基于铵根-电离质谱法检测VOCs氧化中的RO_(2)自由基及其他产物

Detection of RO_(2) radicals and other products in the oxidation of VOCs using NH_(4)^(+) chemical ionization mass spectrometry

准确识别和定量挥发性有机化合物(volatile organic compounds,VOCs)氧化中间产物(包括闭壳产物和有机过氧自由基RO_(2)),对于厘清其降解机理和实现二次物种精准模拟与精细化管控十分关键.本研究基于最新开发的质子转移反应-飞行时间质谱,结合高选择性的铵根离子加成模式,发展了铵根-电离质谱法(ammonium chemical ionization mass spectrometry,NH_(4)^(+)-CIMS),成功实现了对不同种类的RO_(2)自由基以及含氧VOCs物种的高灵敏度检测.通过自主搭建的标定系统,实现了基于质谱法测量RO_(2)自由基的直接标定,有效降低了由于灵敏度替代造成的测量不确定性.该套系统首先应用于近实际大气条件下α-蒎烯臭氧氧化体系的研究,共检测到13种一代反应产物,包括5种RO_(2)自由基和8种闭壳物种,其中来自OH氧化反应生成的过氧自由基C_(10)H_(17)O_(3)占比最大,臭氧氧化α-蒎烯生成的过氧自由基C_(10)H_(15)O_(4)发生多步自氧化及双分子反应,生成含更高氧数的过氧自由基和闭壳产物,证实了α-蒎烯自氧化反应通道的重要性.现有大气化学机理可基本捕捉α-蒎烯臭氧化反应的产物分布,但不同氧化通道产物的相对占比受模型设置的反应速率和反应通道分支比影响存在较大不确定性.此外,氢摘取通道可能对OH自由基氧化α-蒎烯具有重要贡献,但机制待进一步探索.本研究展示的NH_(4)^(+)化学电离质谱技术,具有高灵敏度、高分辨率、低检测限等优点,特别是对中等氧化程度的闭壳产物和RO_(2)自由基的检测.未来,这套系统有望在更复杂的大气环境中揭示VOCs的演化规律和降解机制,为大气环境保护提供重要科学依据.

Volatile organic compounds(VOCs) in the atmosphere undergo oxidation by atmospheric oxidants such as hydroxyl radical(OH), ozone(O_(3)), and nitrate radical(NO_(3)) to form organic peroxy radicals(RO_(2)). The RO_(2)radicals then degrade into secondary pollutants such as O_(3)and fine particulate matter, which would have significant impacts on human health and ecological environment. Accurately identifying and quantifying the oxidation intermediates of VOCs(including closedshell products and RO_(2)radicals) are crucial for elucidating the degradation mechanisms of VOCs, achieving precise simulation of secondary species, and implementing refined control measures.Chemical ionization mass spectrometry(CIMS) is currently the most widely used technique for the simultaneous measurement of RO_(2)radicals and other oxidation products. However, the most popular CIMS technique(e.g., chemical ionization atmospheric pressure interface time-of-flight mass spectrometry, CI-APi-TOF) has limited measurement range which primarily focuses on the measurement of highly oxygenated compounds. In this study, a highly selective ammonium ion-adduct mode based on the latest proton transfer reaction-time-of-flight mass spectrometry was developed named NH_(4)^(+)-CIMS(ammonium chemical ionization mass spectrometry), enabling high-sensitivity detection of various RO_(2)radicals and oxygenated VOCs. Additionally, we established calibration systems for closed-shell products and RO_(2)radicals,respectively. The calibration system achieved the direct calibration of RO_(2)radicals based on CIMS for the first time,effectively reducing measurement uncertainties arising from sensitivity substitutions.This system was first applied to study the gas-phase ozonolysis of α-pinene in our laboratory under near-real atmospheric conditions. In the first-generation oxidation products of α-pinene ozonolysis, a total of thirteen reaction products were detected, including five RO_(2)radicals and eight closed-shell species containing 3 to 7 oxygen atoms. The peroxy radical C_(10)H_(17)O_(3), generated from OH chemistry, exhibited the highest proportion among the oxidation products.The peroxy radical C_(10)H_(15)O_(4), produced from direct ozone oxidation, underwent multi-step autoxidation and bimolecular reactions in subsequent reactions, confirming the importance of the α-pinene autoxidation pathway. The distribution of products from α-pinene ozonolysis remained unchanged with variations in precursor concentrations. The current atmospheric chemistry mechanisms could generally capture the product distribution of the α-pinene ozonolysis reaction.However, there is still significant uncertainty regarding the relative proportions of products from different oxidation pathways, influenced by the reaction rates and branching ratios set in the model. Additionally, the hydrogen abstraction pathway might play a crucial role in the oxidation of α-pinene by OH radicals, but the specific mechanisms require further exploration. The presence of isoprene altered the distribution of oxidation products in the system, potentially resulting in an increased formation of carbonyl-containing compounds.These results indicate that NH4^(+)-CIMS is a promising measurement technique with high sensitivity, high resolution, and low detection limits, especially for moderately oxidized closed-shell products and RO_(2)radicals. With further advancements in the technology, it can be utilized to investigate reactions under more complex atmospheric conditions, aiding in elucidating the evolution patterns, degradation mechanisms, and environmental impacts of VOCs in diverse atmospheric environments.