Kinetic Studies of Reactive Intermediates from the Oxidation of Atmospheric Alkenes

挥发性有机化合物（VOCs）由自然（生物源）和人为来源排放到大气中。烯烃，如异戊二烯，属于一类挥发性有机化合物，由植物排放，构成生物源性排放的主要部分。从大气的最底层对流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。在烯烃氧化过程中会产生短寿命的中间产物，称为克里基中间体和羟基烷基过氧自由基。这些中间物质与大气中的气体分子，如二氧化硫（酸雨的前体）、羧酸、一氧化氮（最简单的氮氧化物气体）和其他低浓度的化学物质发生反应，产生的产物导致OH自由基、臭氧和有机气溶胶颗粒的形成。OH自由基被认为是“大气清洁剂”，因为它负责启动化学去除排放到对流层的大部分挥发性有机化合物。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入射的太阳辐射量和流出的地球辐射量来影响地球气候。气溶胶颗粒可以将周围空气中的水分凝结成云滴，而更大的云层覆盖会影响穿透地面的太阳辐射量。了解导致有机气溶胶颗粒生长的中间物质的反应性可以帮助我们更好地量化烯烃对大气成分和未来变暖的影响。本研究重点关注的反应中间体寿命很短，它们在大气中的浓度太小，无法直接测量。相反，最近已经开发出有效的方法来制备足够高浓度的克里基中间体和羟基烷基过氧自由基，以便在受控的实验室条件下研究它们的反应。这项研究将检验这些中间体与各种微量大气气体反应的速度，并首次研究这些反应的速率如何随温度变化。大多数化学反应随着温度的降低而变慢，但克里吉中间体被怀疑在较低温度下反应更快。这种不寻常的行为特征很重要，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量子化学计算反应路径中中间物质的结构和能量来解释。由此产生的对化学反应机制的深入了解将帮助我们对大气中发生的许多反应的速率作出可靠的预测。这些不同的反应太多了，无法进行详尽的实验室研究。相反，我们将制定反应物质的结构和它们的反应速率之间的关系，从中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了什么产物，因为大气化学通常涉及一系列反应步骤。产品识别将需要与在加州劳伦斯伯克利国家实验室先进光源（ALS）同步加速器设施操作独特仪器的同事合作。然后，产品的结构将被用来估计它们凝结成有机气溶胶颗粒的倾向。全球大气模式STOCHEM-CRI将用于研究我们的新测量结果对预测大气中臭氧、氮氧化物、OH和气溶胶颗粒的全球浓度的影响，其结果将引起科学界、决策者和公众的兴趣。

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.