Kinetic Studies of Reactive Intermediates from the Oxidation of Atmospheric Alkenes

挥发性有机化合物(VOCs)由自然(生物)和人为两种来源排放到大气中。异戊二烯等烯烃是这些VOCs的一类，由植物排放，构成生物排放的主要部分。从对流层(大气的最低层)中去除烯烃的主要机制是与臭氧(O3)和羟基(OH)自由基的氧化反应。在烯烃的氧化过程中会产生称为Criegee中间体和羟基烷基过氧基的短命中间体。这些中间物种与大气中的气体分子，如二氧化硫(酸雨的前身)、羧酸、一氧化氮(最简单的氮氧化物气体)和其他低浓度存在的化学物质反应，产生导致形成OH自由基、臭氧和有机气溶胶粒子的产物。···对流层中的臭氧是一种污染物，对生物有害。大气气溶胶通过改变传入的太阳辐射和传出的地面辐射的量来影响地球气候。气溶胶粒子可以凝结周围空气中的水产生云滴，更大的云层覆盖会影响穿透到地面的太阳辐射量。了解导致有机气溶胶颗粒增长的中间物种的反应性有助于我们更好地量化烯烃对大气组成和未来变暖的影响。本研究关注的活性中间体寿命很短，其在大气中的浓度太小，无法直接测量。相反，最近开发了有效的方法来制备足够高浓度的Criegee中间体和羟烷基过氧基，以在受控的实验室条件下研究它们的反应。这项研究将研究这些中间体与各种微量大气气体的反应速度，并首次研究这些反应的速度如何随温度变化。大多数化学反应随着温度的降低而变慢，但Criegee中间体被怀疑在较低的温度下反应较快。这种不寻常的行为很重要，因为对流层的温度随着海拔的升高而降低。基于实验室的反应速率测量将借助对反应路径上中间物种的结构和能量的量子化学计算来解释。由此对化学反应机理的洞察将有助于我们对大气中发生的许多反应的速率做出可靠的预测。这些不同的反应太多了，无法在实验室里进行详尽的研究。相反，我们将建立反应物种的结构和它们的反应速度之间的关系，根据这些关系可以对未研究的反应做出可靠的预测。我们还需要了解这些化学反应形成什么产物，因为大气化学通常涉及一系列反应步骤。产品鉴定将需要与在加州劳伦斯伯克利国家实验室先进光源(ALS)同步加速器设施中操作独特仪器的同事合作。全球大气模式STOCHEM-CRI将被用来研究我们预测大气中臭氧、NOx、OH和气溶胶粒子全球浓度的新测量的结果，其结果将引起科学界、政策制定者和公众的兴趣。

Volatile organic compounds (VOCs) are emitted into the atmosphere by both natural (biogenic) and human sources. Alkenes such as isoprene, which constitute one class of these VOCs, are emitted by plants and form a major fraction of the biogenic emission. The main mechanisms for removal of alkenes from the troposphere, the lowest layer of the atmosphere, are oxidation reactions with ozone (O3) and hydroxyl (OH) radicals. Short-lived intermediate species called Criegee intermediates and hydroxyalkyl peroxy radicals are created during the oxidation of alkenes. Reactions of these intermediate species with gas molecules in the atmosphere like sulphur dioxide (a precursor to acid rain), carboxylic acids, nitric oxide (the simplest NOx gas) and other chemicals present at low concentrations result in products which lead to the formation of OH radicals, ozone and organic aerosol particles. The OH radical is considered the "cleanser of the atmosphere" because it is responsible for initiating the chemical removal of most of the VOCs emitted into the troposphere. Ozone in the troposphere is a pollutant and is detrimental to living organisms.Atmospheric aerosols affect Earth's climate by changing the amount of incoming solar radiation and outgoing terrestrial radiation. The aerosol particles can condense water from the surrounding air to produce cloud droplets, and greater cloud cover affects the amount of solar radiation penetrating to ground level. Understanding the reactivity of the intermediate species that lead to growth of organic aerosol particles can help us to better quantify the impact of alkenes on both the composition and future warming of the atmosphere. The reactive intermediates upon which this research focuses are short lived and their concentrations in the atmosphere are too small for direct measurement. Efficient methods have instead recently been developed for preparation of high enough concentrations of Criegee intermediates and hydroxyalkyl peroxy radicals to study their reactions under controlled laboratory conditions. This research will examine how quickly these intermediates react with a variety of trace atmospheric gases, and for the first time will study how the rates of these reactions change with temperature. Most chemical reactions become slower as the temperature decreases, but the Criegee intermediates are suspected to show faster reactions at lower temperature. This unusual behaviour is important to characterize because the temperature of the troposphere decreases with altitude.The lab-based measurements of reaction rates will be interpreted with the aid of quantum chemistry calculations of the structures and energies of intermediate species along reaction pathways. The resulting insights into the chemical reaction mechanisms will help us to make reliable predictions for the rates of many reactions taking place in the atmosphere. These different reactions are too numerous for exhaustive laboratory study. Instead, we will formulate relationships between the structures of the reacting species and their reaction rates from which reliable predictions can be made for unstudied reactions.We also need to understand what products are formed by these chemical reactions, because atmospheric chemistry generally involves a sequence of reactive steps. Product identification will require a collaboration with colleagues who operate a unique instrument at the Advanced Light Source (ALS) synchrotron facility at Lawrence Berkeley National Laboratory in California. The structures of the products will then be used to estimate their propensity to condense into organic aerosol particles.The global atmospheric model STOCHEM-CRI will be used to study the consequences of our new measurements for predictions of global concentrations of ozone, NOx, OH and aerosol particles in the atmosphere, with outcomes that will be of interest to the scientific community, policy makers and the general public.