The changing stratosphere

P&me& spscestd.,vol.40,No.213, pp.373-401,m2 ~~~~.

THE CHANGING ST~T~SP~ER~

MICHAEL B. MCELROY, ROSS J. SALAWITCH

and KENNETH MINSCHWANER

Harvanl U~ve~ity, ant of EWh and Planetary Sciences and Division of AppIie&ciences, Cambridge, MA O&38, U.S.A. (Camera-readycopy I-eceived10 December 1991)

Abstract-Large ~~ in 0s c&served in recent years over Anmrctka in spring a~ the cousequence of catalytic re&ons involving industrially related radiis of chlorine and bromine. About 75% of the loss observed in 1987 was due to the Cl0 diief scheme pmposed by Molina and Molina (1987, J. phys. Chem.91,433) with the bslance associated with the ClO-BrO mechanism introduced by M*y ef al. (1986, Narure 321,759). I’& magnitude of 0, loss is sensitive to the extent of ~~~~, the e&ieacy with which HNOs is renmved from the stramsphere by ~~~rn~ in particulate form. It depends also, according to present undemumding,on the relative abundances of ClN@ and HCl in air tmpped originally in the polar vortex in late fall or early winter. High concentmtions of Cl0 and BrO were obser~I also in the Arctic sua@@we during January and early FWuaiyof19!I9. Itisestimetedthasabau1O%ofO~~intheA~iic~~bctwccn about 16 and 20 km WBSlost during the winter of 1989. The extent of dcnitrif~tion and the pcr~ofIhcv~arethekeyfac(orsi~lloncingthemagnioudeof~~inthenorth. Iris shown, based on analysisofdata iWn the Aunosphaic lface MO~!CU~C Specw e~pctim~nl for early May 1985 at 470s (Farmer et al., 1987, JPL PublicW 87-32, JPL., Pamdma, CA), that heterogeneous chemii (boy the react&t of I&& with Hz0 on sulfuric acid dropkts) can have an influence also on the composition of the mid-latitude saafosphere Implications fur midlatitude 4 of consequent changes in the concentmtions of nitrogen, hydrogen and halogen radicals arediscussed. ItissuggesledthatchsngesinUw:abrindanceafOsinthelowerstra~inIhc tropics can have implii also for climate. VE relatively wazmcliiatcs of the Eocene and Cre taceous and the cold climates of recent glacii g?och$ may be associated with shdbwer and deeper ~PssJw~~~, with &ted expans& and contra&on of the symmetric (Hadky) c&da. WWsphe& 0s may be related to va&tkns in stmm#e& circulation corn&omEng to differing kvels of COs. with additi~l contributions for tbc contemporary cnvironmart due to elevated kvels of industrialchlorim and bromine.

373

MICHAEL B. MC ELROYet a/.

374

1. INTRODUCTION It is a pleasure to have the opportunity, once again, to contribute to a volume of papers assembled in honor of Marcel Nicolet. Baton Nicolet’s contributions to atmospheric science and to astrophysics extend over almost six decades. His achievements are extraordinary, both with respect to breadth and depth. It is not the purpose of this paper, however, to revisit the past. Rather, we propose to survey the present and to anticipate the future, harvesting in a number of instances the bounty of seeds planted and tended by Professor Nicolet over the course of a remarkable scientific career. The paper is offered with deep respect to Professor Nicolet on the occasion of his 80th birthday. Our objective is to review and comment on the current state of stratospheric research. The past twenty years have witnessed a remarkable growth in both the quality and pace of research aimed at developing an improved understanding of the processes responsible for control of stratospheric 03. The work has been stimulated in large measure by concerns that human activity in various forms could trigger a reduction in the column abundance of 03, resulting in enhanced penetration of ultraviolet solar radiation, with implications not only for human health but for the environment more generally. Initial alarms were sounded by Cnttzen (1970) and Johnston (1971), who raised the possibility that oxides of nitrogen introduced to the stratosphere as a component of the exhaust gas of commercial supersonic aircraft could result in accelerated loss of 03 by the catalytic sequence NO+03

+NOz+02

(1)

NOa+O_,NO+02. equivalent to Os+O+~+~.

(2)

Concerns heightened with the suggestion by Molina and Rowland (1974) that chlorine radicals formed as by-products of the decomposition of industrial chlorofluorocarbons (CFCs) could have an even mom durable effect on Q as a result of the catalytic sequence Cl+o-j +ClO+@

(3)

Clo+o+Cl+o2. with effect equivalent to (2). It was suggested (Wofsy er al.. 1975: Yung et al., 1980) that further problems could arise as a consequence of bromine radicals formed by decomposition of relatively long lived btominated compounds such as CHsBr; the list of suspected bromine agents has expanded since to include the industrial products CFaClBr and CPsBr (Prather et al., 1984). The sequences invoked for removal of 03 by bromine chemistry included (Wofsy et al., 1975)

Br+Os-+BrO+@

(4)

BrO+O+Br+02, ~~OgOUS to (3) and equivalent to (2). and (Yung et al., 1980)

Br+Os +BrO+Oa cl+03

+ClO+@

BtO+ClO-,Br+Cl+02, equivalent to

(5)

The changingstratospherr-

05)

03+0433q. elaboration of gas phest chemistry mpondde

375

for removsf

of Cg by the cata@tk paths noted above dominated the developmentof stmto@=ric ~Soy until 1985 when a paper appeand bung the discovery of what has come to be known as the ~tarctk OzOneHole @annanet & 198). It was apiary clearthatmodek basedon gas phasechemisrryalone could not accountfor the huge &sea of 03 obr#rvedover Antarcticain spring since the mid 19709;concentradans of the esseaw mdicak3 pdictcd 00 the b&s of such models weR simply too low. Reactions on the surface of partickscomposingpolarsaraosphcricclouds (PSCs)were invoked to provideadditionalGOWXS of halogendical~ @kElmy et al., 1986; Solomon et al., 1986). The reactions proposed included C3NOs+ HCl(s) -+ Cl2 + HNQ(s)

(7)

C3N03+ H@(s) -+ HOC3+ HNO&s)

(8)

N20s + HCl(s) -_)QNOs + HNO&)

(9)

snd N~O~+HZO(S)~~HNO~(~), when (s) ¬es

WV

a speciespresentin the solid phase. Accordingto present views, reaction (7) foI-

lowed by C!12+hv+2C3

(11)

provides the dominant source of chlorine radicalsin the polar stratosphex. Subsequentloss of 03 occursprimarilyby a sequenceinvolving productionand photolysis of the Cl0 dimer (Molina and Mofina, 1987),

Qoo+M4cl+q+M

W)

2[Q+Q_,QO+@], with an edditiond

~n~~ti~ due to #quence (5) (McBhoy et al., 1986). The Chemistry of Antarctic 05 is discus& in Section 2. As will be seen, the importance of (12) and (5) has been established dimctly on the basis of simultaneous measumments of QO, BrO and 03 obtained during the Airbomc Antarctic Oxone Expcrimcnt (AAOE) in 1987 (Andcmon et al., 1989a. 1989b;Anderson ef al., 1991).consistent with msuhs from the National Oxonc Bxporimeni (NO2331) in 1986(de Zafra et al., 1987;P. M. Solomon ef al., 1987; S. soloman cf al., X987).Tbe pxoblemis to account for p&uction and maintenance of the exceptionaliy high levels of Cl0 and BrO observed during these experiments. This provides the topic for the discussion in Section 2. Maintenance of high conoentrations of Cl0 and BrO mquires that the abundance of N& must be low; otherwise, radicals would be converted to more stable forms such as CiNQ and 31N0s. On tht other band, concentranons of NOZmust be high enough to ensun a supply of UNOs sufficient to aBow essentially complete convemion of HCI to Cl2 with subsequent pa0dwi011 of QO @actions (7) and (11) followed by IWWuon of Cl with 03). Persistence of high concentratimrs of Cl0 through the latter half of September appears to requim mmoval of HN& from the stmtqbem, most p&ably by pmcipitstion in psnicuIate form, a phenomenon kknud to in the mccnt literatun as denitrification; othenvise, photolysis of HNO, would provide an unaccqWylargesomceofN@.

376

Tthixa~B. MCEIROY etal.

It is clear from the observations obtained on the Airborne Arctic Stratospheric Expedition 1989 that the conditidns resulting in high concentrations of chlorine and bromine radicals over pica are present also in the winter stratosphere over the Arctic. The observations from AASE covered the period from early January to mid February, eqUhdent in SOW time to the period from early July to mid August. The Arctic mission ended, in solar time, at about the time the Antarctic mission began. Concentrationsof Cl0 and BrO observed over the Arctic in early February were comparable to those observed over Antarctica in September, as discussed in Section 3. Conditionsin the Arctic were primed for rapid loss of 0, at the end of the AASE mission; actualloss of 0, was limited by the n&uively earlier break up of the polar vortex in the north. The major distinction between the Arctic and the Antarcticwith respect to loss of 0s appears to relate to differences in the stability of the vortex in the two regions. The vortex is more persistent in the Antarctic, permitting loss of 03 to continue in the southern region for an additional period as long as several months. The most recent review of stratospheric Os (Scientific Assessment of Ozone Depletion: 1991, un~blis~d rn~~~~), prepared by an in~mation~ group of scientists for the United Nations Environmental Program (UNEP) found evidence for significant loss of 0s in all seasons in both hemispheres over the decade of the 1980s. It is difficult to account for the relatively large loss of 03, about 3% per decade, reported for mid-latitudes. The reduction expected on the basis of gas phase chemistry alone is small, less than 1% per decade. This issue is addressed in Section 4. Specifically, using data obtained by the shuttle-borne Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment (Fanner et al., 1987a),we explore the possible role of heterogeneous reactions taking place on the surface of sulfate aerosols at mid-latitudes. The ATMOS data for sunrise at 47% over the period April 29 to May 6.1985, are consistent with a significant influence due to reaction (10). Reaction (10) provides an important path for conversion of NO, (NO+NOs) to HNOs, through the sequence (AASE) in

No2+03--)No3+4

(13)

N@+N@+M+N205+N, folfowd by (10). As discussed, conversion of NO, to HNOs has implications not only for the abundance of nitrogen radicals, but also for hydrogen, chlorine and bromine. We argue, using laboratory data for the rate of reaction (lo), that changes in the abundance and ~m~sition of radicals associated with reaction (10) could have a significant effect on the rate at which 4 is removed in the lower stratosphere at mid-latitudes, in agreementwith the hypothesisadvancedearlierby Hofinann and Solomon Wg9) andRodriguezet al. (1991). Heterogeneous chemistry could be responsible, in part at lcast, for the trend in 4 identified in the UNBP report for the decade of the 198[k. It is possible that additional ~g~~~~~d~~a~ nquence of variations in the dynamics of the lower stratosphere. Them is evidence from observations of stratospheric CO, for significant temporal fluctuations in the rate at which air is exchanged between the troposphere andstratosphere(Schmidtand Khedim,1991). The UNEP reportinclude a discussion of possible effects on surfacetemperatureof the changes in 03 observedin the lower stratosphere (McCormick et 01.. 1989). It suggests that the increase ln surface temperature expected as a Consequenceof enhancedtransmissionof ultravioletsolar radiation, dting fkomthe nxktion in the a11umndensity of 03, may be offset in part at least, by cooling inducedby changesin transmissionof infrared radiation, relating to perturbations in the height distrlbudon of 0s @tciS et al., 1990). This issue is examined in Section 5 using a new state-of-the-art model for transmission of atmospheric radiation. We suggest that changes ln the upper troposphere, speciflcatlr Changes in the location of the tropopause,may have moresignificantimplications for &late than the direct radiative effect on surface temperature ~~~ by UNBE. It is suggested that reductions

The changing stratosphere

377

in 0~ in the lower stratospherecould result in an inctease in the height of the ttopopause which could conuibute to a change in the circulation of the atmosphere over and above that associatedwith the direct radiative effect of the enhanazd burden of 8teenhouse gases such as CQ, fX&, N20 and the CFCs.

2. CHEMISTRY OF ANTARCTIC 0s The AAOEmission provideddefinitive evidence that the changes in 03 obwwed OVCfAntarctica in spring am the result of the high concentrations of Cl0 and BrO. Anderson et al. (1991) used mcasuredconcentrations of Cl0 snd BrO, combined with laboratory data on rekvant rateconstantsC%mder and Friedl, 1988; Sander et al., 1989)to calculate the time rate of changeof 03 on specific i=ntqic surfaces cormqxmding to regionsof deepest penetration of the ER-2 aircraft into what they termed the chemically pert&& region (CPR). They employed data from IO Bights covering the period 23 August to 22 September, 1987. Their analysis covered a range of potential temperatums from 360 to 450 R corresponding to an altitude interval from about 13 to 18.5 km. Resultsfor potentialtemperaturesof 360,410 and440 K are summarized in Figure 1. The decline in 03 obtained using the empirical approach is in excellent agreement with observation. Approximately 75% of the net loss of 03 is attributed to reaction sequence(12) with the balance due to (5). The ~~n~~ of Cl0 are exceptionallylargein the CPR. To accountfor the observed level of Cl0 it is essential that a large fraction of the available inorganic chlorine be converted to reactive species such as ClO, Cl, and Cl,@. We refer to the combhmtion of reactive species Cl0 + Cl + 2 x Cls@ as Cl’. Total inorganic chlorine is represented by Cl” + HOCl + ClNO, + HCl. We identify the latter combination as Cl,. The fraction of Cl,, present as Cl* may be estimated using procedures developed for and applied to analysis of the AASE data by Salawitch et (Ir. (1990). ‘lhii approach exploits an empirical relation between concentrations of N20 and cl, defined by analysis of data obtained by ATMOS. Measurementsof Cl0 an: used in a photochemical model to infer concentrations of Cl and t&a. ‘Ibis procedure, applied to the data employed by Anderson er al. (1991). using measurcmcnts of NzO by Loewcnstcin et al. (1989), indicates fractional convcnion of Cl, to Cl’ as hi8h as 80% as indicated in Wgure 2. Ihe importanceof the condensedphase as a sink for NO,, (NO + NO2 + 2xNzOs + HNQ + HNOYJ + HNO4 + C&Q + BrNq) and the rule of condensed forms of NO, in the chemistryof Antic 4 was firstsuggestedby Toon et al. (1986) and CIU~IWI and Arnold(1986). They proposed thatstablephases of HN@ could formin the polarstratosphereat temperatures appteciablyabove the frostpointof water,accountingfor whathave come to be known as Type 1 PsCs. Subsequentthermodynamicrn~~~~ by Hansonand Mauersberger(1988), summarizedin Figurr:3, i~ti~ the =kvant compoundas nitricacid trihydrate(NAT), HN@ - 3 H20. Acconling to present understanding, reactionof GINO, with HCl on the surface of NAT, Feaction(7). provides the dominant path for conversionof chlorinefromunreactivereservoirssuch as ClN@ and HCl to morereactiveformssuch as cl,, and eventually cl0 and cl2q. Assuming that reaction(7) provides the key step in the production of &lo&m radicals, it follows that the ultimateyield of Cl* should depend in a sensitive fashion on the abundancesof CXN@ and HQ present in the stratospherewhen the vortex forms in early winter. AXIeventual fractional conversion of 8oRb would imply an abundance of CDT@ relative to Cl, initially equal to 40%. reactionof ClN@

Subsequent

with HCl, thtough reaction(7), would providea sourceof Cl’ equalto twice the initial concentrationof CiNO3,or twice the concentmtionof HCI if HCIis less abunda~ than ClNCb+ The abundance Of CWQ n?lative to HCI should be set by chemical pmcesses ~~~d~~~

MICHAEL B. MCELROY eral.

378

6/25

9/s

s/23

S/IS

FIG obsemd

4

(C3RCLF.S)

and cabbed

a.

1.4

I

m DATE

DATE

VS‘i-E%iE, idoE.

based on in situ Gina

tocyck! (12) (DASHED

WC&. 1987). for removal duo

of Cl0 and BIO during AAOE {AnLINE) and the sum of cycles (12) aad (5) (SOLID LINJZ).

Panels u. b. and c show results fa surfaces of constant potential temperatureequal to 440.410. and 360 K. reJpectively. FromAndersonet al. (1991).

10-I 10-2 10-J %10-’ t ^*lo-a s +lo-4 lo-’ 10-t

\ 10-0-

360 0.4

0.8

I%. 2. FttAtxo~. CONVERSION Cl’ To Cl,. The ratio of reactive chlorine sp@es (Cl’) to tot& inorganic chlorine (Cl,). based on in siru measurements of C.10 (An-on I al., 1991) and N20 (Loewenstein ef al.. 1989) obtained on 23 August 1987 (see text).

100

10”

10-2

10-5

dH2OzO)

10-4

10-a

10-a

(Tad

RG. 3. NIIRICACrD- WATER PIIASE DIAGRAM. Vapor pressums of HNO, and Hz0 in equilibrium with condensed phases for the binury HNO, - i-l20 system. pnsslnts for the three phase ~uilib~um between ‘ice’ (solid s~ibGms of HNOs in ice), trihydmte. & vapor. and the equilibrium between monobydrate. trihydratc, & vapor are representedby the open circles; prcssumsfor the two phase equilibrium between trihydrnte L vapor. at the indicated ~rn~m~~ are represented by the crosses. From Hanson and Mauersberger (1988).

379

The clmging stratospttere

latitudestnatogphcn in fall or early winter. Cl+CH,_,HCI+CH,

(14)

0H+HCl+H20+CI

(1%

ClO+N~+M+ClNO3+M

(16)

hv+ClNO3+Q+N~

(17) (18)

clo+o~cl+~ and ClO+NO+Cl+N~ One might expectthatthe ratioshould dependalso, o

.

w9

discussedin Section 4, on the efficiency of the heterogeneous reaction (10) as a sink for NO,. ‘Ihe ptoblem is comphcated, however, by the fact that the lifetime of HC!lin late fall and early winter is relatively long in the mid to high latitude stratosphete whete the abundance of OH is low. Beaction (10) plays a double role: it contributes to an increase in the abundance of ClNO.3relative to HQ; at the same time, by i~uc~ an ~iti~ source of OH, it results in a decmase in the photochemical lifetime of HQ. The abundances of ClNO3and HC!lpresent initially in the vortex depend not only on chemistry but also on the efficiency of meridional transport in delivering these species from the phomchemically active xone. Morerapid meridional transport, by enhancing the connection between low and high latitudes, would tend to increase the supply of CDQ relative to HCl and would result in more favomble conditions for subsequent removal of 03 in the vortex. The relatively small loss of 03 obsewed over Antarctica in 1988 could nflect less efficient transpott from low to high latitudes in that year at the time of vortex formation. The supply of C&Q relative to HCI should be telatively insensitive to the inner of tea&on (10) in light of the dual role of this process, affecting both the fo~~~ of CiNO3 and the lifetime of HCl. sliming studies, summarized in Figure 4, suggest that the ratio of CINO3 with respect to cl, implied by the AAOE data, 40%. is not unteasonahle for current conditions. The large loss of 0, observed in recent years during spring over Antamtica, reductions in column concentrations to as low as 110 Dobson Units (DU), requites that concentrations of chlorine and bromine radicals remain high to at least the end of September. To maintain the requited high concentrations of Cl0 and BrO it is essential that the abtmdanceof HNO-3be exceptionally low; otherwise, photolysis of HNO3would provide a soumc of NO3 sufficient to convert a major fraction of the available radicals to umeactive forms such as C&JO3and BrNq. In order to account for a low abundance of HNO3,and consequently N4, at the end of September, it is necessary that NOYbe physically mmoved from the stratosphete. Evidence for physical mmoval of NO,, was deduced initially from analysis of infrared obsetvations carried out during the NOZB 1 expedition in 1986 (Farmer et al., 1987b; McElroy et al., 1988). In situ measutoments from the EB-2 aircraft during the AAOE mission in 1987 provided conclusive evidence for removal not only of NO, but also Hz0 @hey et al., 1989a, 1989b). The associated phenomena have come to be known as denittification and dehydration, respectively. A number of recent papers have sought to account for removal of NQ,, and Hz0 on the basis of sedimentationof particles formed in PSCS. While progress has been made, it is clear that a defiitive solution to the probl~is~~~~, Poole and McCormick (1988) assumed that growth of NAT on background sulfate aetosols was inhibited mainly by the Kelvin effect, the dependence of vapor pmssuru on particle size. They found

380

MI-B.

MCESROY

et ol

that about 5% of the background sulfate aerosols were involved in initial COndeTlSStiOn Of NAT; rt%Ub ing particle sizes weir: sm& (radii of about 0.8 tun ) and fall velocities were insufficient to provide app~ia~e loss of NOx. W&y et uZ. (1990a) allowed for effects of the free cncrgy banicr to ntk&ation on growth rates for NAT. They concluded that particles large enough to fall appreciable distances (> 2 tt.mradius) could form if cooling rates were less than about 0.5 to 1.O K day-‘; i.e., if cooling mtcs expeGenccdby adding particles were comparable to those associated with typical synopuc events. If cooling rates were much larger than 1.O K day”, a greater fraction of the available condensation nuclei would grow and the average particle size would be too small to allow appreciable removal of NO,. Ttnco et al. (1989)considemd the opposite extreme, the ~n~~ilib~~ si~a~on in which the atmosphere cools to the frost point of water on a time scale short compared to the time required for nucleation of NAT, Subsequent accretion of NAT on ice crystals would result in this case in simultaneous loss of NO, and HaO. Salawitch er al. (1989), adopting an empirical approach, pointed out that size distributions observed for NAT panicles (Hofinann et al., 1989) implied that only a small fraction of the background condensation nuclei (less than 1%) are activated in conjunction with formation of NAT. They arguedthatby depositing all of the available HN4 on a small number of particles it might be possible to form particles of NAT large enough (greater than 2 pm radius) to remove significant quantities of Nor from the stratosphere, Removal of NO, could be enhanced funher by preferential deposition of water on the larger NAT particles if temperatures were to fati below the frost point, as discussed by Poole and McCormick (1988) and Salawitch et af. (1989). Wofsy et at. (1990) noted the presence of particles too massive to be composed of NAT in clouds observed at tcmperatmes above the water fmst point (Gandrud et al., 1990). They suggested that ice crystals forming in colder regions above the level of observation could acquire a coating of NAT in falling through regions of the atmosphere supersaturatedwith respect to NAT. Accretion of NAT would inhibit evaporation of water from precipitating panicles and could serve to pmscrve the life of these patticlcs while enhancing their efftciencyas scavengers for HNOs. The rn~h~srn envisaged by Wofsy et al. (199Ob)could provide significant denitrification with minimal loss of HaO. Observationsof NO, and Ha0 in 1987 provided evidence for extensive removal of both NO, and Hz0 @ahey et al., 1989a.h 1990). Salawitch et al. (1989) presented a picture, reproduced in Figure 5, Of the relationship between Hz0 and HNOs expected if denitrification and dehydration wem to proceed as sequential processes. Removal of NO, as the atmosphere cools to temperatures below the threshold forproductionof NAT, about 1% K, would initiate loss of NO,. with minimal depletion of H20 (the abundance of Hz0 exceeds that of NO,, initially by a factor of about 250). Mcasumments of the abundsnces of NO, and Ha0 should cluster in this case along tbe tight hand vertical side of the triangle in figure 5. bSS of NO, would be essentially complete by the time temperatures reached the frost point of water, about 190 K; the vapor pressure of HN& would have declined at this time by a factor of about 100 according to the phase diagrsm presented in Figure 3. Subsequent scavenging of Ha0 as tewmatunx dropped below the frost point would result in NO, and Ha0 ~rnbin~o~ along the bottom leg Of the triangle. The dotted lines in Figure 5 represent mixtures of air preserving initial abundances of NO, and Ha0 and air that has lost appreciable concentrations of both NO,, and H20. Assuming that denitrification occurs in advance of dehydration, we expect all of the data to lie in the interior of the triaW$e. In contrast, the mechanism proposed by Tumo et al. (1989). suggesting that loss of NO, occurs as a consequence of dehydration, would squire a significant cluster of points above the triangle. Mcasuremcnts of gas phase NOYand Hz0 included in Figure 5 suggest that, although dchydrafton Wasextensive over Antarctica in 1987, significant loss of NO,, occurmd without associated loss of H20. Data from the AASE mission (Fahey et al., 1990), summarized in Figure 6, provides even mom convincing evidence for denitrification in the Arctic without concurrent dehydration. Data from both the AAOE and AASE missions suggest that while loss of NO,, is large,

381

The changing stratosphere

440

’

’

I

: I

G

: -

:

$ Q 3 400 -

: : :

,,., 420

P

Fto. 4. CALCULATKDRnno. ClNO, TO Cl, The mdo of GINO, to Cl, prior to conversion of chlorine by reactions (7) and (8). Fmctionel abundances were calculated, every 100 in latitude. using a one-dimensional model with observedconccntrutions of 0s and Cl,. for a solar declination of 0”. The calculations were conncctcdalong slant mixing surfaces approximately parallel to the tropopause (Logan ef al.. 1978). Results shown by the SOLID LINE corrcspond to locations along the mixing surfaces where the photochcmical time constant for HCI is 30 days, as would be appropriate for mpld mixing ol air during vortex formation; the DASHED LINE represents locations where the time photochemical time constantfor HCI is 45 days.

i

: :

I

350

1

::

I

$

: : :

360 ’ 0.2

I, 0.3

1.0

0.4

2.0

0.5

3.0

4.0

Hz0 MIXING RATlO (ppm)

I’

1

1.0

2.0

’

1

3.0

’

1

4.0

’

1

‘I

5.0

Hz0 MIXING RATIO (ppm)

FIG. 5. Nnwc Arm - WAIER RELNWNS’IIIP.

23 AUGUST1987. The SOLID LINE shows the relationship between gas phase HNO, and Ha0 for air that undergoes denitrikation followed by dehydration. The DOTTED LINE!3 represent relationships between HNOs and Ha0 that result from mixing of denitrilkd and dehydrated air with unmoditied air having tha initial composition [from Salawitch Cl ol., (1989)). The DATA POINTS rqescnt messunmenuofNO,and Hz0 obtained on 23 August 1987 polewiud of W’S (Fahey cl al.. 1989a)

ho. 6. NIIRICACID - WAIIZR RELATIONSWP. 7 FEURUARY1989. Same as Fig. 5.. except for 7 Febmary 1989. for observationsobtained poleward of 61V (Fahey CI al.. 1990).

6.0

denitrification is not complete; the atmosphere appears to retain a pervasive msiduat concentration of NO,, equivalent to about 2 ppb (Fahey et al,, 1990) as indicated in Figures 5 and 6. The nature of the residual NO, is unclear. According to the phase diagram shown in Figure 3, the mixing ratio of HNO, in equilibriutn with NAT at the water frost point should be exceptionally small, less than 0.1 ppb. The efftciency of reaction (10) makes it unlikely that the residual NO, could be provided by N205. The high abundance of Cl0 preclud& an explanation due to either Na or NO. We can eliminate also ClNOs; a mixing ratio of ClN& of 2 ppb combined with measured concentrations of Cl0 would require an ~accep~ly large abundance of inorganic choke, Cl,,, in excess of 4 ppb. It seems most likely that the residual NO, is provided by HNOa. The agency pervasive ~ckg~und of 2 ppb could result from observationalselectivity. The NO,, instrument on the ER-2 selectively amplifies the signal due to particulateNO,, (Fahey et al., 1989a);the data in Ftgums 5 and 6 wcm mstricled thercforc to occasions when temperatures were too high to pennit the presence of particulate phases. The background level of Nor could reflect an admixture of unprocessed air, possibly from outside the vortex, with air that had been completely denitrified. On the other hand, the relatively constant residual could arise as an intrinsic property o&the process or processes responsible for denitrification. It could be mlated to the biiodal distribution of NAT particle sixes discussed earlier. To the extent that only the large particles precipitate, the background NO, could represent the portion of HN@ sequestered in the smaller, an-p~ci~~ting aerosols. An ~de~~ding of the nature of the residual NO, could provide important clues to the mechanism for denitrification.

3. CHEMISTRY OF ARCTIC 0s The AASE mission provided convincing evidence that the chemical pmcesses responsible for loss of 0s over Antarcticaoccur also in the Arctic. Mixing ratios of Cl0 were observed in excess of 1 ppb in early February, comparable to levels seen in the Antarctic during September on AAOE (Bnme et al., 1990). The abundance of BrO was a factor of 2 higher inside as compared with outside the vortex; peak values approached 10 parts per trillion (ppt), larger than levels observed in the Antarctic during AAOE in 1987 Croohey er al., 1990). Infrared observations of the column abundance of HCl, ClNOs, NOa, and HNOs indicated extensive conversion of NO, to HN4 and conversion of HCt and ClNOs to Cl’ within the vortex (Mankin et al., 1990, Yatteau et al., 1990). In situ observations of NO,, and HsO, as discussed atxrve,demonstrated that important loss of NO, can occur in the Arctic without concurrent removal of Ha0 (Fahey et al., 1990). This section is concerned with implications of the AASE data for ternoval of 0s in the north. We begin with an empirical approach to this problem. The atmosphere cooled significantly over the course of the AASE mission. Air subsides as it cools. The extent of subsidence can be inferred from the decrease in the mixing ratio of NsO observed on surfaces of constant potential temperature (0). Measurements of NsO and 0, for air inside the polar vortex (distinguished by values of potential vorticity, q. larger than 3.4 x lO_’ K m* kg-’ s-t) on a surface defined by 0 = 460f2.5 K are presented in Figure 7. The decrease of the concentration of NsO with time shown in Figure 7a is consistent with diabatic cooling (descent) of air. il reflects the decrease of the mixing ratio of NaO with increasing altitude in the background atmosphere (Schoeherl et al., 1990). Cooling rates implied by the observed dectuase in NsO (- 0.5 K day-t) are consistent with rates calculated using radiative models (Rosengeld ef al., 1990). Descent of air may be expected to result in an increase in the mixing ratio of 03 on a fixed 6 surface, reflecting the increase of the mixing ratio of 0, with altitude. The trend in the mixing ratio of 0s with time expected for the 8 surface included in Figure 7 in the absence of chemical loss is indicated by the dotted line in Figure 7b. ‘Ibe difference between the observed trend, represented by the solid line in Figure 7b, and the trend implied by the

The changing strapsphere

383

dotted lii may be interpretedto yield an empirical estimate of the rate at which 0, was removed by photochemistry. The resultsin Figure 7 imply a loss rate of 0.44% per day averaged over tbe course of tbe AASE mission. Analysisof data for a range of 0 surfaces by Schoeberi et al. (1989) indicated loss rates of 0.44f0.346 per day for the legion of the vorteXsampled by AASE, corresponding to a cumulative loss of about 17%.

so-

l/15

f/Jo

rulS

DATE

Era. 7ia.NzO vs TIME. AASE. Measurements of N20 ob@inedduringAASE (Arctic, 1989) for 8 Sulfa de&d by e = 460f2.5 K, q>3Axl@ Km”kg-* se’ VCZtiCalEnorbars representtha star&d dwiatk of the mcaswments. ~SoL~L~~a~~~~tof~ data.

FIG.7b. 0, vs TIME,AASE. Mcasurcmcnaof O3 obtainedduringAA!%(Arctic, 1989) far a swface d&n& by 8 = 460f2.5 K, 4 > 3.4 x lF5 K m2 kg-’ s-l. Vertical enw bars representthe sumdarddeviationof the mcasufewnts. ~SOL~~isa~~l~~fit~~ ~~~~D~~~~e~~ ~f~rn~l~~~~t~~N~O~~mcntsand the &a&m A(@)/A(N~O)= G3.2~ l$, ohtaincdfmm datacc&c&d dming the dive phaseof the flights(Schoebwl61 al., 1990).

A complementary approach to the problem of 03 loss involves use of measured conccntratious of Cl0 and BrO. This is more divot for AASE than for AAOE. The ~~i~ti~ relates to the fact that the vortex was in the dark for much of the AASE mission a& the m&don sampled only a limited portion of the vortex. The loss rate for 0~ implied by measured concentrations of Cl0 and BrO depends on the history of air masses as they migrate. around the vortex. Under conditions of low illumination, Cl0 is converted to cl& while BrO ikonverted to BrCl by reaction of BrO wirhcx0. Significant loss of 0, nquims. simultaneously, high concentrations of QO and BrO together withlight levels sufficient to ensure efficient ptmtolysis of ClaOa. McKee et al, (1990) described an approach to this problem in which the apadal and temporal evolution of air masses was treated using a tmjectory model defined by observed BeIds of pressum amf tempemture. The chemistry of individual air masses was allowed to adjust in response to changhrg ~~~d~. Initial composhion of air masses was ~to~~o~~~~~~~by~E* ~~~s~~a~~for~ of a~~rna~y 1% per day for the perkd fkom 31 January to 10 February 1989 and about 0.4% per

384

Mrow. B. MCE~ROY u

a/.

day averaged over the 39 day duration of the AASE mission. Isaksen et al. (1990) used a two dimensional model designed to simulate effects of denitriflcation and the heterogeneous reactions (7) through (11). They estimated a cumulative loss of 0s of about 8% for the period up to early February (-0.2% per day). Salawitch et al. (1990) accounted for xonal asymmetries of the flow around the vottex using an approach in which values of Cl* inferred from measurements of Cl0 were mapped onto surfaces of constant 8 and q. They estimated loss rates for 03 averaged over the mission of approximately 0.3% per day. About 60% of the loss was attributed to the sequence (12). with the remainder due to (5). Despite intrinsic uncertainties, loss rates of 03 implied by the different approaches outlined hem ate in reasonable agreement. They suggest a cumulative loss of 03 for the range of altitudes of the Arctic stratosphere sampled by the ER-2 during January and early February of 1989 of between 8 and 16%. The vortex was subjected to a major warming cpisodc approximately one week ah the end of’ AASE. It split into two distinct units which warmed steadily until they disappeared finally in the middle of March (Newman et al., 1990). The ultimate loss of 03 depends on the extent of denitrification and on the persistence and thermal evolution of the vortex. In the absence of denitrification, photolysis of HNOa, as sunlight returns to the polar region at the end of winter, would provide a source of N@ tesulting in efficient conversion of Cl0 to ClNOs, short circuiting the roles of both (5) and (12) in removal of 03. The influence of (12) would diminish with an increase in temperature as thermal decomposition competes with photolysis as a sink for Cl&; the rate of thermal decomposition is equal to the rate of photolysis at a temperature of about 220 K.

4. CHEMISTRY OF MID-LATITUDES Cadle et al. (1975) suggested that the partitioning between N&. NaO5, and l-IN03 could be altered by heterogeneous hydrolysis of gaseous NaOs on sulfate aerosols, arguing that this could lower levels of NOa while raising levels of HNOa. Pioneering observations by Noxon (1975.1976) revealed large reductions in the column abundance of NO2 in air originating at high latitudes during fall and winter. Wofsy (1978). analyzing observations of column amounts of N@ (Noxon 1975, 1976) and HNQ (Murcray et al., 1975), concluded that the deficit of NO2 was consistent with conversion to HNQ by way of the heterogeneous reaction (10) operating at latitudes poleward of about 40°N during winter. Evans et al. (1985) showed that reaction (lo), proceeding on background sulfuric acid aerosols with a macdon cfficicncy (sticking coeftlcient x reaction probability) of 0.1, could explain low values of NaOs and NO2 observed in the high latitude winter stratosphere. Austin et al. (1986) and Jackman et al. (1987) reached similar conclusions based on analyses of HNOs measurements by the LIMS satellite experiment (Gille et al., 1984). Recent laboratory measurements indicate that reaction (10) proceeds rapidly on the surface of concentrated sulfuric acid droplets (Mozurkewich and Calvert, 1988; Van Doren et al.. 1991). with reaction efficiencies ranging fmm about 0.06 to 0.1. Rodriguez et al. (1991) showed that recalibrated measurements of Cl0 at 6O“N, on 13 February 1988 (Bnme et al., 1988) are consistent with reaction efficiencies for (10) within the range of measured values. This section considers the influence of (10) on the suite of nitrogen, hydrogen, and chlorine radicals in the mid-latitude lower stratosphere, using measurements by ATMOS as a basis for comparison of theory and observation. The ATMOS experiment (Fanner et al:. 1987a) provides a particularly useful test of the reliability of pmsent models for the chemistty of the mid-latitude stratosphere. Simultaneous measutements of profiles for Os, NO, NOa. NaO5, HNq, HN04, ClNOs, HCl, H20, and CII., were obtained from 29 April to 6 May 1985 for sunrise at 47’S, and for sunset at 3o”N (Raper et al., 1987; Russell et uf., 1988; Rinsland et al., 1989; Gunson et al., 1990; Zander et al., 1990). Our earlier analysis of ATMOS data

385

The chsngiq strstosphere

indicated signikant discrepanciesbetween theory and observation for NO, IV&, &OS, and HN@ for altitudes below 30 km at 47oS, consistent with the inflqnce of naction (10) (McEhoy and Salawitch, 19tBb). Agreement between theory and observation was good for most species at 30°N, with the exception of GIN@ at altitudes above 30 km, and‘N205 between 25 and 30 km (McElroy and Salawitch, 1989a,b). Similar couclusions we= reache&byAllen and DelitslcyW90,1991) and Natarajan and Callis (1991). Revisions to the ATMOS profiti for CINQ, based on improved spectroscopic parameters,mult in better agreementbetween theory and observation for altitudes greater than 30 km at 3tPN (Zander et al., 1990;Natarajanand Callis, 1991). Data from ATMOS was used to specify vertical profiles for 03, NO,. Cl,, H20, CH4, and tcm~erature. F?ofrlesfor individual chemical species wexe‘obtainedby solving the appropriate set of time dependent ma&on equations (Logan et al., 1978; &athcr et al., 1984; McElroy and Salawitch, 1989a,b),using naction rates and absorption ctoss sections from DeMote et al. (1990). Reaction (10) is assumed to occur on the surfaceof sulfate aerosols, with a rate given by

sA 1 vN,O,

(20) 4 I where ‘Y,SA, and VN,~, represent the reaction efficiency, the density of surface area of sulfate aerosol, and the mean velocity of an NZOJ molecule, respectively. We use a value for y equal to 0.06, consistent with laboratory observations (Mozurkewich and Calven, 1988; Van Doren et al., 1991). A Profile for S& shown in Figure 8, was adopted on the basis of a compilation of SAGE II extinction measumments (G. K. Yue, private communication, 1991, to appear in UNEP 1991 03 Assessment Report). The HlQ produced by (10) is assumed to be released to the gas phase immediately, since the n@on Ofthe stratosphere sampled by ATMOS is undematuratedwith respect to HNQ . ho=Y

28 Rio.8. Auxosot. Sutu;~CnAtuu. A~lsurfI%X~ll&U,eslimale the rateofrex%iou(10)iequatiouG!oIl from a compilation of SAGE11ureasurements(0. K. Yua.privatecommunication,1991).

p 26 ?5 8 24

18 0

0.2

0.4

0.6

0.8

1.0

SURFACE AREA (1 O’* cm* cmw3)

Figure 9 shows a comparison of ATMOS data and theoretical Profiles for HNOs, N@, NO, and N205 between 20 and 30 km at 475, assuming gas Phase chemistry only. The abundance calculated for Hl+Q is lower than the omed value, while concentrations of NOs, NO, and NsO5 appear to be

386

Mnxua

B. MCELROY a

al.

too high. Allowing for reaction (10) brings theory and observation into agreement for I-IN03 and NQ, as shown in Figure 10. Results obtained for NsO5 are in closer agreement with observation Whenwe allow for reaction (10). Increasing the rate of (10) by a factor of 3 at 25 km would improve the agreement between model and observation for Nz05, while maintaining acceptable agreement for HNQ. It would result, however, in a less acceptable simulation of NQ (the calculated vahrc in this casewould he about 50% too low). Discrepancies for NO are not resolved by including reaction (10); the fault here may lie with errors in the computation of photolysis rates for N& at large solar zenith angles. There may be problems also with the procedures used by the ATMOS investigators to correct for variations in NO associated with changes in insolation along the optical path (Russell et al., 1988). Corrections requiredto account for the diurnal variation of NO are large (greater than a factor of 3 compared with the results retrieved in the absence of this correction) for altitudes below 24 km. They depend sensitively on the height profile adopted for NO, which may be influenced in turn by reaction (10). Further investigation is necessary to resolve the cause of the discrepancy for NO. Figures 11 and 12 show a comparison of ATMOS observations for 30’N with theoretical profiles calculated with and without reaction (lo), respectively. Reaction (10) has a relatively small impact on the partitioning of NO,, at 30°N on May 1, since the abundance of N205 is small even for the case allowing for gas phase chemistry alone. The ratio [NzO51/[HNO3], for an altitude of 20 km at sunrise for 30% is equal to 0.095 as compared to a value of 0.26 observed at 47”s. reflecting faster photolysis of N205 in the summer hemisphere. Inclusion of (10) would marginally improve results for NO2 at 3OW but not for HN@. A discrepancy persists for NO at low altitude, similar to that noted ahove for the comparison at 47OS. Calculated values for N205 exceed observed values at 3o”N with and without (10). The concenlralion of N205 at sunset reflects a balance between photolysis and production by sequence (13). AUcn and Delitsky (1990) suggested that the discrepancy for N205 could be resolved if the absorption cross sections were increased by 50 to 75%; howcvcr, most of the uncertainty for the photolysis of N205 appears to involve the quantum yield of N&. and not the value of the absorption cross scclion WeMore er al.. 1990). Reasonable agreement exists between the concentration of N205 calculated for midnight and a single observation obtained on 16 September 1986 (Kunde et al., 1988; McElroy and Salawitch. 1989b). Recent studies of NO3 indicate good agreement between theory and observation for altitudes below 30 km (Smith and Solomon, 1990, Solomon et al., 1989). suggesting that sequence (13) is unlikely to be the source of the discrepancy noted for N205. Further data, both from the laboratory and the field, will be required to resolve the discrepancy for N2O5. Hydrolysis of N2O5 on aerosol surfaces can result in important changes not only in the composition of NO, but also in the abundance of hydrogen and chlorine species. Photolysis of HNO3, with the H alom derived from Hz0 through (lo), provides an additional source of HO, (OH + HQ + 2 x H202) (Cadle er al.. 1975; Rodriguez et al., 1991). Lower levels of NO and N& serve to modify diurnal variations of OH and H&, reducing rates for formation of HN& and HNO4 relative to HO, species near Sunset. The reduction in NO results in an increase in Cl relative to Cl0 as a consequence of reaction (19). This shift, combined with the increase in OH, results in a higher level of GINO, relative to HCl as a consequence of the increase in the rate at which HCl is lost by reaction with OH and the concurrent decrease in the rate at which it is produced by reaction of Cl with C!H.+ The signature of heterogeneous processes is clearly revealed in ratios of concentrations for particular radical and reservoir species. Figure 13 shows the sensitivity of the ratios [N&y[HNO& IcWO3l/[HCll, tH02Y[NO& and [ClOv[HC!l] to the rate of reaction (10) as obtained from a simulation of the ATMOS data for 47OS, 20 km, at sunrise. The ratio [N@]/[HN&] observed at 20 km is about a factor of 4 lower than the ratio calculated assuming gas phase chemistry only. It is consistent with a rate for (10) of about lad m-*. The value observed for the ratio [ClN@y[HCl] is almost a

387

The changing stratosphere

j’j-11

10-10

10-g

lo-*

10-7

VOLUME MIXING RATIO FIG.9.N0,.47”S. SIJNIUSE,GASPHASEGNLY. Calculatrd pmfilcsfor HNOs. NOs, md NO (SOLID LINES, as indicated) and NsOs (DASHED LINE) for the ATMOS simulation at 475. sunrise, assuming gas phase chemistry only. Mcasmwncnts of the four gasesoblpincd by ATMOS am indiitcd by the CIRCLES, which have been conmztcd by dotted linca for convcnicncc of intupnxation. EIKW bars representtic one sigma estimateof the.measunmcnt unccxtaintygiven by Russellci al. (1988).

VOLUME MIXING RATIO

FIG. 11. NO,. 3oON. SUNSET.GAS hIME ONLY. Same as fig. 9.. except for 3tPN at sunset.

VOLUME MIXING RATIO

FtG. 10. NO,. 479. SUIWSE, HYDROLYSLS OF NzOs, Same as for Fig. 9., cxccpt the simulation inclti reaction (10) prouxding with an efficiency of 0.06.

VOLUME MIXING RATIO

FIG. 12. NO,. 3o”N. SUNSET. HYDROLYSIS OFN&. Same as Fig. 10.. except for 3WN at sunset.

388

MICHAEL B. MC ELUOY et al.

factor of 2 larger than that derived on the basis of gas phase chemistry and would appear to imply a slightly larger rate for (10) (-2x led se&). Assuming a frcqucncy for collision of N205 molcculcs with sulfate aerosol of 4x 1W5 se& (G. K. Yue, private communication, 1991). these rates imply a value for y in the range 0.025 to 0.05, consistent with results obtained for sulfuric acid droplets in the laboratory (Mozurkewich and Calvert, 1988; Van Donm et al.. 1991). The ratio [H02Y[N02], assuming a rate for (10) equal to 10d se&. is 9 times larger than the ratio obtained using gas phase chemistry only; the ratio [ClO]fiHCl], with the same rate for (lo), is a factor of 3 larger than the gas phase value, as illustrated in Figure 13. Simultaneous observations of these species (HO, and Cl0 were not detected by ATMOS) would provide an important additional check on the significance of reaction (10). The ratio [HQl/(N&] at noon is about a factor of three less sensitive to (10) than the same ratio at sunrise and sunset, reflecting the change in the diurnal variation of OH and HO2 corresponding to the reductions in NO and NO2 arising as a consequence of (10). Simultaneous measurements of radicals during twilight would serve to further define the role of (10). The ratio [OHy[H@] decreases by about 10% when reaction (10) is included while the ratio of [NOl/lNa] is relatively insen@ive to (10); measurements of these ratios alone ate evidently of limited use as indicators of the significance of the heterogeneous path for conversion of N205 to HNOs. Figure 14 illustrates the sensitivity of the ratios considered above to the rate of (10) for the simulation of the ATMOS data for 30% 20 km, at sunset. Ratios at sunset for 30°N am notably less sensitive to reaction (10) than the corresponding ratios at 47% reflecting faster photolysis of N205 in the summer hemisphere, as discussed above. The [N&Y[HNOs] ratio reported by ATMOS is consistent with a rate for (10) of about 2x lab se&, but results am relatively insensitive to (10). The ratio W.N03I/[HCll for 3O”N provides no useful information about the role of reaction (10). The ratios U-W/tN~I and [ClOy[HCl] are about twice as sensitive to reaction (10) as the ratio [NOJ/[HNOs]. Accurate simultaneous measurements of either HOa (or OH) and N@ (or NO), togcther with Cl0 and HQ, would be required to provide information on the rate for (10) at mid-latitudes in summer. Oxides of chlorine, nitrogen, and hydrogen regulate the abundance of 03 in the mid-latitude stratosphere through catalytic sequences (3). (1), and the sequences 0H+0s+H02+&

(21)

H&+03+OH+202

0H+Os-+H02+02

(22)

HOa+O+OH+02. The abundance of NO, moderates cycles (3) and (20). If HO2 reacts with NO instead of Q or 0, cycling of OH and HQ is given by 0H+03

+H02+02

(23)

HOa+NO_,0H+N02 N02+hv-+NO+0 0+Oa+M+03+M, with no associated loss of odd oxygen. The efficiency of (3) is affected in the lower stratosphere by reaction (16). and in the upper stratosphere by (19). The contribution of the various catalytic cycles to the diurnally averaged rate of loss of 03 for the

‘O” m 10-2 g s 8 70-J

to-si 10-a

10”

10-e RATE (Eec-1)

10-S

1o-4

lo-‘~ lo-”

10-7

10-e RATE(sac-‘)

10-S

10”

390

Mmtm. B. MC ELROV et a!..

k

H lo’: t: “n

-iTi

lOOr A ................................T ......................

10-l 10-a

’ “llllJ

10-7

’ “and

10-8

’ “1*aaJ

10-S

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RATE (se&)

RATE (sac-‘)

- c 10-s 10-e

’ “‘-

d:

’ “11111’ ’ “lal**’ 10"

10-e

’ “*mlaJ

10-S

’ “““k

10-4

10" 10-6

’ “*aad

10"

RATE (set-1)

FIG. 14. CON CBKlRAnON RA'~~.~.~O~N.SUNSET SameasFig.l3..cxceptforWN 81sunset.

’ “11111’ ’ “1*111’ ’ I’10-6

RATE (se')

10-S

10-4

The changing stratosphere

391

ATMOSsimulations at 47% and 3oONis shown in Figures 15 and 16, respectively. Results shown in Figure 15~and 16a assume gas phase chemistry only, while those shown in Figures 1Sb and 166 allow for reaction (10). Cycle (1) mpresents the dominant path for removal of 0, when gas phase reactions alone are considered, with important contributions from HO, sequences [mainly cycle (21)] at low altitudes and cycles (3), reaction (2). and both of the HO, sequences at higher altitudes. Allowing for reaction (10) in the simulation of the winter hemisphere (470s) results in a significant increase in the efficiency of cycles (3), (5), and, especially, the HO, cycles which dominate removal of 0, at low altitudes. With the rate for teaction (10) adopted here (2.3x lad set-’ at 20 km), heterogeneous chemistry has a smaller effect on the catalytic cycles for removal of 03 in the summer as compamd with the winter hemisphere, consistent with the reduced sensitivity of the ratios [HOJ/[NOJ and [ClOl/[HCl] illustratedin Figure 14. An increase in the rate for reaction (10) with respect to the value favored in the present simulation would result in a proportionally larger enhancement of radical concentrations for the summer hemisphere, as indicated by the ttends in Figures 14 and 15. It follows that volcanic eruptions. resulting in significant increases in SA, could have a differential effect on 03 in the summer hemisphere. Rodriguez et al. (1990) estimate an uncertainty in SA of a factor of 2 for the contemporary environment. Hofmann (1990)suggests that the value of SA may have increased by 5096over the past 10 years. This underscoresthe need for additional data to define the value of SA for mid-latitudes. Observationsof N@, IIN@, ClNQ, and HCl by ATMOS suggest that the composition of the lower stratospherein mid-latitudes is affected by reaction (10). Rodriguez er al. (1990). using a twodimensional model, found improved agreement between observed (WMO. 1988; WMO, 1989; Bojkov et al., 1990)and computed trends in 03 both at high and mid-latitudes when they allowed for the iniluence of reaction (10); they attributed the observed loss of @ to the increasing abundance of anthropogenie halogens. Weisensteinet al. (1991) concluded in a complementary study that reaction (10) could play an important role in moderating effects on 03 of water and oxides of nitrogen emitted into the lower stratosphere as components of the exhaust gases of supersonic aircraft Heterogeneous reactions on sulfuric acid aerosols have been invoked also (Hofmano and Solomon, 1989; Brasseur et al., 1990. Michelangeli er al., 1991) to account for observations indicating significant loss of 03 following the eruption of El Chichon in 1982(Adriani et al., 1987). The StratosphericPhotochemistry, Aerosols and Dynamics Expedition (SPADE)planned for September, 1992 will include observations of OH and H& iu addition to the gases measured during AASE, providing for the first time simultaneous, in situ mcasurcmcnts of nitrogen, hydrogen, chlorine and bromine radicals. Data from this mission should allow a definitive analysis of the role of heterogeneous chemistry at mid-latitudes.

5. THE TROPICAL STRATOSPHERE: INPLUENCE ON CLIMATE We discuss in this section the possibility that changes in the concentration of 03 in the lower stratosphere in the tropics could have an effect on global climate. Earlier studies of the influence of stratospheric 03 on climate were directed towards an evaluation of the effect of changes in 0, on radiative forcing of surface tcmpcratures (Ramanathan et al., 1976; Ramanathan and Dickinson, 1979; Wang et al.. 1980;Lacis et al.. 1990). We propose here that changes in the abundance of 03 in the lower tmpical stratosphere could have an additional effect on global climate by modifying the stability of the atmosphere in the ngion of the tropopause. Changes in tmpical 0, could arise as a consequence of changes in stratospheric circulation. Indeed, there is observational evidence for fluctuations in the rate that air is exchanged between the tmposphete and stratosphere (Schmidt and Rhedim, 1991). Reductions in 4 could arise also as a result of chemical loss triggered by enhanced levels of cl,, induced by heterogeneous reactions as discussed

392

Mm

B. MC ELROYeral.

30.

28 -\ 26-

\ \ \

24 -

22 -

,/’ ,...z

20 -

16 1 o-2

loo

10” FRACTION OF TOTAL LOSS

FRACTION OF TOTAL LOSS

l%. 156. ODD OXYGEN SINKS,47OS. HYDROLYSIS01: Nzq. TIIC fractional contribution of the important sinks to the diumully aversgcd loss rate of odd oxygen, computcd using constraints imposed by ATMOS as for Ihc simulation shown in Fig. 10. [reuciion (IO) pmcceding wilh an cfficicncy of 0.06].

FIG. 15~. ODD OXYGEN SINKS,47% GA.S~1A.W WY. The fractional contribution of the dominant sinks to the diumslly avaaged loss rate of odd oxygen, computed using constrsintaimposed by ATMOS as for Le simulstion shown in Fig. 9. (gas phsse chemistry only). Loss due to cycles (21) and (22) is indicated by HO,, that due to cycles (3) and (12) by Cl,., and hat due to cycles (4) nnd (5) by Br, (a profile for inorganic bromine consistent with a mixing ratio of 10 ppt in the upper strstospherewas used).

30

I

28 -

‘ii‘ t

26 -

x

E a

FRACTION OF TOTAL LOSS

FRACTION OF TOTAL LOSS

FIG. I&. ODD OXYGEN SINKS. 304N. GAS PIIAW ONLY. Sume as Fig. 150.. except hr 3fPN.

FIG. 166. ODD OXYGEN SINKS.3CPN. HYDROLYSIS01: NzO,. Snme ss Fig. 1%. cxccpl for 3W’N.

I III

The changingstratosphere

393

by McElroy and salawitch (1989a). or inmrrses in HsO, related, for example, to an increase in the alnu&keofC)4. ‘Ihe present study was motivated by a recent paper (Farrell, 1990) which suggested that major fluctuationsin climate in the past may have been driven, in part at least, by changes in the radiative and dynamical propertiesof the tmpkal atmosphetc i~uencing the spatial extent of the Hadley circulation. It was suggested that equableclimates chamctetistic of the Eocene and Ctetaceous epochs might have arisen as a consequenceof au expansion of the Hadley circulation to higher latitudes, resulting in mom effkknt transport of heat from the tropics to high latitudes. Glacial conditions of the Pleistocene, in contrast, could reflect contractionof the Hadley regime. Models of the symmetric circulation (Held and Hou. 1980; Hou, 1984) indicate that the meridional extent of the Hadley cell, and consequently the efficiency with which heat is transported from the tropics to higher latitudes, depends on the height of the tropopause, H, and on the magnitude of a dimensionkss parameter, I’, summarizing a combination of radiative and dynamical properties of the atmosphere:

where S is a measure of the static stability, zR is the nuliative n&umtion time, 6 is the magnitude of the meridional temperatw gradient appropriate for radiative equilibrium and zA is Ihe time scale for dissi-

pation of relative angular momentum. We shah argue that H and P may be sensitive to conditions in the lower stmmsphere. In particular,we propose that warm conditions (small equator to pole temperature gradknts) may reflect times when the abundance of 0, in the lower stratosphere is tehuively low, msulting in a high tropopause, while cold conditions may correspond to times when concentrations of O~amhighandthetropopauseislow. Indirect empirical support for the hypothesis of a low altitude glacial tropopause may be inferred from a recent analysis of HCHO and C& in polar ice (Staffelbach et al., 1991). The ratio of the concentration of HCHO relative to CHq is expected to vary as a function of the abundance of OH in the Uoposphemaccording to (25)

where&l istherateumstant for reaction of OH with C*, &2is the cotrespondmg quantity for reaction of OH with HCHO, and J is the rate for photolysis of HCHO. Staffelbach et al. (1991) argued that changes in the ratio [HCHO]dq] am due most probably to changes in [OH] (the value of J depends on the intensity of the radiation field below 370 mu and is not expected to vary significantly in response to changes in climate). The change in the ratio [HCHOY[a] observed from interglacial to glacial time was attributed to a decrease in [OH]by about a factor of 2. The lower level of [OH]in glacial time could be a mnce of a higher column density of 0, with a cormspondmgly reduced rate for production of OH. A higher cohtmn density of 0, would be consistent with a lower altitude for the tropopaUSC.

To assess the possible effects of changes in stratospheric 0~ on climate, we consider three postdbk models for 0s summarized in Figure 17. Model A is based on obsetvations of the contemporaty tropkal stratosphere repotted by Barnes et al. (1987). Model B was obtahd assuming photochemi~d equilibrium for Os with the abut&nce of stratosphericCl,, NO,, and Hz0 set equal to values appropriate fortoday. Model C considers an arbitraty mduction in 0s with respectto contemporary conditions in the lower stratosphere, as might arise (under extreme chcumtUances)as a coIlseQueflcteither of an

394

Muxus B. MCJhov

etOL

enhanced diabatic circulation or an anthmpogenically induced increase in chemical loss. A radiative code has been developed that avoids many of the approximations (Curtis Godson, mean transmissivities, dilfusivity factors, etc.) incorporated in earlier models. The model employs a statistical approach to the problem of spectral integrations, a technique referred to in the recent literature as the correlated k method (e.g., Hansen et al., 1983; Goody and Yung, 1989). The principal difficulty in calculating infrared heating rates relates to the rapid variation, orders of magnitude, of the absorption coefficient over smalTsvavenumber intervals. Spectral line profiles in the upper stratosphete am Gaussian, with Doppler hakvidths of approximately 0.001 cm-‘. For line-by-line calculations using a fixed spectml gtid, a minimum of four points per line halfwidth is necessary to resolve individual spectral featums. A total of I$ spectral points is tequired, for example, to cover the 9.6 pm band of 0s alone. The resulting computational effort can be reduced appreciably by a transformation in which a cumulative probability function of the absorption coefficient, k, is employed rather than wavenumber as the independent variable. This involves an organization of quadrature points in order of increasing values of k; nearly identical k values am grouped togcthcr and wcightcd according to their frequency of occuncnce. For a’homogeneous atmosphere (k independent of altitude), the transfonnation is exact; i.e., the spectral dependence of absorption coefficients is conelated at all altitudes. In the real atmosphem, strengths and widths of spectral lines vary with pressure and temperature and absotption coefficients am necessarily altitude dependent. However, to the extent that effective communication of radiation is limited to relatively closely spaced altitude levels charactetiz.ed by similar values of pressure and temperature, the gross pattern of absorptance is maintained and the correlated k method is capable of producing heating rates that agree witb line-by-line results to within 1% (Lacis and Ginas, 1991). A comparison of heating rates calculated using the pmsent model with results from line-by-line model at the Goddard Laboratory for Atmospheres (Ridgway et al, 1991) indicates agmement to within 10% for both the tropical and mid-latitude atmosphere (McClatchey et al., 1971). The model consists of 40 levels separated by -1 km below 20 km, 2 km above, with increased resolution. -0.5 km, near the surface. The mean absorption coefllcient was computed in advance for each layer with line parameters adopted from the AFGL spectral compilation (Rothman et al., 19871, using appropriate reference profiles for temperature and pressure (McClatchey et al., 1971). A variable spectral grid spacing was employed. selected for particular wavenumber intervals by the proximity and width of the neatest spectral lines. We accounted for contributions to the absorption coeffkient from all lines located within 10 cm-t of a particular spectral point, and used the algorithm of Drayson (1976) to compute appropriate Voigt line profiles. The temperature dependence of line strengths was accounted for explicitly through the Boltxmann distribution. Values for line strengths, lower state energies, pmssun-broadened widths at 1 atm and 296 K, and the exponent for the temperature dependence of line widths associated with pressure broadening were taken from Rotbman et al. (1987) for lines in the 15 pm band of COz, the 9.6 pm band of 03 , and for the 6.3 pm and pure rotation bands of HaO. Isotopic forms of these molecules were included. Continuum absorption by water vapor was described using the parameterixation presented by Roberts et al. (1976). The temperature for the atmosphere in radiative equilibrium was obtained assuming that net emission of energy in the infrared is balanced at each level by absorption of solar radiation, mainly in the ultraviolet and visible regions of.the spectrum. Heating rates associated with absorption of solar radiation were calculated for diurnally averaged, equinoctial, conditions at the equator using pmcedures described by Shiiazaki (1985). The heating equation was solved using the Curtis matrix formulation (Rodgers and Walahaw. 1966), with Planck functions and exponential integrals evaluated explicitly. Results shown below were obtained using the convective adjustment scheme (Manabe and Strickler. 1964), in which the temperature lapse rate is replaced by a specified gradient of temperature in regions where the radiative equilibrium temperature profile is found to be statically unstable. In our initial

395

The changing strstosphere

studies, the temperaturelapse rate in regions of instability was set equal to 6.5 K km-t, similar to the observedvalue. Illustrativecalculations were perfont@ using the profiles A. B, and C for @ given in Figure 17. Co~tio~ of water vapor in the troposphere were specified using a fixed xtqmsemtive vertical profile for relativehumidity. The mixing ratioof stratosphericHa0 was held amstat% equal to the value derived for the tmpopause. The computations suppun the conjecture that the structure of the atmosphere may be sensitive to the abundance of 03 in the lower ttopical stratosphere. Wgutu 18 cornpates msults obtained with modelsAand8. Thealtftudeofthttropopauseisreducedby2kmwi~modclBasoompandtr, model A. Use of model C results in an increase in the height of the tmpopause by about 2 km with respect to model A as shownin Wgure 18.

200

240

O3 CONCWTRATlON (cmm3)

Fm 17.h4oDRLPRomEsFQRo&moF+x~. Pro& A (SOLID LINE)corqxm& to obr#rvations

over Natal,Brcrzil(6%) (Bameser af., 1987);prorte B(DOTlEDLINX)comspondstophomckmical equilibriumfor equiuox at the equaton p!ofdC C WlTCQOdtOtUIarbi~ndUC-Ll=.I tionof pfofileA for ahitu&shelow35 km (seetext).

280

320

TEMPERATURE (K)

FIG.18.CALCULATBD TIWPWTURE PROFm??, TRoprcr. calculalai

tmpemlluc

p?ome ror ObLmmi

03 (pm.

ftie A) (SOLIDLlNF$.for increased0, ahuudauces m to photochamii steadystate (proMe B)(DOrlEDLlNE),audforthecascofrc&KX!dc)J abut&Es h&w 35 km (profile c) @AS-ED LINE).

The infrared flux emerging at the top of the atmosphere is equal to 283 W rnB2in all thme cases. The surface temperaturn is 300.0 K for both cases A and C, while for case B the surface temperatute was estimated equal to 300.4 K. Our calculations suggest that changes in surface temperatutu are xelatively i~itive to changea in stratospheric 0s concentrations, in accord with previous studies by Ramanathan er al, (1976). Wang et al. (1980) and La&s er al. (1999). The insensitivity results apparently from the &licate near-balance between ultraviolet abeotption by 0s and absorption and emissionof~radiationinttre9.6CunbPlndof4. Whilethemagnitudeofthechangeinsurface temperature is small, Laeis et al. (1990) showed that the sign of the change may be expected to depend

on the detailsof altitudeprofile of changes in 0, concentraGons. Ramanathanand Dickinson (1979) pointedout thatthe sensitivityof surfacetemperature to a change in stratospheric03 may differ @@iicantlyforthe tropicsss compared with mid-latitudes. tie confusion has arisenin the past concerningthe impactof a change in the abundance and spatial distribution of stratospheric 0s on the radiative forcing of the surface-troposphere System. ‘fhem am three separatebut intemlated effects associated with a reduction in a. A decreasein the column abundance of 0, in th stmtosphen: results in an increase in ~issi~ Of Uhmviolet solar radiation to the troposphere. This tends to drive the surface-tmposphcm system to a higher temperacure;it corresponds thus to positive forcing. Reduction in the abundance of stratospheric 0s provides for more efficient Poisson to space of infrared radiation emitted in the 9.4 pm bands of 4 by the surface-truposphemsystem. The forcing in this case is negative; it may be interpreted as a reduction in the greenhouse effect due to Os. TIE decrease in 6, in the lower stratosphere results in a local reduction in temperature, This arises as a consequence both of the decmase in absorption of uhmviolet solar radiation and the decrease in absorption of infrared radiation at 9.6 pm (the 9.6 j.tmbands are m~ponsible for net heatingof the lower stratospbem)). Colder tempemtums in the lower stmtosphem result in a reduction in the emission of infrared radiation to space. This must be compensated by increased emission andconsequentlyenhancedtempemtumsin the surface-troposphere system below (the total emission of infraredradiationto space is chain to balancethe net absorption of solar Nixon), The relatively small changes in surface temperatums depicted in Figum 18 indicate that positive forcing associatedwith enhanced transmission of ultraviolet radiation is effectively offset by net ncgativc foming in the infrared; the reduction in the gmenhouse effect induced by the change in 4 is more than enough to counteract positive forcing contributed by the decrease in stratospheric temperature. It would be unwise to draw any strong conclusions concerning specific changes in climate on the basis of the simple one-dimensional radiative-convective model described above. At best, the model can be used to highlight potentially important feedbacks to be investigated further in mom realistic two and three dimensional simulations of global climate. An increase in the height of the tmpopauseof the magnitudeimpliedfor case C by the resultsin Figure 18.2 km, would be expected, on the basis of the syrnmctric circulation model employed by Farrell (1990). to result in an expansion of the Hadley mgimc by about 1.5 dcgrccs in latitude. Further changes in tmposphcric d~amics could arise as a consequence of changes in the radiative forcing of the surface-tropospheresystem affecting several or all of the parametersentering into the definition of r in equation (24). An argumentcould bc advanced thatchanges in CX&,by meeting the circulation of the lower stratosphere (Rind et al., 1990),co&d tUWt in changes in the abundanceof 03. Consequenteffects on the dynamicsof the symmetriccircalation,reflecting changes in H and r, could resultin a significantchange in climate. Concentrations of (%2 for the equableclimatesdiscussed by Far&l (1990) were about 2 to 3 times higher than today (Freemanand Hayes, 1991); an increase in r by about a factor of 8 would be required to account for these unusually warm conditions. A relatively sluggish circulationof the stratosphereduring glacial umes, linked perhaps to low concentrations of CQ (about 200 ppm according to Bamola et al., 1987). cNd drive 03 towards the photochemical equilibrium limit envisaged for case B of Figure 17. This could msult in a reduction in H and r, and could contribute to the colder climate of the glacial epoch. As noted earlier, measumments of HCHO and Cl& in polar ice (Staffelbachet ui., 1991) areconsistent with the view that the abundanceof stratospheric0, was significantly higher in glacial time. The Piresentstudy makesthe case, we suggest,thatchangesin 0, in the lower stratospherecuti have a significant impacton climate. A mom definitive study of the ozone-climateconnection is clearly warranted,we believe, in light of the contemporaryevidencefor significantchanges in the abundanceof 0, in the lower stratosphere.

The

changingstratosphm

397

A&wwle&ments-We m i&&ted to S. C. Wofsy and B. E Farrell for stimulatin8discussions. Thii work was suppatedby NSF pant ATM-N-211 19 and NASA 8rantNAGW-1230 to Harvard University.K. Miaschwsnezpatefully acknowledgessupportfrom the AiexandezHostFoundatkm.

REFERENCES Adtiani, A., Fii, G., Gob?&G. P., andCongeduti,F. (1987) Correlatedbehaviorof theaerosol and oxone conteatsof thestrstogphscaftezthe El Chiihon cqtion. J. geophys.Res. 92.8365. Allen. M. andDelitsky,M. L. (1990) Stmto@~& NO, N&, and N~OJ:8 comparisonof mode1resultswith Space&b 3 AfmaspbesicTraceMolecule (ATMOS) mea!auwnalts.J. geopirys.Rcs. 95,14077. Allen, M. and DeUsky. M. L (1991) Infaring the abundaslctsof Cl0 and H@ from Spacelab 3 Atmospheric Trace Molecule Spectmscopyobocmationr.3. geophys.Res. 96.2913. Andemon,J. G., Bnme, W. ft. Lloyd, S. A., Toobey, D. W., Sander,S. P., Starr,W. L., Loewens&in,M., and Fodoiske,J. R. (1989a) Kinuics of Os de&u&on withinthe antarcticvortex: an analysisbased on in titu ER-2 data.i. gu#yr. Res. 94.11480. Anderson,3. G., Bnme, W. H., and Proffitt, M, H. (lw9b) OzMle deslructioo by chlorine radicalswithin the Antarcticvortex: the spat&land temporalevolutionof CiO-0, anticorrelationbased on in sifu ER-2 data.J. geiq&ys.Ress 94.11465, Andemon,J. 0.. Toohey, D. W., and BN~IC,W. H, (1991) Free radicalswithin the Antarcticvortex: the role of CFCs in Anwctic oxone loss. Science 251.39. Austin,J. R,, G&a, R., Russell,J. M., III, Solomon, S., and Tuck, A. F. (1986) On the photochemistryof nitric acid. J. geophys. Rcs. 91.5477. Barnes,R A..,Holland,A. C.. andKiihoff, V. (1987) Equator&lozone pro&s from thegroundto 52 km during SouthemHe&phem autuma.J. geop&s. Res. 92,5573. Banrola,J. U. Raynaud,D., Korotkevich,Y. S.. and Lo&s. C. (1!%7) Vostok ice core provides 16O,ooo-year record of atmo@e& a. Nature329,408. BojkOV, R, Bishop,L., Hill, W. J., Reinsel,G. C., and Tii, G. C. (1990) A statisticaltrendanalysisof revised Dobson totalozone dataover theNorthemHemisphere.j. geophys.Res. 95.9785. Brasscut,G. P.. Granier,C., aad Walters,S. (1990) Futurechanges in stm&qh& oxone and the role of hctezo8eneouschemistry.Nature348,626. Bmne, W. H., Toohey. D. W., Anderson,J. G.. and Ghan,K. R. (1990) In stiu observationsof Cl0 in the Arctic strauwphae ER-2 aircraftresultsfrom 59”N to 8O%llatitude.Geophys.Res. Lerr. 17,505. Bnme, W. H., Toohey, D. W., Am&son, J. G., Starr,W. L., Veddcr, J. F., Da&&on, E. F., and Heidt,L. E. (1988) Jn Bitllobservstiuw of CK3, 4, BIO, NsO, CFC-11. and CFC-12 ia the once lower stmiospherebetween37% and61W l&&e. S&ace 242,558. Cadle,R. D., Crutzen,P., andEhhalt,D. (1975) Hv chemical reactionsin the stratosphae.J. gcophys. Res. 80,338l. titzen, P. J. (1970) The influenceof nitro8enoxides on the atmos+ic ozone content.Q. J. R. Met. Sot. 96, 320. CNtzcn, P. 1. and Amold, E (1986) Nitricacid cloud fomtationin the cold Antanxicstratos~ A majorcause for Ihospringtimekone hole”.Nature 324,651. DeMore, W. B., Sander,S. P., GoMen, D. U. Molina, M. J., Hampson,R. F., Kurylo, M. J., Howard,C. J., and Ravisha&am, A. R. (1990) Chemical kineticsand photochemicaJdata for use in suatosphericmodel@, EvaluationNO.9. JPL Peon W-1, JetPmp&ioa Lab.. Pas&aa, CA de zafm, R. L., Jammillo,M., Parrish,A., Solomon,P., Conner,B., and Barre%J. (1987) Hi8hconceatratia~sof chlorine monoxideat low altitudesin UteAntarcticspring&ram: diumalvariation.Nurtwe329.126. Dmyson. S. R. (1976) Rapidcomputationof theVoigt profile. /. Qu& S~GCWOSC. R&at. Tranfiv 16,611. Evans,W. E J., McElroy, C. T., and Galbally,I. E. (1985) The conversicmof N20s to HNt& M hi$~ latitudesin winter.G&s. Res. &u. 12,825. Fahey.D. W., Kelly. K. K., Fary, 0. V., m L. R., W&on, J. C., Murphy,D. M., Loemn, M. and Chan, K. R. (1989a) In s&umeanrmmentsof~nactivenitrogen,toralwaret,andaerosolhrapoEer~~-. #ieric cloud in the Antarctic.1. geophys. Res. 94.11299. F&Y, D. W., Kelly, K. K., Kawa, S. R., Tuck, A. F., Loewenslein, N., Ghan. K. R, and Heidt,L. E. (1990)

398

MI-B.

MCELROY

et ai.

Observations of denitrification and dehydration in the winter polar stratospheres. Nafare 344 321. Fahey, D. W.. Murphy, D. M., Kelly, K. k, Ko. M. K. W.. Proffht, M. H.. Eubank C. S.. Ferry. G. V.. Loewenstein, M. and Chan, K. R. (1989b) Measurements of nitric oxide and total reactive nitrogen in the Antarctic stratosphere: observations and chemical implications. J. geophys. ReS. 94.16665. Farman, J. C., Gardmer, B. G., and Shanklin, J. D. (1985) Large losses of total Ozone in Antamuca reveal sea sonal ClO,/NO, interaction.Narure 315,207. Farmer, C. B., Raper, 0. F., o’callaghan. F. G. (1987a) Final report on the first flight of the ATMOS instrument during the Spacelab 3 mission, -April 29 through May 6.1985. JPL Publication87-32, Jet Propulsion Lab., Pasadena, CA. Farmer, C. B., Toon. G. C., S&per, P. W., Blavier. J.-F., and Lowes. L. L. (1987b) Stratospheric trace gases in the spring 1986 Antarctic atmosphere. Nature 329.126. Farrell, B. F. (1990) Equable Climate Dynamics. J. annos. Sci. 47.2986. Freeman, K. H. and Hayes, J. M. (1991) Fractionation of carbon isotopes by phytoplankton and &mates of ancient CGa levels. Clokal Bio. Cycles. (in ptess). Gandrud B. W., Dye, J. E., Baumgardner. D., Ferry, G. V., Loewenstein. M., Chan, K. R., Sanford, L., Gary, B. L. and Kelly, K. K. (1990) The January 30.1989 Arctic polar stratospheric cloud (PSC) event: evidence for the mechanism of &hydration. Gcophys. Res. L&r. 17,457. Gille, J. C., Russell, J. M., III, Bailey, P. L.. Rem&erg. E. E., Gordley, L. L.. Evans, W. F. J., Fischer, J-J.,Gandrud. B. W., Giranl, A., Harries, J. E.. and Beck, S. A. (1984) Accuracy and precision of the nitric acid concentrations determined by the Lib Jnfrated Monitor of the Stratosphere Experiment on Nimbus 7. .I. geophys. Res. 89.5 179. Goody, R. M. and Y. L. Yung (1989) Amospheric Radiation: Theoretical Basis, 2nd ed.. Oxford University Press. New Yodc. Gunson, M. R., Farmer, C. B., Norton, R. H., Zander, R., Rinsland, C. P., Shaw, J. H., and Gao, B.-C. (1990) Measurements of CH.,, NsO, Co, HaO, and 0, in the middle atmosphere by the Atmospheric Trace Molcculc Spectroscopy cxpcrimcnt on Spacelab 3. J. geophys. Res. 95.13867. Hansen, J., Russet. G., Rind. D., Stone, P., Lacis, A., Lebedeff, S.. Ruedy, R., and Travis, L. (1983) Efticient threedimensional global models for climate studies: models I and II. Mon. WeatherRev. 111.609. Hanson, D. and Mauersberger, K. (1988) Laboratory studies of the nitric acid trihydratc: implications for the south polar stratosphere. Geophys. Res. Left. 15,855. Held, I. M. and Hou. A. Y. (1980) Non-linear axially symmetric circulations in a nearly inviscid atmosphere J. atmos. Sci. 37.515. Hofmann, D. J. (1990) Increase in the stratospheric background sulfuric acid aerosol mass in the past 10 years. Science 248,996. Hofmann, D. J., Rosen, J. M., Harder. J. W.. and Hereford, J. V. (1989) Balloon-borne measurements of aerosol, condensation nuclei, and cloud particles in the sttatosphere at McMurdo station during the spring of 1987. J. geophys. Res. 94.11253. HoFmann,D. J. and Solomon, S. (1989) Oxone destruction through heterogeneous chemistry following the eruption of El Chichon. J. geophys. Res. 94.5029. HOU.A. Y. (1984) Axisymmetric circulations forced by heat and momentum sources: a simple model applicable to the Venus atmosphere. J. atmos.Sci. 41.3437. Jsakn, J. S. A., Rognerud, B., Strodal. F., Coffey, M. T.. and Mankin, W. G. (1990) Studies of arctic stratospheric oxone in a 2-D model including some effects of zonal asymmetries. Geophys.Res. Lett. 17.557. Jackman. C. H.. Guthrie, P. D.. and Kaye, J. A. (1987) An intercomparison of nitrogen-containing species in Nimbus 7 LJMS and SAMS data J. geophys. Res. 92.995. Johnston, H. S. (1971) Reduction of stratospheric oxone by nitrogen oxide catalysts from supersonic transport exhaust. Science 173,517. Kunde. V. G.. Bmstmas. J. C.. Maguire, W. C., Herman. J. R., Massie, S. T.. Abbas, M. M.. Herath. L. W.. and Shaffet, W. A. (1988) Measurement of nighttime stratospheric NaOs from infrared emission spectra. Geophys. Res. Let. 15.1177. J-acis. A. A. and Oinas, V. (1991) A description of the correlated k distribution method for modeling nongray gase0u.s absorp~On, thennat emission, and multiple scat&ring in vertically inhomogeneous atmosphema. J. geophys. Res. 96.9027. La& A. A., Wuebbles, D. J., and Logan, J. A. (1990) Forcing of climate by changes in the vertical distribution

The

changingstratcsphete

399

of ozone. I. geophys. Ra. 95,997 1. Loewenstein,M.. Podokke,3.R., Ghan, K. R., and Strahq, S. E. (1989) Nitrous oxideas a aYnamid tram in the1987AirbonwAnmrctkOune Expczimen~J. gcoplrys.Rim 94 11589. Logrm, J. A., Rathex, M. J., Wofsy. S. C., and McElroy, M-B. (1978) A-c chanistry: =tqo-e to human influaus J%if. ‘J’rans~R. Sot. A2!HJ,187. Manabe, S. and Suickler, R. F. (1964) ‘lkqnal equilibeiuDil of the atmosphem with a convective~adjustment. J. rmnoJ. Sci. 21,361. Ma&m, W. G, Coffey, M T., Go A., Sdmbed. M. R., Lait, L. R., and Newman, I’. A. (19901 Airbonz measurements of sframspheric constituents over the Arctic in the winks of 1989. Geophys. Res. Len. 17, 473. McCWchcy, R. A., Femt, R. W., Seiby, 3. E. A., Volz, F. E., and Garirtg, J. S. (1971) optical ptopaies of the atmolpphae. Rep. AFCRL-72- 279, Air Force Cambridge Res. Lab.,Bedford,MA. USA. McCormick,M. P., Zawodny,1. M., Vciga, R. E., m, J. C.. and Wang, P. H. (1989) An overview of SAGE I and II (zsn& measmunents. Plan&. sprrcc sei. 37.1567. McElroy, M. B. and Saiawitch, R. J. (1989a) Changiog compositionof the global stratot#a’e. Science 243,763. McElroy, M. B. and &&witch, R. J. (1989b) Stmm@szs ozone: impact of human activity. Plunet. Space Sci. 37.1653. McElroy. M. B.. S&witch. R. J., and Wof$y, S. C. (1988) Chemistry of the An-tic stnttosphem. Pfunez. Spuce sci. 36.73. McElroy, M. B., Salwitch, R. J., Wofsy, S. C., and Logan, J. A. (1986) Reductionsof Antarcticozone due to sywgistic interactionsafchlorine and brcunine.Nurse 321,759. McKenna, D. S.. Jones, R. t, Poole. L. R. Solomon, S., Fahey, D. W., Kelly, K. K., RuKitt, M. H., Brune, W. f-l., Loewenstein, M.. andGhan,K. R. (1990) Calculationsof ozone destn~tion during the 1988/1989 arctic winter. Gmphys. Res. Iat. 17,553. Miihehmgeli, D. V., Altar, M., and Yung, Y. L. (1991) Hetemgeneous leactioos with N&l in the El Chichon volcanic aan&?. Geofiys. Res. La. 111,673. MoIina, L. T. and Molina, M. J. (1987) Froduction of Cl$& from the self-reaction of the ClG radii. J. p&s. them. 91,433. chloline atom catalyzed Motina M. J. and Rowland, F. S. (1974) Stm@#esic sink fix chlorofl -s: destruction of ozone. Natwe 249,810. Mozurkwich, M. and Calve% J. G. (1988) Reaction probability of NzOs on aqueous aexosols. I: gcophys. Res. 93.15889. MWCXSY, D. G, Bark%,D. B., Bnxks, J. N., Goldman, A., and Willis. W. J. (1975) Seasmal and latitudinal variationsof the stmto@& concentration of HN03. Gcopihys, Res. Lea 6,223. Nararejan, M. and callis. L B. (1991) Stratosphek photochemical studii with Atmosphuic Trace Mokmb Spectrascow (ATMOS) meas-rs. J. geophys. Res. %,9361. Newman, P. A., La& L. R., Scboeberi, M. R.. and Nagatani, R. M. (199Q) Stratospheric tempcxatures during the 88-89 Ncrthem Hem&&e winur. Geophys. Res. I&r. 17,329. Noxon. J. E (1975) NO, in the stmmspheze and troposphexe mc~smed by gmund based absorptim spectroscopy. science lb, 547. Noxoo. J. F. 0976) Seamal andiatitudinalbehaviorof stmmqhericN@, paperpresentedat the meetingof Im, Int. Ass. of Mcttxxol. and Amos. Phys., Logan, Utah. l’ook L. R. ani M&xmii, M. P. (1988) IWarstmto@& clouds and the Antwctic (1~01~: hole. J. geopkys Rcs. 93.8423. M. M. J.. McBlmy, M. B., and Wofsy, S. C. (1984) Reductions in ozone at high concentmtionsof stratos-

phcrichabgens. Nuzure312,227. Ramanathan.V., callis, L B., and Bougfiner,R. E. (1976) sensitivity of surface&mpemtmeand atmosphuic temptraulreU,ptubatiaw in the stratosphericconcentrationof ozol~:and nitqen dioxide.J. QI~OS. Sci. 33.1092. Ramanathan,V.andD~nson,R.E(1979)Theroleofsttalosphaicozx#leinthe~antseasonalradiative eIKI%Iybalana of the earth-tqoqhem systan. 1. atinos. Sci. 36, IOW. Rapet, 0. F., Famux C. B., Zandq R., and Park, J. H. (1987) Inftared spccuoscopk -entsofhalo~~~~~ ~inthcstretoaphrre~breATMc)sinstnaneat.J.g~p~.Rts.9~~1. =dgway, W. L.. Harshvardhan, Ark&, A. (1991) Computation of longwave cooliig rates by exact and

apptcximate methods. J. geophys. Res. 96.8969. Rind, D., Snoxso, R., Bah&mdmn. N. K., and Pmther, M. 3, (t99@ CIimane Change in the middk: a-e Part I: the doubled COs climate. J. atmos. Sci. 47,475. Rinsland,C. P., Toon, G. C.. Farmer, C. B., Norton, R. H., and Namlrung, J. S. 0989) Smtosphaic NzOs PrOt&s at sunrise and sunset from further analysis of the ATMOWSpacelab 3 solar spectra J. geophys. KS. 94,

18341.

Roberts, R. E., Bibennan, L. M., and Selby, J. E. A. (1976) Infrared continuum absorption by atmospheric water vapor in the 8-10 pm window. Appi. Opt. 15.2085. Rodgers, C. D. and Walshaw C. D. (1966) The ~~~ of infnved cooling rates in Planetary a~osphermQm. J. Roy. Methyl. Sot. 92.97. Rodriguez, J. M., Ko, M. E. W., and Sxe, N. D. (1991) Role of heterqteneous conversion of NZOS on sulphate aerosols in global oxone losses. Norun 352,134. Rosenfuzld,J. E., Schoeberl, M. R., Lait, L. R.. Newman, P. A., Roffitt M H., and Kelly, K. K.. (1990) Radiative heating rates during the Airbomc StratospMc Experiment Geophys. Res. I&t. 11,345. Rothman, L. S., Gamache, R. R., Goldman, A., Brown, L. R., Toth. R. A., Fickeu, H. M.. Poyntcr, R. L., Fhd, J. M., Gamy-Peynx, C., Barbe, A., Husson, N., Rinsland. C. P., and Smith. M. A. (1987) HJTRAN databa~ 1986edition.Appl. Opt. 26.4058.

Russell,J. M., JE, Farmer, C. B., Rinsland, C. P., Zander, R.. Froidevaux, L., Toon, G. C.. Gao, B., Shaw, J., and Gunson, M. (1988) Measurements of odd nitrogen compounds in the stratosphere by the ATMOS experiment on SpaceRb 3. J. geophys. Res. 93.1718. Salawitch,R. J., Gobbi. G. P., Wofsy, S. C., and McElroy, M. B. (1989) ~i~~~ in the Antarctic stmtosphere. Nature 339,525. Salawitch, R. J., McElroy, M. B., Yatuau, 3. H., Wofsy, S. C., Schoebe& M. R., La& L. R., Newman, P. A., Chan, K. R., Loewenstein, M., Podolske, J. R.. Stmhan, S. E., and Froffitt, M. H. (1990) Loss of oxone in the Arctic vortex for the winter of 1989. Geophys. Res. Lett. 17,561. Sander, S. P. and Fried&R. R. (1988) Kinetics and product studii of the CIO+BrO reaction: Implications for Antarctic chemistry. Geophys. Res. Lett. 15,887. Sander, S. P.. Friedl, R. R., and Yung, Y. L. (1989) Rate of formation of the Cl0 dimer in the polar sbatospherez implications for oxone loss. Science 245,1095. Schmidt, U. and Khedim, A. (1991) In s&u measorem ems of carbon dioxide in the winter Arctic vortex and at rn~ati~ an indicetor of the ‘age’ of stranx#eric air. Geophys.Res. Z&t. 18,763. Schoeberl, M. R., Proffht, M. H., Kelly, K. K., Lait, L. R., Newt, P. A., R~n~e~, J. E., Loewenstein, M., Podolske, J. R.. Suahan. S. E., and Chan, K. R. (1990) Stmm@eric constitnent trends from RR-2 protile data. Geophys.Res. &cc. 17,469. Shimazaki,‘I’.(1985) Minor Constituentsin the Middle Atmosphere, Terra Scientific Pub. Co., Tokyo, Japan. Smith, J. P. and Solomon, S. (1990) Atmosphe& NOs 3. Sunrise drqqeamnce and the stratospheric protIle. J. geophys. Res. 95.13819. SoJomon,P. M., Conner, B., de Zafra, R. IL.,Fan&h, A., Barrett, J., and Jaramillo, M. (1987) High concentrations of chlorine monoxide at low altitudes in the Antarctic spring stratosphere: secular va&tion. Nature 328,

411. Solomon, S., Garcia, R. R., Rowland, F. S., and Wuebblcs, D, J. (1986) On the depletion of Antarctic 07m. Nature 321,755. SokXnOn,S., Miltcr, H. L., Smith, J. P., Sanders, R. W.. Mount. G. H., Schmchckopf. A. L.. and Noxon. J. E (1989) Atmospheric NO, 1. Measurement Technique and annual cycle at 4tPN. J. geophys. Res. 94,11041. Solomon, S., Mount. G. H., Sanders. R. W., and Schmeltekopf, A. L. (1987) Visible specuoscopy at McMurdo Station, Antamtica. 2. Observations of OClO. J. geophys. Res. 92.8329. Staffelbach, T., Neftel, A., Stat&r. B., and Jacob, D. (1991) A record of the atmospheric methane sink from for-

maldehyde in polar ice cores. Nurrwe349,603. Toohey, D. W.. Anderson, J. G., Brune, W. H., and Ghan. K. R. (1990) In situ measurements of BrO in the mctic stratosphere. Geophys. Res. Lett. 17.513. Toon. 0. E., Hamill. P., Turco, R. P., and Pinto, J. (1986) Condensation of HNOs and HCl in the winter polar stratospheres. Geophys. Res. L&t. 13,12&1. Turco. R P., Toon, 0. B.. and Hamill, P. (1989) Highs ~~~~~~~ of the polar oxone hoIe. J, geophys. Res. 94.16493.

The changing stratosphere

401

Van Doren. J. M., Watson, L. R., Davidovits, P., Wasanop, D. R.. Zahniacr, M. S., andKolb, C. E. (1991) Wake of NzOs and HNGs by aqteous aulfuric acid dmpiets. J. phys. Chem 95.1684. Wang, W. C., Pinto, J. P., and Yttag, Y. L (1980) Climatic effects due to haloge.nated contpounda in the Eatth’s atmosphere. J. alms. Sci. 37,333. Weisanatcin, D. K., Ko, M. K. W., Rudrigucz. J. M., and Sue, N.-D. (1991) Impact of heterogateous chemistry on mo&l-calculatcd ozone change due to civil tmnspott ;tsllrcraft.GeopIrys. Res. Lat. 18.1991. Wofay, S. C. (1978) Tamporal and latitudhtal variationa of stmmqksic trace gases: a critical comfmriaon between than-y and expathnsnt. J. geophys. Rcs. 83,364. Wofay, S. C., Gobbi, 0. P., Salawitch. R. J, and McEhoy, M. B. (19!Xk) Nuckation and growth of HN03 -Hz0 ~~~~~~ J. atnms.Sci. 41, uK)Q. Wofay, S. C., McBhny, M B., and Yung, Y. L. (1975) Tlta chemistry of atmoapherk bromine. Geophys. Res. L&t. 2,215. Wofay, S. C., Sahnvitch R. J.. McElroy M. B., Yatteau J. H., Gandrud B. W., Dye J. E. and Baumgatdnar, D. (199ob) Condensation of HNOs on falling ice particles: me&aniam for da&if~ation of tha polar s&amqham. Geophys.RGS.La. 17,449. World Mctrxxolngical Organizatimt (1988) Global saute msc#ch and ~i~ng project, Report No. 18. Report dthe ilu-ewnol mat? tr&pwwl: m8, Geneva, swim World Mcbmologicd Organii (1989) Global oznna msearuh and mot&o&g project, Reptut No. 20. Scitntijk assessmetu ofslrarosphsric owne: 1!%9, Geneva, Switzerland. Yattaau, J. H.. Wofsy, S. C., Salawitch, R. J., McEhny, M. B., Schoabml. M. R.. Lait, L. R.. Newman, P. A., Tones, A., Jnqensen, T., Mankin, W. G., Coffey, M. T.. Toon, G. C., Loewenatein, M.. Fodlskc. J. R., Stmhan, S. IL, Ghan, L R, and Ptnffttt, M. H. (1990) Bffccts of tranapott on column abundamxs of n&r+ gen and chlorine compounds in the Arctic ~. Gcophys. Rw, La 17,533. Yung. Y. L., pinto, J. P.. Watsat. R T.. and Sander, S. P. (1980) Aunn@eric bromine and ozone parturbatkws in the lower w. 1. tmos. Sci. 37,339. zander, R., Gunson. M. R.. Fosta, J. C., Rinsland, C. P., and Namkung. J. (1990) Stratoaphesic ClONOs, HCI, HF cnncantration ptofiles derived from Atmoqksic Trace Molecule Spectrnscopy experiment Spacelab 3 observations: an update. J. geophys.Res. 95.20519.