Cl引发α-蒎烯的大气氧化机制及动力学

Atmospheric oxidation mechanism and kinetics ofα-pinene initiated by·Cl

蒎烯是大气中含量最丰富的单萜烯类挥发性有机化合物,与氯自由基(·Cl)反应是其重要的转化途径.然而,·Cl引发α-蒎烯大气氧化的分子机制尚不清楚.本研究采用量子化学计算与微观动力学模拟相结合的方法,研究了·Cl与α-蒎烯的引发反应以及引发反应生成的主要α-蒎烯自由基的后续转化机制与动力学.结果发现,·Cl反式加成到α-蒎烯的C2位置生成加成中间体IM1-9-a是最可行的反应途径.在298 K和1.013×10^(5)Pa条件下,计算得到·Cl与α-蒎烯的反应速率常数(kCl)为3.1×10^(-10)cm^(3)·molecule^(-1)·s^(-1),与kCl的实验值(4.6±1.3)×10^(-10)cm^(3)·molecule^(-1)·s^(-1)一致.生成的IM1-9-a进一步与O_(2)反应生成过氧自由基IM3-1-a,在2.5×10^(9)cm^(-3)(100 ppt)NO和1.25×10^(9)cm^(-3)(50 ppt)HO_(2)·条件下,IM3-1-a后续主要与NO和HO_(2)·反应生成有机硝酸酯(C_(10)H_(16)ClNO_(3)和C_(10)H_(16)ClNO_(4))、氢过氧化物(C_(10)H_(17)ClO_(2)和C_(10)H_(17)ClO_(3))和烷氧自由基(C10H16ClO_(2)·).生成的闭壳层产物与母体α-蒎烯相比具有较高的O∶C比,可能具有较低的蒸气压,进而贡献二次有机气溶胶.本研究结果可为全面评价α-蒎烯的大气化学反应对大气质量的影响提供理论支撑.

α-pinene is the most abundant monoterpene volatile organic compounds in the atmosphere,and reaction with chlorine radicals(·Cl)has been considered an important sink for it.However,the atmospheric oxidation mechanism of·Cl initiated reactions ofα-pinene remains unknown.Herein,the initial reaction ofα-pinene with·Cl,and subsequent reactions of resultingα-pinene-radicals,were investigated by a combination of quantum chemical calculations and kinetics modeling.The results indicate that·Cl anti-addition to the C2position ofα-pinene forming intermediate IM1-9-ais the dominant pathway for the initial·Cl+α-pinene reaction.At 298 K and 1.013×10^(5)Pa,the calculated overall reaction rate constant(kCl)for the·Cl+α-pinene reaction is 3.1×10^(-10)cm^(3)·molecule^(-1)·s^(-1),which is close to the available experimental value(4.6±1.3)×10^(-10)cm^(3)·molecule^(-1)·s^(-1).The favorably formed I-M1-9-afurther react with O_(2)to produce peroxy radicals IM3-1-a,which mainly react with NO and HO_(2)·under 2.5×10~9 cm-(~3100 ppt)NOand 1.25×10~9 cm^(3)(50 ppt)HO_(2)·conditions,to form organonitrates(C_(10)H_(16)ClNO_(3)/C_(10)H_(16)ClNO_(4)),hydroperoxide products(C_(10)H_(17)ClO_(2)/C_(10)H_(17)ClO_(3))and alkoxy radicals(C_(10)H_(16)ClO_(2)·).The formed closed-shell products have high oxygen-to-carbon ratios compared to the parentα-pinene and therefore are expected to have lower vapor pressures,which can contribute to the formation of secondary organic aerosol.Overall,our work provides theoretical support for a comprehensive evaluation of the impacts of atmospheric chemical reactions ofα-pinene on air quality.