Gas phase studies of the kinetics of Criegee Intermediates

The Earth's atmosphere is a complex mixture of gases, liquids and even solids. This mainly gaseous envelope around us performs many vital functions e.g. it protects us from harmful ultraviolet light (high energy light) from the Sun through the stratospheric ozone layer. Through the water cycle, clouds are formed in the atmosphere that redistributes water in the Earth systems. These clouds also cool the planet by acting like mirrors, reflecting some of the energy from the Sun back to outer space. This cooling mechanism is essential to the Earth system to permit an equitable surface temperature to exist that allows all the diverse life forms on it to exist. As well as this natural cooling mechanism, so called greenhouse gases in the atmosphere (e.g. carbon dioxide, CO2 and methane CH4) absorb infrared energy released by the Earth and trap some of it, very similar to the analogy of putting on a blanket and warm the Earth's surface up. Both these natural cooling and warming mechanisms are essential to a habitable surface and as long as they stay in balance, the surface temperature will remain reasonably constant. However, over the last 200 years humans have been increasing the level of greenhouse gases in the atmosphere by burning fossil fuels and evidence shows that this is leading to an overall warming of the surface of the Earth. The consequences of even a modest increase in average global surface temperature are significant for human, animal and plant life.

It is known that chemicals released naturally by plants (unsaturated organic molecules such as alkenes) can react with oxidants in the atmosphere to produce extremely fast reacting intermediates, so called Criegee intermediates (CI). However, recent studies by us have shown that these Criegee intermediates react rapidly with a number of species present in the atmosphere such as sulphur dioxide (SO2). Ultimately, these reactions lead to the formation of sulphuric acid, which is very good at promoting aerosol formation (cloud precursors). Under polluted environments, aerosol formation may have detrimental effects on health but in the background atmosphere, promotion of cloud formation leads to a cooling of the Earth's surface. We have assessed the possible impact of these natural emissions of chemicals using computer models of the atmosphere and it appears that this process may be very important in producing cloud precursors and therefore be having an important impact on the Earth's climate (cooling it).

However, we have only been able to investigate the reactions of two possible Criegee intermediates and there are potentially thousands of different ones. Whilst it would be impossible to study them all and indeed not a sensible endeavour, it is important to study different types of Criegee intermediates. If they all have a similar reactivity then the impact on the atmosphere is likely to be true and would then be important to include in climate models. In order to investigate how quickly these Criegee intermediates react with species such as SO2 we have devised an experiment in the laboratory that takes advantage of recent developments in optics. Using laser light to generate these Criegee intermediates we will be able to detect them using a highly sensitive technique called cavity ringdown spectroscopy (CRDS). In the experiment the Criegee intermediate is generated in a closed system where light is trapped between two highly reflective mirrors. As the light bounces backwards and forwards between the mirrors it may be absorbed by the Criegee intermediate and so less light is left. The greater the level of Criegee intermediate made the less light is reflected back and forth and so we have a way to measure this species. In this way we will be able to investigate how fast these Criegee intermediates react with a number of important gases in the Earth's atmosphere.

Beyond academic beneficiaries identified myriad other groups will benefit from this research. First, this work addresses the issue of air quality through proposing new routes to aerosol formation via Criegee biradical mediated oxidation of SO2. Since a major source of alkenes in the urban environment is from natural systems such as trees and these are precursors to Criegee biradical formation, groups such as urban planners, forestry commissions, environment agencies, air quality management groups and ultimately groups such as Defra in the UK will want to understand the implications of this research. Outside the UK, Environment agencies in other countries will also want to assess the impact of this research on their urban air quality strategies. Hence these data should impact on policy makers.

Second, these data will potentially impact on the indoor environment and may alter our perception about what are poor combinations of chemicals in these environments with respect to secondary aerosol formation. Architects and building specialists will want to understand how these new data affect the impact of emissions from a variety of materials on indoor aerosol and how ventilation schemes and outdoor pollutant levels will combine with indoor emissions to generate aerosol. A variety of indoor environments may potentially be candidates for inspection where high volatile organic loadings and SO2 and O3 are collocated. These environments extend beyond buildings inhabited by humans and will include animal pens and a range of botanic settings. Therefore, a wide range of stakeholders could potentially be interested by these data and indoor habitats be impacted by them.

Third, based on preliminary model analysis, it appears that the reactions of Criegee biradicals with SO2 could be a very important additional oxidation pathway to form SO3 and ultimately sulphuric acid in the atmosphere. The implications of these data could be significant to our understanding of past, present and future climates. Although much further testing is required, ultimately groups such as the Met. Office in the UK will want to assess these impacts in detailed climate models. If the impact persists on detailed inspection then this will factor in our understanding of past climates as well as current and future ones. In all this work may eventually point to an even more important role for terrestrial and aquatic ecosystems in off-setting climate change and this will be of considerable benefit to groups such as IPCC as they continue to refine their assessments about the direction and speed of climate change.

Therefore, we believe that the primary impact will be on our understanding of the atmosphere and that this may affect future air quality and climate policies.

In the Education sector, where we place considerable emphasis, studies of this kind provide considerable stimulus for future young scientists and provide teachers with new material to broaden curricula. We have set out ways we hope to support the dissemination of these studies to this group in a direct and effective manner.

Economic benefits can be envisaged, e.g. through analytical scientists, new detection techniques may emerge for biradical species and these may lead to new instruments with potential economic benefits. If these data do highlight important 'new' processes taking place in the atmosphere then there may be subsequent economic benefits from policies that preserve good air quality and address (even in a modest way) the issue of climate change on regional, national and even global scales.