Kinetic Studies of Reactive Intermediates from the Oxidation of Atmospheric Alkenes

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.

The results of the proposed research will be useful to various academic communities as described in the Academic Beneficiaries section above. This benefit will be in the short to medium term. We will ensure prompt dissemination to interested academics through publication of our research in open-access journals, provision of open access to the data from our studies (with data sets allocated DOIs for ease of retrieval), communication of outcomes via our website, and presentation at international and national conferences. We will also communicate highlights of our research and its consequences through the University of Bristol's Cabot Institute which brings together scientists and social scientists from diverse disciplines with a common interest in global change.

Beyond the University research sector, research scientists working for/in institutes and organizations such as NCAS, the UK Met Office, NOAA, NASA, the Max Planck Institute for Chemistry in Mainz, and many others will be interested in the outcomes of this work. The laboratory data will be appropriate for incorporation into chemical kinetics and photochemistry reference tabulations and databases, for use in the medium to long-term by atmospheric modellers. The project also involves methodological development in spectroscopic analysis of gaseous mixtures, specifically in the near-IR region which is rich in precise chemical signatures for organic molecules and radicals. In the medium term, researchers involved in spectroscopy and kinetics of reactive intermediates in research institutes and industry, working in combustion and plasma science as well as atmospheric chemistry, will benefit from these advances. Combustion and plasma processes have numerous and diverse industrial applications, e.g. in plasma deposition and processing of advanced technological materials, or in combustion for air, sea and land transportation).

Extensive effort will be invested in public engagement in science. The general public has a growing interest in climate change and its consequences in the UK and internationally. Advances in understanding the chemistry and composition of the Earth's atmosphere inform the predictions for future climate change and therefore impact on policy development for emissions from energy generation, industry and transportation. Members of the research team, in association with Bristol ChemLabS and the School of Chemistry's Outreach Director Tim Harrison, will present the "A Pollutants Tale" talks at various schools outreach events and science festivals in the UK (e.g. Cheltenham, Manchester, Brighton) aimed at the general public. We will update these talks with the latest findings from the proposed research. We will also engage in the Schools Outreach Programme managed through Bristol ChemLabs, with contributions that recognize the importance of air quality and climate change as topics in the new 21st Century Science curriculum at GCSE and A level in the UK. The Research Co-Investigator will become a STEM ambassador and will help to deliver the RSC's Spectroscopy in a Suitcase outreach activity which gives school students a chance to learn about spectroscopy through hands-on experience. These activities will help the students to understand and use methods of scientific inquiry in the classroom and outside. The Research Co-I will also take Bristol ChemLabS outreach to schools in Nepal, his country of origin, in collaboration with Kathmandu University. A secondary aim will be to seek collaborative opportunities with researchers in Nepal, and contribute towards advancement of science in the developing world. Nepal is geographically important because the Himalayan mountain range significantly perturbs the weather in the Indian sub-continent and South East Asia. Thus, research links with Nepal could open new opportunities in the future for UK geophysical researchers.