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以数值模式的预测为指导。这些模型包含了对自然和人类活动产生的微量气体和气溶胶的未来排放量、它们在整个大气中的扩散以及它们向各种二次产物的化学转化的估计。对流层的光氧化非常复杂，由短命自由基引发，白天以羟基自由基OH为主，而晚上则由NO3自由基或臭氧主导。化学氧化循环去除对人类直接有害的主要排放的微量物质（如CO）或对更广泛的环境有害的温室气体（如CH4）。然而，大气光氧化产生的许多二次产物也是直接有害的，例如O3、NO2、酸性和多功能物质，其中许多挥发性低，能够有效地分配到凝聚相，形成二次有机气溶胶（SOA）。为了计算OH的丰度，从而计算大气中其他微量气体的寿命，有必要对产生和去除OH的过程的反应速率有基本的了解。OH可以与有机和无机物质发生反应，由于空气中存在成千上万的活性挥发性有机化合物（VOCs），因此前者在OH计算中会产生很大的不确定性。实际上，识别空气中存在的所有挥发性有机化合物是不可能的，即使以全面的方式进行了尝试，也经常缺少有关反应速率的动力学数据。VOCs的这种复杂性和有限的相关数据使得观测到的OH浓度和OH反应性与模型计算的一致性本质上是困难的。在尝试时，观察到明显的不匹配，突出了我们模拟对流层化学的能力中的一些基本缺陷。最近对OH反应性的测量，结合对VOCs的测量，可以对缺失的OH汇的大小进行量化，但不能确定它们的化学性质。其他测量结果表明，采用综合二维气相色谱法测量时，环境空气中存在许多不明有机成分。该提案首次将约克的Lewis和Hamilton开发的超高分辨率VOC测量与利兹的Heard， Whalley和Rickard开发的FAGE自由基测量和MCM建模技术相结合。我们将确定有助于去除OH的缺失有机物质的身份，并评估其氧化形成的降解产物。这将通过将综合二维气相色谱与飞行时间质谱仪和火焰电离检测器与OH化学反应器相结合来实现。通过将环境空气样品暴露在含有增强OH自由基的受控环境中，并通过观察未识别物种的色谱峰强度相对于已知VOC物种（并且与OH的反应性已知）的强度变化，将确定未识别物种的OH反应性。使用质谱检测器将使我们能够积极地识别那些我们观察到的对OH汇有重要贡献的物种，并对形成的二次产物的形成速率进行评估。通过对造成损失的主要物种进行功能群分类，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。了解缺失反应性的功能将使有关复杂排放（如溶剂和石化蒸发）的政策变化对空气质量和气候的影响能够得到更好的评估。

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.