Identification of missing organic reactivity in the urban troposphere

国际社会对空气质量恶化和气候变化的反应是由数值模型的预测指导的。这些模型包含了对自然和人类活动产生的痕量气体和气溶胶的未来排放、其在大气中的扩散以及其转化为各种次级产品的化学变化的估计。对流层中的光氧化是高度复杂的，并且由短寿命的自由基物种引发，在白天由羟基自由基（OH）主导，并且在夜间由NO3自由基或臭氧主导。化学氧化循环可去除对人类（如CO）或更广泛的环境（如CH4等温室气体）直接有害的主要排放痕量物质。然而，大气光氧化产生的许多次级产物也是直接有害的，例如O3、NO2、酸性和多功能物质，其中许多具有低挥发性并且能够有效地分配到冷凝相，从而产生次级有机气溶胶（SOA）。有必要对产生和除去OH的过程的反应速率有基本的了解。OH与有机和无机物质反应，前者在任何OH计算中产生显著的不确定性，因为空气中存在数千种活性挥发性有机化合物（VOC）。实际上，不可能识别空气中存在的所有VOC，即使尝试以全面的方式进行识别，反应速率的动力学数据也经常丢失。VOC的这种复杂性和有限的相关数据使得观察到的OH浓度和OH反应性与模型计算难以协调。当尝试时，观察到显著的不匹配，突出了我们模拟对流层化学的能力中的一些基本缺陷。最近的测量OH反应性，结合测量挥发性有机化合物，使失踪的OH汇的大小进行量化，但不是他们的化学身份。其他测量表明，许多不明的有机成分存在于环境空气中时，全面的二维气相色谱法作为测量技术，这个建议首次结合超高分辨率VOC测量由刘易斯和汉密尔顿在约克与FAGE自由基测量和MCM建模技术开发的赫德，沃利和里卡德在利兹。我们将确定有助于去除OH的缺失有机材料的身份，并评估其氧化形成的降解产物。这将通过耦合全面的二维气相色谱与飞行时间质谱仪和火焰离子化检测器与OH化学反应器来实现。通过将环境空气样品暴露于含有增强的OH自由基的受控环境，并通过观察未识别物质的色谱峰强度相对于已知VOC物质（并且已知其与OH的反应性）的强度变化的相对变化，将确定未识别物质的OH反应性。使用质谱检测器将使我们能够积极地识别那些物种，我们观察到的贡献显着的OH汇，并提供形成的二次产品的形成率的评估。利用造成损失的主要物种的功能群分类，我们将创建用于MCM的替代参数化机制，以便更准确地描述控制城市OH和O3的过程。了解缺失的反应性的功能，将能够使大气对空气质量和气候产生影响，这是由于有关复杂排放的政策变化造成的（如溶剂和石化蒸发）进行更好的评估。

The international societal response to deteriorating air quality and the changing climate is guided by the predictions of numerical models. These models contain estimates of future emissions of trace gases and aerosols from natural and human activity, their dispersal throughout the atmosphere, and their chemical transformations into a wide range of secondary products. Photo-oxidation in the troposphere is highly complex, and is initiated by short lived radical species, in the daytime dominated by the hydroxyl radical, OH, and at night by either NO3 radicals or ozone. Chemical oxidation cycles remove primary emitted trace species which are directly harmful to humans (e.g. CO) or to the wider environment (greenhouse gases e.g. CH4,). However, many of the secondary products produced by atmospheric photo-oxidation are also directly harmful, for example O3, NO2, acidic and multifunctional species, many of which are of low volatility and are able to partition effectively to the condensed phase, creating secondary organic aerosol (SOA).In order to calculate the abundance of OH, and hence the lifetimes of other trace gases in the atmosphere, it is necessary to have fundamental knowledge of the rates of reaction of the processes that generate and remove OH. OH reacts with both organic and inorganic species, with the former generating significant uncertainty in any OH calculation, since many thousands of reactive volatile organic compounds (VOCs) exist in air. Practically, it has been impossible to identify all VOCs present in air and even where this has been attempted in a comprehensive manner, kinetic data on reaction rates are often missing. This complexity of VOCs, and limited associated data make it intrinsically difficult to reconcile observed OH concentrations and OH reactivity with model calculations. When attempted, significant mismatches are observed, highlighting some basic flaws in our ability to simulate the chemistry of the troposphere. Recent measurements of OH reactivity, combined with measurements of VOCs, have enabled the magnitude of missing OH sinks to be quantified, but not their chemical identity. Other measurements have shown that many unidentified organic components exist in ambient air when comprehensive two dimensional gas chromatography is used as the measurement technique.This proposal combines for the first time ultra high resolution VOC measurements developed by Lewis and Hamilton in York with the FAGE free-radical measurement and MCM modelling techniques developed by Heard, Whalley and Rickard in Leeds. We will determine the identity of missing organic material that contributes towards the removal of OH, and assess the formation of degradation products from their oxidation. This will be achieved by coupling comprehensive two-dimensional gas chromatography with a time-of-flight mass spectrometer and flame ionization detector with an OH chemical reactor. By exposing ambient air samples to a controlled environment containing enhanced OH radicals, and by observing the relative change of chromatographic peak intensity for unidentified species relative to the change in intensity for known VOC species (and for which the reactivity with OH is known) the OH reactivity of the unidentified species will be determined. The use a mass spectral detector will allow us to positively identify those species which we observe as contributing significantly as OH sinks, and, provide an assessment of the formation rates of secondary products formed. Using a functional group classification of the major species contributing to losses, we will create surrogate parameterized mechanisms for use in the MCM to allow a more accurate description of processes controlling urban OH and O3 Understanding the functionality of the missing reactivity will enable the atmospheric effects upon air quality and climate due to policy changes regarding complex emissions (such as solvent and petrochemical evaporation) to be better assessed.