-xylene (oX) initiated by hydroxyl (OH) radicals has been investigated by using quantum chemistry, transition state theory, and unimolecular theory (RRKMME) calculations. Molecular structures of reactants, transition states, and products are optimized at M06-2X/6-311++G(2df, 2p) level, and the electronic energies are calculated at the ROCBS-QB3 level. The classical transition state theory is employed to predict the rates or rate constants for all the reaction steps as well as the branching ratios of the reaction pathways. RRKM-ME calculations are employed to explore the pressure-dependence of the reaction kinetics. Under atmospheric conditions, the oxidation of o-Xylene is dominated by OH addition to the C1 and C3 positions, forming adducts oX-1-OH (R1) and oX-3-OH (R3), which will readily react with atmospheric oxygen. The reactions of R1 and R3 with O2 can proceed by irreversible H-abstraction to dimethylphenols (R3 only), or by reversible addition to form bicyclic radicals, which recombine with atmospheric oxygen to form bicyclic peroxy radicals (BPRs). BPRs will react with NO and/or HO2 in the atmosphere, forming organonitrate, hydroperoxides (ROOH), and bicyclic alkoxy radicals (BARs), of which the BARs eventually transfer to the final products, including biacetyl, butenedial, methylglyoxal, 4-oxo-2-pentenal, epoxy-2, 3-butenedial, and a small amount of glyoxal. The products ROOH and methylglyoxal are considered to contribute to the formation of secondary organic aerosols. A new oxidation mechanism of oX in the atmosphere is proposed, based on the current theoretical predictions and previous experimental measurements, and the predicted product yields under high NO conditions are compared with previous experimental measurements. The effect of temperature on the oxidation mechanism is also discussed.

Key words: o-Xylene, Atmospheric oxidation mechanism, Unimolecular Reaction, Temperature effect