Millet Dylan B, Alwe Hariprasad D, Chen Xin, Deventer Malte Julian, Griffis Timothy J, Holzinger Rupert, Bertman Steven B, Rickly Pamela S, Stevens Philip S, Léonardis Thierry, Locoge Nadine, Dusanter Sébastien, Tyndall Geoffrey S, Alvarez Sergio L, Erickson Matthew H, Flynn James H
University of Minnesota, Saint Paul, Minnesota 55108, United States.
Utrecht University, Utrecht 3584 CC, The Netherlands.
ACS Earth Space Chem. 2018 Aug 16;2(8):764-777. doi: 10.1021/acsearthspacechem.8b00061. Epub 2018 Jun 14.
Terrestrial ecosystems are simultaneously the largest source and a major sink of volatile organic compounds (VOCs) to the global atmosphere, and these two-way fluxes are an important source of uncertainty in current models. Here, we apply high-resolution mass spectrometry (proton transfer reaction-quadrupole interface time-of-flight; PTR-QiTOF) to measure ecosystem-atmosphere VOC fluxes across the entire detected mass range 0-335) over a mixed temperate forest and use the results to test how well a state-of-science chemical transport model (GEOS-Chem CTM) is able to represent the observed reactive carbon exchange. We show that ambient humidity fluctuations can give rise to spurious VOC fluxes with PTR-based techniques and present a method to screen for such effects. After doing so, 377 of the 636 detected ions exhibited detectable gross fluxes during the study, implying a large number of species with active ecosystem-atmosphere exchange. We introduce the reactivity flux as a measure of how Earth-atmosphere fluxes influence ambient OH reactivity and show that the upward total VOC (VOC) carbon and reactivity fluxes are carried by a far smaller number of species than the downward fluxes. The model underpredicts the VOC carbon and reactivity fluxes by 40-60% on average. However, the observed net fluxes are dominated (90% on a carbon basis, 95% on a reactivity basis) by known VOCs explicitly included in the CTM. As a result, the largest CTM uncertainties in simulating VOC carbon and reactivity exchange for this environment are associated with known rather than unrepresented species. This conclusion pertains to the set of species detectable by PTR-TOF techniques, which likely represents the majority in terms of carbon mass and OH reactivity, but not necessarily in terms of aerosol formation potential. In the case of oxygenated VOCs, the model severely underpredicts the gross fluxes and the net exchange. Here, unrepresented VOCs play a larger role, accounting for ~30% of the carbon flux and ~50% of the reactivity flux. The resulting CTM biases, however, are still smaller than those that arise from uncertainties for known and represented compounds.
陆地生态系统既是挥发性有机化合物（VOCs）进入全球大气的最大来源，也是其主要汇，而这两种双向通量是当前模型不确定性的一个重要来源。在此，我们应用高分辨率质谱法（质子转移反应-四极杆接口飞行时间质谱仪；PTR-QiTOF）来测量一个混合温带森林中整个检测质量范围（0 - 335）内生态系统与大气之间的VOC通量，并利用这些结果来检验一个科学水平的化学传输模型（GEOS-Chem CTM）在多大程度上能够代表观测到的活性碳交换情况。我们表明，基于质子转移反应（PTR）的技术中，环境湿度波动会导致虚假的VOC通量，并提出了一种筛选此类影响的方法。在此之后，在研究期间检测到的636种离子中有377种表现出可检测的总通量，这意味着大量物种存在活跃的生态系统-大气交换。我们引入反应性通量作为衡量地球-大气通量如何影响环境中羟基（OH）反应性的指标，并表明向上的总VOC（VOC）碳通量和反应性通量所涉及的物种数量远少于向下的通量。该模型平均低估了VOC碳通量和反应性通量40% - 60%。然而，观测到的净通量在很大程度上（以碳计为90%，以反应性计为95%）由CTM中明确包含的已知VOC主导。因此，在模拟该环境中VOC碳和反应性交换时，CTM的最大不确定性与已知物种而非未被表征的物种相关。这一结论适用于通过PTR-TOF技术可检测到的物种集合，就碳质量和OH反应性而言，这些物种可能占大多数，但就气溶胶形成潜力而言不一定如此。对于含氧VOCs，该模型严重低估了总通量和净交换量。在这里，未被表征的VOCs起到了更大的作用，占碳通量的约30%和反应性通量的约50%。然而，由此产生的CTM偏差仍然小于已知和已表征化合物的不确定性所导致的偏差。
