Multi-generation OH oxidation as a source for highly oxygenated organic molecules from aromatics
Multi-generation OH oxidation as a source for highly oxygenated organic molecules from aromatics
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DOI：
10.5194/acp-20-515-2020
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发表时间：
2020-01-15
影响因子：
6.3
通讯作者：
Ehn, Mikael
中科院分区：
文献类型：
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作者：
Garmash, Olga;Rissanen, Matti P.;Ehn, Mikael
最近的研究已经认识到大气中的高含氧有机分子（HOM）在二次有机气溶胶（SOA）的形成中起着重要作用。大量的研究集中在HOM的形成从氧化的生物排放的单萜。然而，HOM的形成，从人为蒸汽到目前为止受到的关注要少得多。以前的研究已经确定了芳香族挥发性有机化合物（VOCs）的SOA形成的重要性。在这项研究中，我们研究了几种芳香族化合物，苯（C6 H6），甲苯（C7 H8），萘（C10 H8），为他们的潜力，形成HOMs后与羟基自由基（OH）反应。我们进行了流管实验与所有三种挥发性有机化合物，并专注于详细的苯HOM的形成在Julich工厂大气室（JPAC）。在JPAC中，我们还研究了HOMs对NOx和种子气溶胶的响应。使用基于硝酸盐的化学电离质谱仪（CI-APi-TOF），我们观察到的HOMs的形成在流动反应器中的苯氧化从第一OH攻击。然而，在甲苯和萘的氧化中，其以较低浓度注入，多代OH氧化似乎影响HOM组合物。我们对JPAC中的苯系统进行了更详细的测试，这使得研究停留时间更长。结果表明，在我们的实验条件下，苯HOM的表观摩尔产率在4.1%至14.0%之间变化，具有强烈的依赖于OH浓度，表明大多数观察到的HOM通过多个OH-氧化步骤形成。质谱中鉴定的HOM的组成也支持这一假设。通过仅将苯酚注入腔室中，我们发现苯酚氧化不能单独对苯实验中观察到的HOM负责。当NOx被添加到腔室中时，HOM组成改变，并且在CI-APi-TOF中观察到许多含氧的含氮产物。在种子气溶胶注入后，HOM损失率高于不可逆冷凝的预测，表明一些未检测到的含氧中间体也冷凝到种子气溶胶上，这与一些HOM在多代OH氧化中形成的假设一致。根据我们的研究结果，我们得出结论，HOM产率和组成的芳香族系统强烈依赖于OH和VOC的浓度，需要更多的研究，以充分了解这种影响的HOMs的形成，因此，SOA。我们还建议，HOM产量对腔室条件的依赖性可以解释在文献中报道的SOA产量的变异性的一部分，并强烈建议在未来的SOA研究中监测HOM。
Recent studies have recognised highly oxygenated organic molecules (HOMs) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOCs) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOMs upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOCs and focused in detail on benzene HOM formation in the Julich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOMs to NOx and seed aerosol. Using a nitrate-based chemical ionisation mass spectrometer (CI-APi-TOF), we observed the formation of HOMs in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of toluene and naphthalene, which were injected at lower concentrations, multi-generation OH oxidation seemed to impact the HOM composition. We tested this in more detail for the benzene system in the JPAC, which allowed for studying longer residence times. The results showed that the apparent molar benzene HOM yield under our experimental conditions varied from 4.1% to 14.0%, with a strong dependence on the OH concentration, indicating that the majority of observed HOMs formed through multiple OH-oxidation steps. The composition of the identified HOMs in the mass spectrum also supported this hypothesis. By injecting only phenol into the chamber, we found that phenol oxidation cannot be solely responsible for the observed HOMs in benzene experiments. When NOx was added to the chamber, HOM composition changed and many oxygenated nitrogen-containing products were observed in CI-APi-TOF. Upon seed aerosol injection, the HOM loss rate was higher than predicted by irreversible condensation, suggesting that some undetected oxygenated intermediates also condensed onto seed aerosol, which is in line with the hypothesis that some of the HOMs were formed in multi-generation OH oxidation. Based on our results, we conclude that HOM yield and composition in aromatic systems strongly depend on OH and VOC concentration and more studies are needed to fully understand this effect on the formation of HOMs and, consequently, SOA. We also suggest that the dependence of HOM yield on chamber conditions may explain part of the variability in SOA yields reported in the literature and strongly advise monitoring HOMs in future SOA studies.