Total Radical Production and Degradation Products from Alkene Ozonolysis

大气成分是由排放和大气中的化学作用共同决定的。大多数排放到大气中的碳氢化合物的去除是通过与羟基自由基(OH)的反应而开始的。接下来是一系列的降解步骤，最终导致二氧化碳和水的产生。在氮氧化物存在的情况下，碳氢化合物的氧化会导致产生臭氧/一种对人类健康、植物和材料有害的污染物。臭氧形成的程度取决于碳氢化合物氧化步骤的细节。OH是通过阳光作用于臭氧而自然形成的。与碳氢化合物反应后，OH被转化为HO2和RO2、氢基和有机过氧基。如果存在中等水平的NO，则RO2和HO2可以转化回OH/这是自由基循环的一个例子。OH水平控制着污染物和甲烷等全球变暖气体的丰度，并限制臭氧的产生速度。为了量化这些影响，我们需要了解控制OH水平的过程，而为了预测给定碳氢化合物对大气的影响，我们必须确定其降解产物/例如，评估它们是否具有足够长的寿命，可以从其起源点运输。不饱和化合物，即烯烃，是那些具有一个或多个双键的化合物。它们通过工业过程排放，也来自植被(森林是由1-3个或更多个单位的异戊二烯C5H8组成的烯烃分子的大量来源)。烯烃与臭氧反应，有两种影响：产生自由基物种，包括羟基，导致其他化合物更快的氧化，以及烯烃-臭氧反应产生更多的碳氢化合物(降解产物)，这些碳氢化合物可以参与大气反应循环，潜在地产生臭氧。然而，这两种影响都有相当大的不确定性，本项目旨在解决这一问题。臭氧-烯烃反应除了产生OH外，还产生HO2和RO2自由基。在大气中，RO2和HO2很容易转化为OH；因此，任何RO2或HO2的产生都会导致OH水平的提高。到目前为止，HO2和RO2的产量都是间接推断出来的。最近，我们已经能够直接测量HO2和RO2，有证据表明，烯烃臭氧分解产生的HO2和RO2(因此，最终产生的OH)比以前认为的要高得多，并且随着湿度的变化而变化。该项目的第一个目标是测量一系列具有天然和人工意义的烯烃的臭氧分解产生的总自由基产率(OH、HO2和RO2)。烯烃-臭氧反应会产生一系列降解产物。对于生物来源的烯烃(如月桂烯等萜类)，高达90%的这些气相降解产物是未知的。该项目的第二个目标是结合使用常规仪器和两种新的方法来识别这些物种：化学电离飞行时间质谱仪，特别适用于大型含氧碳氢化合物的识别，以及测量未识别物种的总反应性(相对于与羟基的反应)，以进一步限制它们的可能性质和对大气的意义(寿命)。实验工作将在西班牙巴伦西亚的欧洲光反应堆设施(EUPHORE)进行。EUPHORE包括一个200立方米的模拟室，配有一系列测量仪器，并将由莱斯特大学的CIR-TOF-MS和过氧基化学放大器补充，用于测量RO2。在实验工作之后，我们将更新大气模型(主化学机制)中的自由基产生和烯烃降解机制，并在从欧洲到英国的受污染空气传输的标准模拟中，使用修订后的模型重新评估烯烃对臭氧产生的影响。

Atmospheric composition is determined by a combination of emissions and chemical processing within the atmosphere. The removal of most hydrocarbons emitted to the atmosphere is initiated by reaction with the hydroxyl radical (OH). A series of degradation steps follows, leading eventually to CO2 and water. In the presence of nitrogen oxides, hydrocarbon oxidation leads to production of ozone / a pollutant harmful to human health, vegetation and materials. The extent to which ozone formation occurs depends upon details of the hydrocarbon oxidation steps. OH is formed naturally through the action of sunlight upon ozone. Following reaction with hydrocarbons, OH is converted into HO2 and RO2, hydro and organic peroxy radicals. RO2 and HO2 can then be converted back into OH if moderate levels of NO are present / an example of radical cycling. OH levels control the abundance of pollutants and global warming gases such as methane, and limit the rate of ozone production. To quantify these effects, we need to understand the processes which govern OH levels, while to predict the atmospheric impact of a given hydrocarbon, we must identify its degradation products / e.g., to assess if they have a long enough lifetime to be transported from their point of origin. Unsaturated compounds, alkenes, are those with one or more double bonds. They are emitted through industrial processes, and also from vegetation (forests are a large source of alkene molecules made up of 1-3 or more units of isoprene, C5H8). Alkenes react with ozone, with two effects: Radical species, including OH, are produced (without the need for sunlight), leading to faster oxidation of other compounds, and the alkene-ozone reaction produces further hydrocarbon species (degradation products), which can participate in atmospheric reaction cycles, potentially producing ozone. However, both of these effects have considerable uncertainties, which this project aims to address. The ozone-alkene reactions produce HO2 and RO2 radicals in addition to OH. RO2 and HO2 are readily converted into OH in the atmosphere; therefore, any production of RO2 or HO2 will lead to enhanced OH levels. Hitherto, the HO2 and RO2 yields have been inferred indirectly. Recently, we have been able to directly measure HO2 and RO2, and evidence has emerged that HO2 and RO2 yields (and hence, ultimately, OH production) from alkene ozonolysis is rather higher than previously thought, and varies with humidity. The first objective of this project is to measure total radical yields (OH, HO2 and RO2) from ozonolysis of a range of alkenes of both natural and man-made significance. Alkene-ozone reactions produce a range of degradation products. For biogenic alkenes (terpenes such as myrcene) up to 90 % of these gas-phase degradation products are unidentified. The second objective of this project is to identify these species, using a combination of conventional instruments together with two new approaches: A Chemical Ionisation Time-of-Flight Mass Spectrometer, uniquely suited to the identification of large oxygenated hydrocarbons, and measurement of the total reactivity of the unidentified species (with respect to reaction with OH), to further constrain their likely nature and atmospheric significance (lifetime). The experimental work will be carried out in the European Photoreactor facility (EUPHORE), in Valencia, Spain. EUPHORE consists of a 200 m3 simulation chamber, with a range of measurement instrumentation, which will be supplemented by the CIR-TOF-MS and a Peroxy Radical Chemical Amplifier, for measurement of RO2, from the University of Leicester. Following the experimental work, we will update the radical production and alkene degradation mechanism in an atmospheric model (the Master Chemical Mechanism), and use the revised model to reassess the impact of alkenes upon ozone production, in standard simulations of the transport of polluted air from Europe to the UK.