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									The roles of stratospheric ozone in past and future tropospheric ozone budgets
and trends
Kengo Sudo,1 Masaaki Takahashi,2,1 and Hajime Akimoto1
    Frontier Research System for Global Change, Yokohama, Japan
    Center for Climate System Research, University of Tokyo, Tokyo, Japan

Abstract. We perform historical simulations of
tropospheric ozone (O3) distributions and budgets during
                                                                      Model and experiments
1850-2000 using a chemistry coupled climate model. In                     This study employs the coupled tropospheric chemistry
this paper, we focus on the impacts of stratospheric O3               climate model CHASER [Sudo et al., 2002] which has
changes as observed since the late 1970’s on tropospheric             been developed in the framework of the Center for Climate
O3. Our simulations show that the stratospheric O3                    System Research/National Institute for Environment
depletion since ~1980 has a significant negative effect on            Studies (CCSR/NIES) atmospheric GCM [Numaguti et al.,
the tropospheric O3 trend. In response to the stratospheric           1995]. For this study, the horizontal resolution of T42
O3 depletion during 1970 to 2000, stratospheric O3 input to
                                                                      (2.8ox2.8o) is adopted with 32 vertical layers from the
the troposphere decreases significantly, and global mean
chemical lifetime of tropospheric O3 also decreases by                surface to about 40 km altitude. The model considers a
5-20% owing to increased UV radiation. We also conduct                detailed on-line simulation of tropospheric chemistry
future simulations of tropospheric O3 for 1990 to 2100                involving O3-HOx-NOx-CH4-CO system and oxidation of
with/without climate change using the IPCC SRES-A2                    nonmethane hydrocarbons (NMHCs), and includes detailed
scenario. Our simulations show large increases in                     dry and wet deposition schemes also. The concentrations of
stratospheric O3 transport to the troposphere (by ~80%                stratospheric O3 and NOy species above ~20 km altitude
during 1990-2100) as a result of enhancement in the                   are nudged to the monthly mean satellite data from the
tropospheric and stratospheric circulation with climate               Halogen Occultation Experiment project (HALOE) [Russel
change in the model.                                                  et al., 1993] and output data from the 3-D stratospheric
                                                                      chemistry model [Takigawa et al., 1999].
Introduction                                                              With the CHASER model, past and future simulations
    Many previous studies have suggested that tropospheric            of tropospheric O3 are performed. We set up time-slice
O3 increases significantly since preindustrial times, in              simulations for every ten years of 1850-2000 in the past
accordance with dramatic increases in anthropogenic                   simulation and of 1990-2100 in the future one.
emissions especially in the northern hemisphere (NH) [e.g.,               In the past simulation, we use the historical emission
WMO, 1990; Crutzen and Zimmerman, 1991]. As these                     database (HYDE) of the EDGAR inventory for
studies imply, changes in tropospheric O3 appear to depend            anthropogenic emissions. The changes in stratospheric O3
most principally on emissions of O3 precursors (NOx, CO,              during 1970-2000 are prescribed with the SPARC O3 trend
and hydrocarbons NMHCs) and CH4. However, effects of                  estimate [D. Karoly and D. Sexton]. This past simulation
other factors such as meteorological conditions and                   also considers climate change during 1850-2000
stratospheric O3 abundance should be also evaluated. For              diagnostically using historical data of sea surface
example, stratospheric O3 can be an important O3 source               temperatures (SSTs) and greenhouse gas concentrations.
for the troposphere (especially for the upper troposphere),               In the future simulation, we conduct two experiments:
and can also control tropospheric photochemistry altering             (Exp1) a control experiment only with emission changes
UV incidence into the troposphere. In this paper, we assess           and (Exp2) a climate change experiment with emission
the impacts of changes in stratospheric O3 and its transport          changes. In Exp1 GCM simulates present-day climate, but
                                                                      in Exp2 it simulates climate change using the IPCC
to the troposphere on past and future tropospheric O 3
                                                                      SRES-A2 scenario. In both experiments, future
distributions and budgets in our historical (past) and future
                                                                      anthropogenic emissions of O3 precursors and CH4 are
simulations of tropospheric O3.
                                                                      specified by the A2 scenario.

Figure 1. Temporal evolution (1850-2000) of global mean O3 mixing ratios calculated at distinct altitudes (surface, 500hPa, and
300hPa). Shown are net O3 (solid lines) and O3 from the stratosphere O3(S) (dashed lines).
Results and discussions

Past simulation: influences of stratospheric ozone
depletion on tropospheric ozone
      Figure 1 displays the time evolution of global mean
tropospheric O3 from 1850 to 2000 calculated in the past
simulation. The model calculates increases in O3 at all
altitudes toward the present-day; for example, global mean
surface O3 level increases from 16 ppbv in 1850 to 27 ppbv
in 2000 (~+70%). More rapid O3 increases are calculated
after 1950 reflecting the higher increasing rates of O3
precursors emissions after ~1950. O3 increases, however,
appear to be reduced significantly after 1980. This can be
attributed to the prescribed stratospheric O3 depletion
which causes (1) a decrease in O3 input to the troposphere
and (2) a shorter photochemical lifetime of tropospheric O 3
with increased tropospheric UV radiation. O3 of                 Figure 2. Temporal evolutions of (a) net STE O3 flux to the
stratospheric origin O3(S), a tracer separate from net O3 in    troposphere for 1990-2100 calculated in the climate change
the model, decreases after 1980 (especially in the upper        experiment (Exp2, solid) and in the control experiment (Exp1,
troposphere: 300hPa) reflecting reduced O3 transport from       dashed) and (b) global and annual mean surface air temperature
the stratosphere associated with the depleted stratospheric     change in Exp2 relative to 1990. Net STE is shown on a global
O3. In response to the stratospheric O3 depletion, the global   and annual basis (TgO3/yr). The decreases in net stratospheric
mean chemical lifetime of tropospheric O3 also decreases        O3 input in Exp1 are due to increases in tropospheric O3 with
                                                                the emission increases.
by 5-20% in the mid-high latitudes during 1980-2000. The
decreased chemical lifetime of O3 appears to be most
responsible for the reduced O3 increases at the surface after   enhanced in the climate change experiment (Exp2). The net
1980.                                                           chemical O3 production within the troposphere estimated
      The calculated O3 trend as shown in Figure 1 is           for 2100 is 740 TgO3/yr in Exp1 but - 4.7 TgO3/yr in Exp2
basically consistent with historical O3 observations at the     reflecting enhanced O3 destruction due to the water vapor
surface. The simulated surface O3 levels in NH in ~1900,        increases especially in the lower troposphere and increased
however, appear to be overestimated. The historical             O3 input from the stratosphere in Exp2 as described above.
emission trend specified in this study should be further             In future works, we will further evaluate our future
evaluated to validate our past simulation of tropospheric       simulation including the other IPCC scenarios. The effect
O3 .                                                            of future stratospheric O3 change (O3 recovery), not
                                                                considered in this study, will be also included.
Future simulation: impacts of future climate change on
                                                                   Acknowledgments       We wish to thank T. Nozawa and T.
stratospheric ozone transport to the troposphere                Nagashima at the NIES for providing the data from the
                                                                CCSR/NIES coupled ocean GCM and for their useful comments.
      In this paper, we focus on future changes in              We also thank J. Kurokawa at the FIP for cooperating in
stratosphere-troposphere exchange (STE) associated with         compiling the emission dataset. This study has used the SX-6
climate change and their impacts on tropospheric O3 in our      computing system at the NIES.
future simulation of tropospheric O3 using the SRES-A2
scenario [Sudo et al., 2003]. As Figure 2 shows, the            References
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