FAGE measurements of OH, HO2 and IO during COBRA

The COBRA (Impact of combined iodine and bromine release on the Arctic atmosphere) consortium project is already funded via an International Polar Year (IPY) grant, with the main objectives of improving the understanding of Arctic ozone and mercury depletion events through reducing the uncertainties in important chemical species and chemical and physical mechanisms, both in the gas phase and on surfaces of snow and sea-ice. Ozone is a significant greenhouse gas whilst mercury is a toxic, persistent and bioaccumulative pollutant. Hence it is important to understand the depletion events, which can be sustained and occur in spring (polar sunrise), as they have implications for the atmospheric oxidising capacity, climate and health. One of the major activities within COBRA is a field campaign at Kuujjuarapik on the south-eastern shore of Hudson Bay in spring 2008, a site with an established record of ozone and mercury depletion episodes (a bromine 'hotspot'). Measurements by existing consortium members will be made of BrO, Br2, BrCl, IO, I2, halocarbons, other VOCs, CO, O3, aerosol iodine, as well as flux measurements of halogens, particles and ozone from a variety of frozen surfaces. The project, which involves 6 UK institutions and 3 overseas partners with strong track records in halogen and polar chemistry and physics, will be underpinned by 1D and 3D chemical modelling, the latter using remotely-sensed sea-ice formation. In this small grant we propose to participate in the Kuujjuarapik field campaign and make in situ measurements of the concentrations of OH, HO2 and IO radicals by laser-induced fluorescence spectroscopy using the small, compact aircraft FAGE instrument developed at Leeds. These measurements will enhance significantly the capability of COBRA, as OH provides a measure of the oxidising capacity and HO2 and OH are intimately involved in ozone depleting cycles involving bromine and iodine. Little is known about the role and sources of iodine in polar boundary layer chemistry but during the recent CHABLIS campaign at Halley, Antarctica, elevated concentrations of IO radicals were observed suggesting a widespread and likely abiotic/photochemical source of iodine to the polar atmosphere. During CHABLIS we also made measurements of OH and HO2 radicals, and the striking conclusion was that the budgets of both species were dominated by coupling with halogen species. COBRA represents an ideal opportunity to test the hypothesis that these couplings play a major role in the Arctic. Measurements of OH, HO2 and IO would provide ideal targets for comparison with detailed model calculations based on our current understanding of heterogeneous and gas-phase chemical mechanisms of HOx and halogens, and relevant ocean-snow-ice-atmosphere interactions. An important goal is to improve models of Arctic chemistry and emissions and their effect and feedbacks on regional/global atmospheric chemistry and climate. Making measurements in this polar environment presents significant challenges and we will take advantage of the existing framework of the COBRA consortium, which will organise all campaign logistics. Our involvement at low-cost represents significant gearing of the investment already made by NERC in the COBRA IPY project.