GEOPHYSICAL RESEARCH LETTERS, VOL. 21, NO. 13, PAGES 1471-1474, JUNE 22, 1994
'Three dimensional modelling of chlorine activation i the
Arctic stratosphere
J. A. Kettleborough, G. D. Carver, D. J. Lary, J. A. Pyle
Centre for Atmospheric Science, Department of Chemistry, Cambridge, United Kingdom
P. A. Scott
Department of Meteorology, JCMB, Kings Buildings, Edinburgh, United Kingdom
Abstract. The IlK 11niversities Global Atmospheric Mod-
elling Progra,mne (UGAMP) General Circulation Model
has been used to study the chemical evolution of the
northern hemisphere polar vortex. A run that includes
a parametrisa.tion of the heterogeneous reactions occur-
ring on polar stratospheric clouds (PSCs) has been ini-
tia. lised with the ECMWF analyses of the 5th of January
1992. This run prodsaces high values of active chlorine
within the polar vortex, for example mixing ratios of C1Ox
(=C1+C, 10+2xC, 1202) of 1.8ppbv are calculated after six
days of integration, with correspondingly low values of HC1
and C1ONO2. During warm periods of this model run,
when the PS(' reactions are not active, the C1Ox is con-
verted slowly back into C1ONO2. C, hemical loss of ozone
in January in the model is small.
,.
INTRODUCTION
The conversion of the chlorine reservoirs H C1 and
C1ONO2 into more reactive forms of chlorine on the sur-
faces of PSCs is an important stage in priming the south-
ern hemisphere winter stratospheric polar vortex for ozone
destruction [Anderson et a.1 1991]. In the northern hemi-
sphere similar processes occur and high levels of active chlo-
rine are observed [Brune et a.1 1990]. However the consider-
able ozone loss that occurs in the southern hemisphere has
not. yet been observed in the northern hemisphere. The
root, of the difference is the greater amount of wave a.c-
tivity in the boreal hemisl)here compared to the austral
hemisphere [Schoebe,'l et al 1992]. Wa.rmer temperatures,
a more disturbed zonally a.symmetric vortex, and strato-
spheric warntings are all related to this wave activity and
can affect the chemistry in different ways. For instance
if the temperatures are cold early in winter then active
chlorine may be produced. Large meridional excursions of
the vortex may actually expose this activated air to sunlit
conditions even in midwinter and some ozone loss could oc-
cur. In contrast to this, enhanced mixing associated with
a disturbed northern hemisphere vortex could mean that
this C1Ox will be removed into reservoir species before the
return of sunlight and so will not destroy ozone.
To understand and quantify these effects models that
include the full 3-dimensiona.1 dynamical, chemical and ra.-
Copyright 1994 by the American Geophysical Union.
Paper number 93GL03048
0094-8534/94/93GL-03048503.00
diative aspects of the problem are needed. Here we present
some results from one such model. After giving an outline
of the model, results ot' a. run initialised in early January
1992 are l)rescnt('d. References are made to measurenents
made during EASOE in order to indicate those results that
match the evolution of the real stratosphere. The main
emphasis here is on the production and evolution of active
chlorine although we also consider the ozone distribution.
MODEL OUTLINE
The UGAMP general circulation model is based on a. ver-
sion of the ECMWF spectral model [Tibaldi et al 1990]. A
tracer advection scheme and gas phase chemistry scheme
have leen added to the model [La.ry et al 1993]. This in-
chides the tracers Ox(=O(D)+ O(3p)+ O3), NOx(=N+
NO + NO2+ NO3). HNO3, HONO3, N20.. N20, ClOx(=
('l + CIO +2x('1202). ITC1, HOC1 and CIONO2. The par-
titioning among the fanlily members is achieved by using
photochemical steady state expressions. For more details
of the reactions included see Lary and Pyle [1991]. Photoi-
s'sis cross sections and reaction coefficients are taken fi'om
DeMote el al [19.90]. A enhancement factor lookup table
is used to calculate photolysis coefficients [Meier et a,1 1982;
Lary and Pyle 1991]. hitegration of the chemical rates of
change is by..' an Euler Backward scheme [Stott and Har-
wood 1993]. The time step varies from 30 seconds during
sunset, and sunrise to 15 minutes a.t nighttime.
As well as the gas pha, se rea. ctions the model also includes
a. simple parametrisation of the reactions occurring on the
surface of PS(',s. This kind of parameterisation has already
been employed in studies such as that of Eckma, net al
[1993]. The reactions
C1ONO(g) + HCl(psc) - C12(g) + HNOa(psc) (1)
C1ONO2(g) + HO(psc)- HOCI(g)+ HNOa(psc) (2)
N2Os(g) + H20(psc)- 2HNO0(psc) (3)
are given a rate of 4.6 x 10-Ss - when the temperature
drops below 195K. The C12 produced by the reaction (1) is
assumed to dissociate immediately to produce C1. There
are no tracers in the model corresponding to solid phase
nitric acid and so the nitric acid produced from the reac-
tions (1), (2), (3) is put straight into the gas phase. This
has two consequences. Firsfly there is no sedimentation of
PSCs, and so no irreversible removal of HNO3. Secondly
all of the nitric acid produced by the heterogeneous re-
actions will be available for photolysis in the presence of
1471

1472 Kettlel')orougl et al.' Three Dimensional Modelling
sunlight. The model 1'1111 described here does not have a
prolonged cold spell and so denitrification is not, expected.
Furthermore, PSCs are most abundant in early January
when photolysis rates are slowest.
The dynamical varial)les and tracers are advected using
a spectral scheme in the horizontal and a. finite difference
scheme in the vertical [Hoskins and Simmons 1975]. A
triangular truncation of 21 wave numbers has been used
for the results presented liere. This approximates to a
6 ø x 6 ø grid. Although the spectral scheme is accurate
[Rood 1987], sharp gradients in constituents, uch as those
associated with a processed polar vortex, can restilt in nu-
merical errors. These appea.r in two ways. The first is that
the advection scheme appears to be over diffusive [Chip-
perfield et al 1992]. At low resolutions the sharp gradients
in constituents, such as those observed at the edge of the
'chemically perturbed region', cannot be maintained in the
model. This smoothing of sharp gradients is also seen in
dynamical variables such as PV (figtire la). However us-
ing a spectral method t.o advect the tracers is consistent
with the numerical scheme used for the dyamica.1 vari-
ables. The second effect of the numerical errors is that
negatives occur in the mixing ratios of some species. Sixice
these are physically unrealistic they are set to zero for the
sake of calculating the clemical contribution to the tracer
continuity equations, while leaving the tracer values them-
selves unchanged. This method is conserving and prevents
the appearance of any spurious sources and sinks.
The initial meteorological fields have been taken from the
ECMWF analysis for January 5th 1992. Tracer fields were
generated using a method similar to that of Douglass et al
[1991]. The initialisation takes a tracer data set froin a 2D
model and transforms it into a vortex following coordinate
system more appropriate for wave-disturbed flows. For a
more detailed discussion of the method see LaW et al (Sub-
mitted Manuscript) The resulting fields are then stepped
forward for a single day using just the chemistry scheme.
MODEL METEOROLOGY
The model has been run for 30 days. The fields pro-
duced are forecast fields and after initia]isation are not
constrained by the analyses in any way. For the first few
days of the run the model dynamical fields compare well
with those observed. In a case study of a period later in
[CONTOUR FROM .1 TO 1.8 BY .1 (X 10- [CONTOUR FROM .1 TO 1.6 BY .1 (X
Fig. 2. Modelled distributions of active chlorine mixing
ratios on 11th January, 12.00GMT, a.t 475K. (a) C1Ox=
(CI q- CIO +2x C120) (b) CIO, solid line corresponds to
90 ø zenith angle.
January, ('arver et al [lhis issue] suggest hat the model
has a good forecast skill for periods of up to about 5 days.
Although the model drifts h'om the observed fields it does
reproduce some of the general features of January 1992.
For instance in both the real and model atmospheres af-
ter a. cold period ]asling for the first half of January the
temperatures warm to above the PSC threshold. The anal-
ysed temperatures then drop below the PSC threshold for
a. short period in late January. The model temperatures
also dropped 1)elow tlle PSC threshold towards the end of
the run. However the model temperatures are too low, the
areal extent of PSCs somewhat too large and the second
period when PSC, s were present is prolonged compared to
the analyses.
The model temperatures for the 11th of January, inter-
polated onto the 475K surface, are shown in figure lb. The
temperatures at which PSC, s form lie over the Norwegian
Sea and Northern Scandanavia. Temperatures are much
higher over tle Aleutian,s and NE Canada. Model PV and
winds are shown in figure lb. Although the gradients of
PV at the vortex edge are not as steep as those in the
analyses, there is still a clear polar stratospheric jet. The
flow of air, a.t this time, is through the cold temperatures
over Scandanavia. across the pole and through the warm
teml)eratures over NE ('anada.
DISTRIBUTION OF CHLORINE
Fig. 1. Model 475K fields at i2.00 GMT on the 11th
of January 1992. (a) PV (PVU)and winds (ms -) (b)
Temperature (K)
The C1Ox distribution on initialisation of the model re-
flects the cold temperatures through the PSC pa.rametri-
sation. The highest mixing ratio of C1Ox is 0.7ppbv. By
January llth, six days into the run, the whole of the
model vortex contains high mixing ratios of active forms of
chlorine and correspondingly low values of the 'reservoir'
species HC1 and C1ONO2. The total C1Ox peaks at around
1.8ppbv (figure 2a) with about 1.6ppbv of this as C10 (fig-
ure 2b). The 0.2ppbv C1Ox contour is contained almost
completely within the 30 PVU contour. The tongue of
C10 over NE C, anada results fi'om thermal decomposition
of the dimer in the warm region of air shown in figure lb.
This elevated C10 is in darkness and consequently there
should be no associated ozone destruction. The vortex air
is depleted in N2Os, indicating denoxification. High values

Kettleborough et al.' Tlree Dilnensional Modelling 1473
of HOC1 are also seen in the vortex. In the model these
a.re produced by reaction (2). If the rea,ction of HOC1 with
HC1 on the stirfaces of clouds ha, d been included, we expect
that such high values would not be present and the 010
mixing ratios would have been further elevated [C, rutzen
et a,1 199'2' Pra, ther 1992]. These resttits for early Ja.nuary
within the polar vortex agree with the observations of a,
low HC1 column [Bell et al, this issue] and the Microwave
Limb Sounder (MLS) measurements of C10 which showed
peak mixing ratios of ClO at about 45øE wit l decreasing
values towards 180øE [X.'Vaters et al 19.93].
As the model run proceeds there is a build tip of a, collar
of C1ONO2 around the vortex edge (figure 3a). This is
consistent with the observations of Teen et al [1992]. The
collar resttits from the rapid reaction of C, 10 with NOa. Tile
ca.use of the collar could be: in-situ production of C10 in
extra.-vortical air. or incomplete denoxification of air a.t tile
edge of the vortex, or mixing processes across the vortex
bouldarv.
An idea of the temporal evolution of chlorine species in
the vortex can be gained 1)5' considering the time series
for various species at .175I( vertically above Heiss Island
(58E.80N) (figure 4). Tle PV values seen at Heiss Island
which lie in lhe range 50-67 PVU, while the values of PV
characteristic of vortex air (o1' at least of activated C1Ox
in the model) were greater than about 30 PV[I. Heiss Is-
land therefore appears to be representative of vortex air
throughout the model run. Of course, the evolution of
the mixing ratios a,t a. point on a,n isentropic surface not
only reflects the chemical changes occurring but a,lso ha.s
contributions from qua.st-horizontal motions a, nd dia, batic
effects. The PV, Ox and Cly (- C1Ox+ HC1 + HOC1 +
C1ONO2) give seine indication of these motions. The grad-
ua,1 increa, se in Ox, for exa, mple is consistent with diabatic
descent.
The temperatures at Heiss Isla, nd start off below the
threshold for PSC formation but gra, dua, lly rise. During
the period 20th-23rd Janua, ry the whole vortex became
warmer and PSC, s were not 1)resent. It can be seen that tip
to this time tlere is a general increa,se in ClOx (figtire 4b);
in fact valtes as high as '2.2ppl)v are reached. During the
warmer 1)erio(l o1' lhe ru lhe heterogelmous reactions are
not, activated an,ywhere in the vortex a,nd the chemical sink
for C1ONO2 is reduced. ('orrespondingly the model shows
an increase in C, 1ONO.2 during this period while at, the same
CONTOUR F OM .1 TO 1 BY .1 (X 10-')l [CONTOUR F OM 1.1 TO 3.8 BY ,1 (X 10-'
Fig. 3. Modelled mixing ratios on 11th January 1992,
12.00 (IMT a ,4751<. (a) C1ONO2, (b) Ox (-03 + O(3p)
+ O('D))
TIME SERIES, 475K, HEISS ISLAND
I, I, I, I, l.._, I ,[ [ / N,
200 6 
T.P
' ' I ' I ' I '  ........ PV
5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35
TIME (DAY8)
' ' ' ' ' ' 16
CIONO2
HNO3
5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35
TIME (DAYS)
Fig. 4. Time series at.-175K above Heiss Island (58E,80N)
(a.) Temperature (K), Ox (l)pbv)a.nd PV (PVI/)(b) the
mixing raios of cl1orine species.
time the HNO3 decreases. This is due in part to HNO3 pho-
tolysis, although there is not enough ra.dia,tion at this time
of the year for the 3ppl)v loss to be a result, of photoly-
sis alone. Itorizontal motions of the vortex account for the
rest of the cliange. The NO, produced as a result of HNO3
photolysis, then reacts with the C10 giving C, 1ONOa. How-
ever in the G('M the recovery of the activated chlorine may
be slightly accelerated due to the diffusivity of the spectral
advection scheme. This may enhance NO2 in the vortex,
and the NO2 will react rapidly with the ('10.
The modelled HNO3 is high within the vortex for most
of the run. Murcray et al [tltis issue] measured low HNO3
when PSC, s were l)reset at Kiruna, on January 9t, h 199l.
In mid .Ianuary. after temperatures had risen and the PSCs
eval)orate(l. as iraloll as 201)pl)v HNO3 were measured. The
discrel)a]cv 1)elween llodel and n_ea,nrelYtent is due to the
1nodel PS(' scheme wlicl does not include condensed ni-
tric acid. During EASOE Oelhaf et al [this issue] mea-
sured ('1ONO2 witlin tle polar vortex. In early January
the ('1ONO2 mixing ratio was very low. but rose to about
3ppbv by early NIarch. Although the mode] has not been
run to NIarch there is some evidence for recovery of C1Ox
to C1ONO2 with mixing ratios of 1.,Sppbv at lteiss Island
1)y the beginning of Fe])rtlar.y.
OZONE DISTRIBUTION
The ozone distribution for the 1 lth of Ja.nua.ry is shown
in figure 3b. Qualitatively simila, r fea,tures a.re seen in the
MLS observa, tions [Waters et a,1 1993] with higher mixing
ratios of 03 in the vortex compared to mid-latitudes. There
is no obvious signal of the 03 loss suggested by Wa,ters et
in the region of high C1Ox. Of course, the dia.ba,tic descent
shown in figure 4a. makes the qua.ntifica.tion of ozone loss
difficult in both the model and the a,tmosphere, a. point we
a, re currently investigating. Compa.rison of the 03 fields
fronl this run with those from a run tha, t did not included
the parametrisa.tion of PS½' reactions indica.tes only a. sma.ll
amout of 1)herochemical O3 loss.
S U M M A RY
A simple PSC, schene conpled with detailed chelnistry
implemented in a GC5! indica.tes that high levels of active

1474 Kettleborough et a.l' Three Dimensional Modelling
chlorine can be present within the northern hemisphere
polar vortex. These model results are in broad agreement
with observations made during the EASOE winter e.g., the
MLS C10 mea.surenaent. s, Waters [1993], and column HC1,
Bell et al [this issue]. The detailed gas phase chemistry al-
lows the determination of the subsequent evolution of this
chlorine. In periods when the PSC reactions are not acti-
vated, C1ONO2 becomes the dominant chlorine reservoir.
Only a small chemical ozone loss is apparent during Jan-
uary, consistent with the weak illumination a.t this time of
year.
Acknowledgeme,rs. This work was funded by DGXII of
the CEC under contract STEP-CT91-0139 for the support
of EASOE. The diagnosis and development of the model
forins part of our contribution to NERC's UGAMP project.
The authors would like to thank the EC, MWF for providing
the analyses from which the model has been initialised and
the Atlas (',omputer C,enter at the Rutherford Appleton
La.borator 3,for ensuring a quick turn around of jobs during
the period of the canqaign. For funding, GDC, DJL and
PAS thank UGAMP, JAK thanks NERC for a studentship.
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J.A. Kettleborough. G.D. Carver, D.J. Lary, J.A. Pyle,
(',entre for Atmospheric Science, Department of Chemistry,
Lensfield Road, (',arabridge, CB2 1EW, UK.
P.A. Stott, Department of Meteorology, JCMB, Kings
Road, Edinburgh. EIt9 3JZ, UK.
Recieved: November 27, 1992
Revised' April 22, 1993
Accepted' October 29, 1993