The Role of Short-lived Species in the Tropical Atmosphere

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The halogens are vital components in the chemistry of the atmosphere, influencing ozone concentrations in the stratosphere and the troposphere. Currently, the supply of very short-lived halocarbons (VSLH) to the stratosphere is a major uncertainty in the stratospheric bromine budget, with VSLH providing up to 20% of the total bromine loading. The future supply to the stratosphere will likely be affected by climate-related changes in the emissions of VSLH arising from increases in sea surface temperatures and convection. VSLH emissions can also play a large role in determining O3 concentrations as high BrO levels could lead to depletion of O3 in the boundary layer and free troposphere. A significantly improved understanding of the marine, chemical and climatic factors affecting VSLH emission and atmospheric concentration is required to predict their importance as climate changes in the coming decades. The tropics with its large area, regions of convective intensity and highly productive oceans is critical. High values of CHBr3, CH2Br2 and other VSLH have been observed in the tropical Pacific and Atlantic oceans, providing evidence for the importance of emission hotspots in otherwise pristine atmospheres. Measurements of atmospheric concentrations of VSLH in the Western Pacific will be made from 2008-2012 under a recently awarded NERC grant on which I am the Principal Investigator. These time series will form a unique record of VSLH in this region, as they are continuous and highly time-resolved. The collection and interpretation of the measurements provide an excellent opportunity to make real breakthroughs in our understanding of scientific issues including the role of halogens in the global oxidative capacity and the role of iodocarbons in the production of aerosols and clouds in the marine boundary layer in this region of the tropics. I will broaden my research interests in the measurement of VSLH to provide increased knowledge and understanding of how short-lived halocarbons and hydrocarbons influence the tropical troposphere and stratosphere. The new, continuous measurements will be interpreted together with other measurements, principally the less frequent but 20 year record collected at the Univ Calif Irvine. Joint interpretation of the halocarbon data with meteorological and local marine records will enable the influence of climatic features such as El Nino and the Madden Julian Oscillation to be understood. These analyses of existing and new data will be used to design model studies using the NCAS/Met Office UKCA model. Data analysis and interpretation will be extended to include measurements from existing stations (Mace Head, Cape Verde, Weyborne and those in the AGAGE and NOAA networks) and from tropical campaigns such as the PEM series and OP3. The measurement system developed here with robust, autonomous instruments operating at remote sites with minimal human intervention has great potential to be extended to include other instruments based on the same philosophy and I will extend the measurements made by developing and deploying other robust instrument packages for autonomous use. From an atmospheric perspective, the West Pacific will be fascinating over the coming decades. Emissions of VSLH and hydrocarbons are likely to change significantly as a result of changes to natural habitats and increased industrialisation. The chemical mix in this region, strongly affected as it is by the pristine air over the Pacific Ocean itself, will evolve in a currently unpredictable way, with emissions, chemistry, meteorology and oceanic conditions all likely to change significantly. Overall, I will establish a world-leading programme interpreting measurements to understand the role of halo- and hydrocarbons in this region and in the global atmosphere. Once developed, this interpretative expertise can be used to address a much broader range of atmospheric change issues.