Pronounced Minima in Tropospheric Ozone and OH above the Tropical West Pacific and their Role for Stratospheric Composition

Tuesday, 16 December 2014
Markus Rex1, Ingo Wohltmann1, Ralph Lehmann1, Karen Hepler Rosenlof2, Paul O Wennberg3, Debra K Weisenstein4, Justus Notholt5, Kirstin Krüger6, Viktoria Mohr7 and Susann Tegtmeier7, (1)Alfred Wegener Institute Helmholtz-Center for Polar and Marine Research Potsdam, Potsdam, Germany, (2)NOAA ESRL CSD, Boulder, CO, United States, (3)California Institute of Technology, Pasadena, CA, United States, (4)Atmospheric and Environmental Research, Inc., Lexington, MA, United States, (5)University of Bremen, Bremen, Germany, (6)University of Oslo, Oslo, Norway, (7)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Hundreds of organic species are emitted into the atmosphere mostly from biogenic processes. The rapid breakdown by reactions with OH radicals prevents most of them from reaching the stratosphere. Hence, the omnipresent layer of OH in the troposphere shields the stratosphere from these emissions and is particularly relevant for those species that do not photolyse efficiently. Reactions involving ozone are a strong source of OH in clean tropical air. Hence the OH concentration is closely coupled to ozone abundances. The Western Pacific warm pool is key for troposphere to stratosphere exchange. We report measurements of 14 ozonesondes launched during the Transbrom ship cruise through the center of the warm pool. During a 2500km portion of the ship track between 10S and 15N we found ozone concentrations below the detection limit of the sondes throughout the troposphere. We will discuss the uncertainties of ozonesonde measurements at very low ozone concentrations, the robustness of our observations and the upper limit of the ozone concentration that would be consistent with our raw data. Based on modelling and measurements of OH on the ER-2 during the STRAT campaign we suggest that there also is a pronounced minimum in the tropospheric column of OH over the tropical West Pacific. We show that this increases the lifetime of chemical species and has the potential to amplify the impact of surface emissions on the stratospheric composition. Specifically, we discuss the role of emissions of biogenic halogenated species from this geographic region for stratospheric ozone depletion. Also, we discuss the potential role of increasing anthropogenic emissions of SO2 in South East Asia or from minor volcanic eruptions for the stratospheric aerosol budget.