Kinetic Studies of Reactive Intermediates from the Oxidation of Atmospheric Alkenes

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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.