The purpose is to identify major discrepancies between empirical models and theoretical models and to stress the need for additional observations in the atmosphere and for further laboratory work, since these differences suggest either problems associated with observation techniques or errors in chemical kinetics data (or the existence of unknown processes which appear to play an important role). The model used for this investigation extends from the earth's surface to the lower thermosphere. It includes the important chemical and photochemical processes related to the oxygen, hydrogen, carbon, nitrogen and chlorine families. The chemical code is coupled with a radiative scheme which provides the heating rate due to absorption of solar radiation by ozone and the cooling rate due to the emission and absorption of terrestrial radiation by CO2, H2O and O3. The vertical transport of the species is expressed by an eddy diffusion parameterization

Brasseur, G.

Derudder, A.

English

NASA Technical Reports Server

f_
N91-27641
9
COMPARISON BETWEEN OBSERVED AND CALCULATED DISTRIBUTIONS
OF TRACE SPECIES IN THE MIDDLE ATMOSPHERE
G. Brass_ur* and A. DcRudder**
*National Center for Atmospheric Research, Boulder, CO 80307
**Belgian lnstituu _.for Space Aeronomy, 1180 Brussels. Belgium
117
_TRODUCTION
The number density of atmospheric minor constituents iJ characterized by large temporal and spatial
variability. [n the c_ of long-lived species luch as the "source gases" (N20, CB4, the chlorofluorocar-
boris, etc.), tr_port processes may account for much of this va_riabi}ity. In the case of fast-re•ariel
species SuCh u chemical radicals (OH, HO2, O, NO, CI, etc.), a large fraction of the v_riabiliD" is
produced by the dittrnal and seuonal vLrlation of the solar insolation. However, u these radicals are
usually produced by chemical or photochem.ical decomposition of long-lived species, their distribution is
also indirectly controlled by trar_port processes. Finally, in the c._ of species whose chemical lifetime Ls
apprt_imately equal to the trar_sport characteristic time of the atmosphere (o_ne ai_d nitric acid in the
middle stratosphere, terrpora_y reservoirs such u HO_NO2, CIONO_, HOCI in liven altitude rLnge_),
chem_try and dynamos play an equally important role.
With the measurement, over a significant period of time and over a wide spatial range, of • number
of trare species concentrations, it has become poesible to produce climatological distributions of thee
¢ompotmds and even. for some of them, to inter reliable empirical models. As most of these models resu}t
from averaging a large number of observatinnJ, they may be compared to theoretical models which intend
to simulate global average conditions by solving the conservation equations baaed on chemical, radiative
and dynamical considerations. Such comparison allows the v'_lidation of both observational data a._d
theoretical calculations. Moreover, s_ch study leads to • better understanding of the basic pro<e_es
which control the observed distributions and to the identification of inconsistencies between theor)* a_d
observations.
Ideally, in order to investigate El pro<esses involved, a comparison between theory and observations re_
quire on the one hand mx:ltidimensional model, and on the other hand atmospheric data sets covering the
entire ea.,'th. However, because the data aw.ilable are limited and accurate multidimensional transpor_
schemes are computationaJ}y expem..wve and d]iScult to achieve, "first order" va}idation of the curre_dy
known chemical processes in the stratosphere can be based on simpler one-dimensional calculationJ.
The ptw_:4e of this short paper is to idem_Lfy major discrepancies between _ models and theore_ic_i
models and to stress the need for additional observatior_ in the atmosphere and for further laboratory
work, since these dit]'erences suggest either problemJ associated with observation techJtiques or errors m
chemJc_l kinetics data (or the exhstence of unknown prOCesses which appear to play s_n important role).
The model treed for thb investigation [1! extends from the earth's surface to the |ower thermosphere. ]t
includes the important cnealical and photochemical processes related to the oxygen, hydrogen, carbon,
nitrogen and chlorhae fs_aailies. The chemical code is coupled with a r,_liative scheme which provides
the heating rate due to absorption of solar ra_Jiaticn by ozone and the cooling rate due to the emission
and absorption of t_rrestrial radiation by CO_, H=O and Oa.I2) The vertical transport of the species is
expressed by an eddy diffusion parameterlzation.
COMPARISON BETWEEN THEORETICAL MODELS AND OBSERVATIONS
A_ the model used hereafter is one-dimensional and produces global average vertical profiles, the present
• National Center for Atmospheric Research b funded by the National Science Foundation
https://ntrs.nasa.gov/search.jsp?R=19910018327 2020-03-19T16:35:03+00:00Z
118
<
N20
_$A-5 .:'.%._' ",
r _a e" i '_¢.
AcJe'-_'" e- _ '9.s5 ,_]* _.
. d , e-
Fig. 1. Comp_ison 5e:_een several observed dLstribu-
uons or nitrou.s oxide i3, 4, 5, 6_ Lnd • 1-D theoretical
prof_e.
Fig. 2. C.omp_rison between observed
distribution, of methine [3, 4, "i _nd
1-D theoretic_lprofiJe.
35.
3O
25
e 20
.x
10
_ n i _ i , i i I v i ; u , , i_ I |
, "_h,_, work CFCt_
[- _ lIFe 11)
!ooo.o0.o.o
_ _ 0o %
o UBaLloon-oorne ohserva/,ons &
° _ go
o
o
l I I n Ixl
loo
_, [(,n WHO. 1982)
• Fab,an [51obus. 191_])
• Schm,dt (Globus. 1983)
o Lal et al, 198&
/
VOLUH( MIXING RATIO lpptv}
Fig. 3 Comparison between observed distributions of CFC-11 I3, 5) Lnd • I-D theoretical
profile.
119
study wal fo<u* essentla}iy on the lOnll-lived '.r--e limes. However, some importLm s.nd _nexplained
dira:repLucies concerning the fMt-rea.:'in| species will a.ho be mentioned.
S¢_¢¢ QMes
The c,lcullte4 distributior_ of N20. CHd. CCI,. CH_CCI3. CFC-II Lnd CFC-12. s_e di-played in
Figures 14, The agreement between %heoreticaJ Lnd observed vertic-] distributions hi good for N_O
L_
E ){;
E
<
'°I
'i , ; 1,,,,
, _-' _.._ CF2CI2
ICF[ 121
i " "";'Z,
• L.. e_ ,r, 1,15.
,ib
VOL_H[ _IIXIN_ _I,I,_I0 IpDt,}
%000
Fig. 4. Comp_hon between observed dlqributions of CFC-12 i3, 5] Lnd • 1-D theoretical
profile,
. " ;¢?
Z$ :-_ ' I
!
J
,o
,,- bo':_l'* *' a '_t9
I" j
Fig. 5. Compaxhon between observed distribution, of c_bon tetra.chloride {8, gl • od •
I-D theoretical profile.
120
20
_6
I
0
Q o _ o _%o o oo T_'..711,
o oO oo
'-
CH3CCI] "::._
o.
&
, ....... L L_, , ,,LL,I L L ....... " -?'
VOLU_C "IXIN6 RATIO tppt,_
= .....
_000
Fig. 6. Compa.rison between observed distributiorm of methyl chloroform I8, 9] lmd a I-D
_.heoretictl profile.
mad for CFI4. In the latter c_e however I_ge difTerer, ces in the observations exit =bore 30 km, mLki_.g
the comparison between model and observation difficult. The re!_tiveJy good _greement, in the c_se
of N=O, i_ not =urpriting u the eddy diffusion coeffmiemt "_'hic. are used in the modeL_ !including the
preaent mode]) are _u_,lly tuned to fit the vertic|d profile o r t}-,tt p_ticul_ gas. For the precursor g=e_
of _¢tive chlorine (e.g., the CFCI), the model tends to overestimcte the mixmg r_tio, especiali)' in the
hi&her ]evel_, except for CFC-12. Such discrepancy which =ppe_s in e.mentially _[I 1-D models h= not
yet been re_olved. It c_m be due either to the u_ of tn inadequate eddy diffusion coefficient or to
underestimated lots r,,te {or to both). Indeed, it hM been shown from theoretical cot_ider=tion_ il0!
th,,t the epecifed v'Mue of the 1-D eddy diff'a_ion coefficient should be • function of the lifetime of the
tr=ce-con*tituent. Moreover, uncertaintie_ remain in the c*tlcu]_;ion of the penetration of sunlight in the
SchumL,'m-Runge bands, leading to uncertain photod_ci_tion rates or" the chlorofluoroc_bor_.
The cMculated lifetime of the _ource g_e* p]aying _ major role in the stratosphere it given in T_ble 1.
TABLE 1 Calcu]_ted Lifetime of the Source Gcs_
Species Lifetime (yrs)
NsO 165.6
CH4 i0.0
C_lsC) 1.3
CCI_ 68.8
CHsCCI_ 6.6
CFCL_(CFC-II) 86.6
CF=CI_(CFC-12) 154.3
CFCI_CF_CI(CFC-113) 129.8
CHF_CI(CFC-22) ]612
121_
Active G_es a.nd Temporary Reservoirs
The concentration of active gasessuch a.sOH, HO2, O, CI, C]O, etc. isdL_cult to rneuure sincetheir
concentration _ low and their chernicai re_ti_ ,ty very high. A reliable comparison between theoretical
mode] resu}t.s lind the few a_'_Hab]e data "zequi:_ the know}edge of the so}_ _nhh _.ng]e at the time of
the meuurement and the concentration in the observed L_.r mass of _.he traxusport dependent long-live
spe<ies which are the progenitor of the fa_t reacting compounds. From a.n e_nation of Figures T and
8, it can however be deduced tha_ the most recent measurements of the OH radica.1 [11, 12, 13, 141 have
t.he same order of m_gnitude than v_ues provided by theoretica] modeis but that, Ln the cu, of"HO_. the
vMue* reported by Helten et M. (151 are m the }ower stratoephere a face.or 100 )_ger the: redicted by
V_'LUN[ NI_(IN_ I_A![O I_plvl
Fig. 7. Comp_bon between observed or indirectly deduced mixing ratio of'OF[ [II, 12, 13,
141 _ad theoretical profiies [24 hour average amd daytime average: m_d-Jatitude; equinox).
)S
L
2_
1s
0_
H 0 2 _h,__,0_
• 9-26-80 /// ii
4_ 9-20-83 -'""
ThIS _0rk //f"
daytime / // !---'_t _' _
I 10 10_ 1000
VOLU_[ /"HXIN_ _,TIO (;]_tv)
Fig. B. Comp_ison between observed mixing ratio of HO_ I15'; and theoretictl profiles
(24 hour average and daytime average; rnld-l_.titude; equinox).
o, !:'00_ QUALITY
122
theory. U' additional me_urt=e:.:. • "t r.d to con_rm these data. the ;"_er, tly _cepled chemical scheme
u, Lnerror for the hydrogtn spec._ _t ]e_t, in the atmospheric l_._r where "he ozone concentration Ls
the lUlest.
Efforts to meuuze the vertical dlscr_ution of tempora_ry, reservoirs have been reported only recently.
Figures 9 and 10 show tha, t, especiadi._ /or C]O._O2, the d•ts deduced from i_rg-red meu_ements, for
example from the ATMOS e.xperlmenI., ire corx_Lstent with • 24 hour-averaged model calculation.
I
s .. ?-
.u
C: _.._..--/
V_'._"q: l"dX_', RATIO IOip_v)
Fig. 9. Compa.rLson 5etween observ'ationJ of HO_NO2 I16i end a 24-hour aveJ'•ge theoret-
ical pro61e (1:_d-|•:i_.ude, equiDox I,
.I
........i-. ElON_?
I """ \\
_- ', -......\
t
, ---o-_ _ _-
/ 11 I .......i''I''_°"'
;5 _o IS _o
_'I!X_F,_ _AIIO Ippbv)
Fig. lO. Compirbion between observations of ClO]_O_ 117, 18, 191 and a 24-hour averege
theoretical profile (mld-I_titude, equinox). The dotted line and the duhed lme refer to
ATMO5 d•ti_ •t 30"_ (s_nset) at 47"S (eumrh, e) respectively.
123
Nitric Acld _.nd Ozone
Finally, • comp_u'ison he:ween C_eor3" and o_ser_'mtions is performed for 2 gases (ENO3 and 03) which
are produced in the s_r_taspher_ and ,_hose lifetime varies signi_ca.ntl:, with aititude and latitude. In
the cue of ENOs CFigu_e 11}, the .-g.'eement is fLLrly Iood be_e<n theory _md ob4erv_tion below 30
km but above this hei|ht, most mode_.s seem to overestimL_e the HNO_ mixing ratio. ThiJ discrepancy
is emphasized by the t'Lct thLt • new treLtment of the HNO3 LLMS data 120] indicates that the mixing
ratio retrieved in the upper stratosphere should be reduced by a.s much a.s • fLctor 2-5.
Fig. ll, Comparison be:ween observations of nitric acid (balloon-borne experiments a_
LLM5 data) and a theoretk_ proCqe (24 hour average, mid-_atit_ce, equinox).
Ozone has been measured rather sys',ematicilty and by different techniques over a number of yea_,'•.
The vertic_ profile provided by the L_ StLnd_d Atmosphere 121'_ which is in c/_e agreement wi:h
other da:a bases is comp_ed in Figure 12 with a model CLIculLtion. The "heoretical concentrations are
obviouab' 20 to 40% lower th_..,_ the oLs_rved values in the upper stratosphere. This ozone imb_a.nce
which wu noted in several investigatio.-ts !22, 23) is not yet explained. It could be due either to unknown
addition,,] production processes of ozone or to errors in some chemic_l or phol.ochemlca.I pa.rLmeS,ers. This
problem is • major question as it reflecus some unknown processes occurring in the atm_pheric _gion
where pho1_c..hemlcLJ conditions -pply and where the largest rtls'ive ozone depietinns s.rt predicted u
• response to the emi_ion of CFC•.
CONCLUSIONS
Models reproduce most of the observed distrihutinns of the tr_e species belonging to *.he oxygen,
hydrogen, nitrogen and chJorine families. Some discrepancies however remain, which _ect errors or
uncertainties in the chemical scheme currently adopted in the models. Mort work is thu_ needed to
identify- the physical or chemical processes which could explain the cause of these discrepancies, A
more detail comport•on between observations and theory, which should a_:count for the latitudinal and
seLson•l vaziatinn of the trLce species concentration, should involve muhi-dimension•l models.
124
50' _ 0
o l 2 3 4 S $ ? • 11
YOLuk_'_.MIXING RATIO [PI_+)
Fi I. 12. Comp_i.son between the U.5. Stand_d itmosphere model of ozone [211 and 8
theoreticld ver_icad di_tribuT.ion.
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Comparison between observed and calculated distributions of trace species in the middle atmosphere

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