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Atmos. Chem. Phys., 11, 1221712226, 2011www.atmos-chem-phys.net/11/12217/2011/doi:10.5194/acp-11-12217-2011 Author(s) 2011. CC Attribution 3.0 License.
Stratospheric ozone chemistry in the Antarctic: what determines thelowest ozone values reached and their recovery?
J.-U. Groo1, K. Brautzsch1, R. Pommrich1,2,3, S. Solomon4, and R. Muller1
1Institut fur Energie- und Klimaforschung Stratosphare (IEK-7), Forschungszentrum Julich, Julich, Germany2Laboratoire dAerologie, CNRS/INSU, Universite de Toulouse, Toulouse, France3Groupe detude de lAtmosphere Meteorologique, CNRM-GAME, Meteo-France, Toulouse, France4Department of Atmospheric and Oceanic Science, University of Colorado, Boulder, CO, USA
Received: 27 July 2011 Published in Atmos. Chem. Phys. Discuss.: 5 August 2011Revised: 18 November 2011 Accepted: 25 November 2011 Published: 7 December 2011
Abstract. Balloon-borne observations of ozone from theSouth Pole Station have been reported to reach ozone mix-ing ratios below the detection limit of about 10 ppbv at the70 hPa level by late September. After reaching a minimum,ozone mixing ratios increase to above 1 ppmv on the 70 hPalevel by late December. While the basic mechanisms caus-ing the ozone hole have been known for more than 20 yr, thedetailed chemical processes determining how low the localconcentration can fall, and how it recovers from the mini-mum have not been explored so far. Both of these aspectsare investigated here by analysing results from the ChemicalLagrangian Model of the Stratosphere (CLaMS). As ozonefalls below about 0.5 ppmv, a balance is maintained by gasphase production of both HCl and HOCl followed by het-erogeneous reaction between these two compounds in thesesimulations. Thereafter, a very rapid, irreversible chlorinedeactivation into HCl can occur, either when ozone drops tovalues low enough for gas phase HCl production to exceedchlorine activation processes or when temperatures increaseabove the polar stratospheric cloud (PSC) threshold. As aconsequence, the timing and mixing ratio of the minimumozone depends sensitively on model parameters, includingthe ozone initialisation. The subsequent ozone increase be-tween October and December is linked mainly to photochem-ical ozone production, caused by oxygen photolysis and bythe oxidation of carbon monoxide and methane.
Correspondence to: J.-U. Groo(email@example.com)
Since the discovery of the ozone hole (Farman et al., 1985),the chemical mechanisms causing ozone depletion have beenclarified in increasing detail (e.g., Solomon, 1999; WMO,2011). These mechanisms lead to an almost complete de-struction of ozone at certain altitudes (1520 km) in theAntarctic stratosphere, as observed in measurements byozone sondes over four decades (Solomon et al., 2005). Fig-ure 1 shows the ozone observations from the South Pole Sta-tion at the 70 hPa level. The logarithmic ordinate highlightsthe massive loss of ozone at this level, with ozone mixingratios dropping to and even below the approximate instru-ment detection limit of 10 ppbv (see Vomel and Diaz, 2010)by late September and early October above the South PoleStation. There are some observations that do not displayozone depletion over the South Pole Station, e.g. betweenlate September and early November. These data correspondto times when the polar vortex was not located over the pole,particularly in 2002.
The graph reveals a lower envelope of the observations thathas remained nearly the same in different years since 1990,characterising the chemical decay of ozone to values as lowas 10 ppbv around the end of September and increasing againafterwards. While the basic mechanisms for the rapid polarozone loss are understood (Solomon, 1999), the explanationfor the ozone depletion near and below the detection limit ofabout 10 ppbv have not yet been elucidated, nor has the fac-tor driving the subsequent increase of ozone been identified.Here we use simulations of the Chemical Lagrangian Modelof the Stratosphere (CLaMS) to investigate these processes.
Published by Copernicus Publications on behalf of the European Geosciences Union.
12218 J.-U. Groo et al.: Stratospheric ozone chemistry at lowest ozone values
South Pole Ozone at 70 hPa
Jul 1 Sep 1 Nov 1 Jan 1109
19901999 20002004 20052010
Fig. 1. Ozone sonde observations at 70 hPa from South Pole Stationin winter and spring since 1990. Updated from Fig. 7a of Solomonet al. (2005).
2 Model description
The simulations presented here are performed with CLaMS(McKenna et al., 2002a,b; Konopka et al., 2004; Groo et al.,2005a). The model is used in two different modes. In thebox-model mode, stratospheric chemistry is calculated for afew representative air parcels along their trajectory whereasthe global mode is the Lagrangian 3-dimensional ChemistryTransport Model.
In the box-model mode, the air parcels are defined hereby the location and time of ozone soundings in the ozonehole period, from which trajectories are calculated both back-ward to June and forward to December. Simulations withthe CLaMS chemistry module are then performed forwardin time using the combination of these two trajectories. Thetrajectories of the air parcels were calculated using wind andtemperature data from the ECMWF operational analysis in a11 resolution. The diabatic descent (or ascent) rates werecalculated from a radiation code (Morcrette, 1991; Zhongand Haigh, 1995) for a cloud-free atmosphere based on tem-peratures from the ECMWF operational analyses and clima-tological ozone and water vapour profiles (Groo and Rus-sell, 2005). CLaMS includes options for different solversto integrate the system of stiff ordinary differential equationsdescribing the chemistry (Carver et al., 1997; McKenna et al.,2002b). The chemical kinetic data are taken from Sanderet al. (2006), except for the Cl2O2 cross sections which aretaken from von Hobe et al. (2009), scaled by a factor of 1.48to match the observation of Lien et al. (2009). The photolysisrates are calculated in spherical geometry (Meier et al., 1982;Becker et al., 2000) for every hour using a climatologicalozone profile for ozone hole conditions from HALOE mea-surements (Groo and Russell, 2005). Heterogeneous chem-istry is calculated on ice, nitric acid trihydrate (NAT), andliquid ternary H2O/H2SO4/HNO3 particles. The parametri-sations of the temperature-dependent uptake coefficient on
liquid aerosols are derived from measurements by Hanson(1998) and those on NAT particles are taken from Carslawand Peter (1997) based on lab measurements derived fromHanson and Ravishankara (1993). NAT formation is as-sumed to occur at a HNO3 supersaturation of a factor of10, corresponding approximately to 3 K supercooling belowTNAT. The NAT particle density is assumed to be 1 cm3.
In the box-model mode, we employ the solver SVODE(Brown et al., 1989) that does not use the family approxima-tion, which could become invalid at very low ozone mixingratios. Since no denitrification parametrisation was used inthe box-model mode, the chemical consequences of the den-itrification were addressed by sensitivity studies. The chem-ical initialisation for the air parcels in the box-model modeis interpolated from the 3-dimensional CLaMS simulationsdescribed below. The results from box-model studies do notinclude mixing between neighbouring air masses nor denitri-fication due to sedimentation of HNO3-containing particles.Over the long time span from June to November, a trajectoryshould certainly not be viewed as representing the exact lo-cation of a single air mass. Rather it should be interpretedas one of the possible histories of an air parcel that reacheslow ozone mixing ratios. Thus, the results of this trajectory-box-model can be used to investigate the chemical processesaround the observed minimum ozone mixing ratios.
The CLaMS 3-D simulations for the ozone hole in the year2003 were described in detail in a dissertation for a univer-sity diploma (Walter, 2005), which focused on the time pe-riod up to November 2003. These simulations are continueduntil the end of December in this work. The CLaMS sim-ulations were initialised on 1 May 2003 using observationsfrom MIPAS-Envisat data for O3, N2O, and HNO3. Corre-lations with N2O were used to initialise Cly (Groo et al.,2002), Bry (Groo et al., 2002)+10 %, and NOy Groo et al.(2005b). The remaining species and the family partitioningwere initialised from the Mainz 2-D model (Groo, 1996).Denitrification by sedimenting NAT particles was simulatedwith the scheme used for the Arctic winter 2002/2003 (Grooet al., 2005b). The horizontal resolution of this simulation is100 km between 40 and 90 S. Here, the solver IMPACT wasused based on the family concept approximation (Carver andScott, 2000).
For investigations of the chemical processes around theozone minimum in late September, a trajectory was chosenthat exactly intersects an observation from the South PoleStation of about 10 ppbv ozone at the pressure level of 70 hPaon 24 September 2003. The box-model simulation was per-formed along this trajectory for the time period from 1 Juneto 30 November 2003. The trajectory stays in the vortex coreat an average equivalent latitude of 77 S with a standard de-viation of 5. This trajectory should be considered as an
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aPotential Temperature Trajectory Sonde