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Sunday, January 8, 2012

2011 Network for the Detection of Atmospheric Composition Change Symposium. Session 5 - Tropospheric Observations and Analyses: Is there a hole in the global OH shield over the tropical western Pacific warm pool? by Markus Rex et al.

Is There a Hole in the Global OH Shield Over the Tropical Western Pacific Warm Pool? 

Markus Rex (Alfred Wegener Institute for Polar and Marine Research), Theo Ridder (Institute of Environmental Physics, University of Bremen), Justus Notholt (Institute of Environmental Physics, University of Bremen), Franz Immler (GRUAN lead centre, German Meteorological Service), Kirstin KrügerViktoria Mohr and Susann Tegtmeier (IFM-GEOMAR)

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. The dominant source of OH in clean tropical air are reactions involving ozone. Hence the OH concentration is closely coupled to ozone abundances. Biogenic halogenated species, biogenic species containing sulphur and perhaps anthropogenic SO2 emissions play an important role in the stratospheric composition and ozone chemistry. Changes in their abundance and tropospheric breakdown processes provide an ozone climate feedback mechanism. The dominant source of OH in clean tropical air is the photolysis of ozone at wavelengths shorter than about 340 nm producing O(1D) radicals followed by the reaction O(1D) + H2O -> OH + OH. This couples the OH concentration and hence the oxidizing capacity of tropospheric air closely to the concentration of ozone. The area of the Western Pacific warm pool is known to be key for troposphere to stratosphere exchange (e.g., Newell & Gould-Stewart, JAS, 1981; Fueglistaler et al., JGR, 2004). Vertical profiles of tropospheric ozone or OH from that part of the Pacific are not available so far. Measurements from the south east edge of the warm pool area during the Central Equatorial Pacific Experiment in 1993 (Kley et al., Science, 1996) and individual profiles from the station Samoa in the same geographical area (Solomon et al., GRL, 2005) showed extremely low ozone concentrations in the marine boundary layer and at tropopause level but still significant amounts of ozone in most of the free troposphere, where most of the oxidation of biogenic species occurs. We report ship based ozonesonde measurements from the center of the Western Pacific warm pool in October 2009. During a 2500-km portion of the ship track between 10S and 15N we found ozone concentrations below the detection limit of the sondes throughout the troposphere. Based on comprehensive CTM modelling we show that the observations suggest the existence of a pronounced minimum in the tropospheric OH column well correlated with the region where most of the vertical transport of air into the stratosphere occurs. The consequences for the tropospheric lifetimes of chemical species in that key geographical area and implications for the transport of emissions from that area into the stratosphere will be discussed. The results highlight the importance of setting up longer term observational  capabilities in the westernmost part of the tropical Pacific.

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