CO2 - LASER RADIATION ABSORPTION IN 1000 CM-1 WATER VAPOR CONTINUUM

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CO2 - LASER RADIATION ABSORPTION IN 1000

CM-1 WATER VAPOR CONTINUUM

V. Aref’Ev, N. Sizov, V. Dianov-Klokov, V. Ivanov

To cite this version:

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V.N. Aref'ev, N.I. Sizov, V.I. ~ianov-~lokov* and V.M. lvanovf.

,Institute of Experimental MeteoroZogy, Obninsk, U. S. S . R.

.

I n s t i t u t e of Atmospheric Physics, Moscow, U. S. S. R.

Abstract.- Water vapor absorption of C02-laser radiation at R 4 0 , R 10, P 20, P 32 lines of the

1 0 . 4 ~ band and R 10, R 18, R 44 lines of the 9.4y band has been investigated in the laboratory. The results obtained are compared with the measurements of 10-13~ solar radiation attenuation. The experimental facts strongly support the assumption that water dimers play an important role in the 8 - 1 3 ~ region. The analysis of field and laboratory experiments allow the following conclusion : under most meteorological conditions occuring in a cloudless atmosphere the continuum absorption by water vapor must be taken into account in the C02

-

laser radiation absorption. Estimates of the attenuation can be successfully obtained by making use of approximate expressions deduced from the laboratory data.

The 8 - 1 3 ~ 4 atmospheric window plays The labora tory-scale experiments an important r o l e i n i n f r a r e d r a d i a t i o n were c a r r i e d out i n a W t e o p t i c a l multi- t r a n s f e r i n the atmosphere and has been pass c e l l using a ~ 0 ~ 1 a s e 1 8 . Radiation studied extensively during recent decades t r a n m i a s i o n Was measured using a two- because of the development o f l a s e r com- channel 88%-up with an e r r o r of 8-10%. munication systeme,laser remote mcnito- ~ e a s u r e m e n t s of with water va- r i n g of the atmosphere,laser navigation, por a t temperatures @= 284-353 K and wlth

etc. a mixture of water vapor with nitrogen a t

Linear dependence of o p t i c a l thicknessTon water vapor content w

.

This conjecture was confirmed i n laboratory s t u d i e s 2-5.

Penner e t a1. (196812, Bignell ( 1 9 7 0 ) ~ and Burch ( 1 9 7 0 ) ~ a l s o obtained a negative tempsrature dependence f o r the absorption c o e f f i c i e n t , while Moskalenlco e t a l .

6

(1972) i n t e r p r e t e d laboratory d a t a assu- ming a l i n e a r dependence of -'L- on W and concluded t h a t the absorption increased with temperature. The same a s s e r t i o n was made by Trusty e t a1.(1973)~. These con- t r a d i c t o r y r e s u l t s have l e d us t o m d e r take a s e r i e s of complex laboratory and f i e l d s t u d i e s of r a d i a t i o n a t t e n u a t i o n i n the 8-13/2 region.

Laboratory s t u d i e s

revealed t h a t (1) the e f f e c t of broadening gas pressure (P = 1000 mbar) on the ab-

N2

sorption by water vaporwasnegligible over the e n t i r e range of absolute humidities i n v e s t i g a t e d 6(=0.1-25 g m'3) and t h a t (2) an i n c r e a s e i n the temperature ( i n the ran- ge of@=284-353 K ) r e s u l t s i n a decrease of o p t i c a l thickness r = - l n ~

.

These ex-

perimental r e s u l t s can be described by the approxima t i on formula

T ~ ~ . ~ = ~ x ~ [ - ~ / k ~ a ( l + d * p )+

N2

+%a2ew(- A VRQ# (1

where R i s the universal gas constant, -6 6 Kl=1.?6 x 10-3m3/g km, %=0.42 r 10 JII

/

g2km,d=1 -78 x ~o-~mbar-', d Ha-4546 c a l l mol being the matcbjng parameters found by the method of l e a s t squarea. A standard

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C9- 102 JOURNAL DE PHYSIQUE

deviation of d'T=0.08.

leasurements of Td f o r 9.16,9.28, 9.32,10.12,10.32 and 10072y have revealed the absorption t o be unaltered i n charac- t e r with these wavelengths a s well. It is, theref ore, possible t o modify the expressi- on (1 ) using the c o e f f i c i e n t C J f o r the e n t i r e 8-1 3 f 1 region.

5

= C , ) Z 10.6 (2

The values of the c o e f f i c i e n t C A =0.69,

0.71,0.72,0.87,0.92,1.04 were determined r e s p e c t i v e l y 9.16,9.28,9.32,10~12,10~32,

10.72fion the b a s i s of new data (Burch) and

9

Roberts e t a 1

.

The coinciding values of C2 s 0.66,0.73,0.72,0.8,0.92,1.04 f o r the corresponding wave lengths have been obtained experimentally. The measurements allow one t o describe the dependence of

5,

on

/?

over the 9-1 I,$/ region by the approximate formula

C, = 0.56

+

769.2 exp(- 79.1 A - ' )

0)

(standard d e v i a t i o n i s 0.03).

The r o l e of aerosol was estimated f o r 10.6$1' i n s p e c i a l s t u d i e s when the values

Of '10.6 and '0.63 versus r e l a t i v e humidi- t y r were measured, The dependence of Q,63 on r i s t y p i c a l of water o r water-coated aerosol, t h e r e being almost no a t t e n u a t i o n when r

<

30-35%, a slow increase f o r 3582 2: r.475% and a rapid i n c r e a s e when r>75%, t h e zones of p o i n t s which correspond t o the transmission T~ 6 f o r d i f f e r e n t Q - * s ,

. 3

t u r n out t o be p r a c t i c a l l y overlapping.

!&is r e f l e c t s t h e r o l e played by r e l a t i v e humidity i n the processes of aerosol forma- t i o n i n c o n t r a s t w i t h this, TlOo6 i s atro- ngly influenced by temperature which con-

firms the d i f f e r e n c e between the mechanism

of a t t e n u a t i o n of 1 0 . 6 g r a d i a t i o n due i n the main t o molecular absorption and t h a t of a t t e n u a t i o n of 0 . 6 3 ~ r a d i a t i o n , where there i s only s c a t t e r i n g of r a d i a t i o n by water aerosol.

Besides, our atmospheric-air contai- ning c e l l measurements have shown t h a t the c o n t r i b u t i o n of n a t u r a l a e r o s o l t o

210.6

does not exceed 10%

2

Penner e t a1. (1968) and Bignell ( 1 9 7 0 ) ~ suggested t h a t water dimers might play a r o l e i n the a t t e n u a t i o n of 8-13y

r a d i a t i o n . Our r e s u l t s support t h i s conjecture.

It i s c r u c i a l f o r the understanding of the continuum absorption mechanism t h a t the i n t r o d u c t i o n i n t o the c e l l of aubetan-

t l a l amounts of f o r e i g n gas (PI$ 50 P ~ ~ ~ ) weakly a f f e c t s the value of Tloo6. This

means t h a t the deformation of the water- vapor absorption l i n e s due t o t h e i r c o l l i - s i o n broadening by nitrogen o r a i r a f f e c t s the transparency of t h e mixture very

s l i g h t l y . It follows t h a t the c o n t r i b u t i o n of the l i n e component of the spectrum t o the a t t e n u a t i o n involved i s small.

A second e s s e n t i a l f e a t u r e i s a very s t r o n g negative temperature dependence of The second term i n (1) may be shown t o vary with temperature a s

@

-'4

which i s much s t e e p e r than

8-n,

n = 0.5-2,0, the values occuring i n d e s c r i p t i o n s of temperature deformations of l i n e shapecr. This suggests the conclusion t h a t the temperature dependence o f 2 r e f l e c t s i n the main t h e v a r i a t i o n i n the number of absor- bing p a r t i c l e s .

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t

ned by molecuiar absorption spectnun unde- formable by a f o r e i g n gas. For high humi- d i t i e s i t s i n t e n s i t y i n c r e a s e s a s the squ- a r e of humidity and decreases with increa- s i n g temperature a t a r a t e whose absolute value exceeds the values c h a r a c t e r i s t i c of

temperature deformation of i n d i v i d u a l li-

nes and those of the preexponential fac- t o r s i n the expressions f o r the l e v e l d i s - t r i b u t i o n of p a r t i c l e s . It i s reasonable t h e r e f o r e t o r e l a t e -this spectrum t o d i - mere whose content a t low concentrations i s p r a c t i c a l l y proportional t o the square of absolute humidity. !Sfhe obtained value

of A B.i-4.5 kcaljmole may then be regarded

a s an estimate of the energy required f o r a dimer t o be formed. This value i s i n good agreement with the range o f the indepen- dent e s t i m a t e s f o r the energy of dimer formation, A E=3-6 kcal/mole.

F i e l d s t u d i e s

F i e l d measurements i n the 8-13Jlregi- i

on were made' using the long Bouguer method with a r e s o l u t i o n not worse than 2-3 cm",

The e r r o r of de t e m i n i n g

re

was w i t h i n 0.01-0.03 depending on the season and me- t e o r o l o g i c a l conditions.

A n a n a l y s i s of our r e s u l t s h a s shown t h a t t h e o p t i c a l thickness

re

determined under f i e l d conditions i s nonlinearly de- pendent on w and i s characterized by a ne- g a t i v e temperature dependence. We have a l s o shown t h a t i n analyeing

re

i t i s ne- cessary t o consider r e a l a l t i t u d e p r o f i l e s of humidity, temperature and sir pressure.

In comparing f i e l d measurements i n t h e h o r i z o n t a l paths with the l a b o r a t o r y

i n t o account (2). llhe c a l c u l a t e d values of

<

were found by i n t e g r a t i n g these expressions.

When c a l c u l a t i n g

Cc

use was made. -of the r e s u l t s of synchronous measuremen* of the p r o f i l e s of the atmospheric meteorolo- g i c a l parameters r e l a t i n g t o the place where o p t i c a l observations were made. 'Phe

e n t i r e s e t of experimental d a t a

(%)

and,. of the corresponding c a l c u l a t e d d a t a

(q)

was represented a s a dependence of

re

on

%

or a s t h a t of

/

PC

on

'q

.

I n a d d i t i - o n , i t turned out possible t o o b t a i n a ge- n e r a l ' c h a r a c t e r i s a t i o n of the correspon- dence between both groups of d a t a by avoi- ding the d i f f i c u l t i e s a r i s i n g from, 'he d i f f e r e n t meteorological condition's and d i f f e r e n t wavelengths. (Che c o n t r i b u t i o n of aerosol and t h a t of the o t h e r components of the r e a l atmosphere, neglected i n the c a l c u l a t i o n s based on formulas (1)-(21, were estimated f o r s l a n t p a t h s from

A T =

=g-

Tc.

I n analyzing the r e s u l t s only those obtained under steady a n t i c y c l o n i c condi- t i o n s i n the

cloudless

atmosphere w e r e used. The atmospheric a e r o s o l s t a t e was monitored by synchronous measurement6 of

standard meteorological v i s i b i l i t y

%

o r

ee

i n the v i s i b l e region of the spectrum. leasurementa i n the A n t a r c t i c were made under conditions of extremely low ae- k s o l content. !Che v a l u e s of

re

f o r 0 . 6 3 ~ on the average exceeded the Rayleigh va-

l u e s only by 1%.

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C9- 104 JOURNAL DE PHYSIQUE

the i n f r a r e d and those of

,re

i n the v i s i - b l e region. The estimates of a e r o s o l a t t e - nuation have shown t h a t the c o n t r i b u t i o n of a e r o s o l t o Ce i n the 8-13$/ region does not exceed 3%.

There i s a good agreement between

re

and 2, i n the 10-13$/ region (Fig. I )

.

Fig. 1. Measurements versus calcula- t i o n s of o p t i c a l thickness f o r the Antarc- t i c atmosphere.

The standard deviation of the p o i n t s from the s t r a i g h t l i n e

Tee

<

Zs 0.007. This i s c l o s e t o the observation error. It may t h e r e f o r e be considered t h a t under the conditions of a low water content i n the p r a c t i c a l l y aerosol-free atmosphere above the Antarctic Continent t h e a t t e n u a t i o n of r a d i a t i o n i n the l b - 1 3 ~ windows

i s

described, within the experimental e r r o r , by the continuum molecular absorption.

A s i m i l a r p i c t u r e emerges f o r high water contents i n t h e t r o p i c and the north

tempera$e zon'e. Observations i n the lO-l3$~ yindod i n the t r o p i c s a s w e l l a s summer and w i n t e r obsemationa i n t h e n o r t h tem- p e r a t e zone f o r

%

2

10

km

have revealed

ce/

r e .

to-.bs..aqual t o

anitx

wA-khy..8.c/rz

s0.15 i n a very wide range of Tc 4

0.05-1 i n the t r o p i c s and i n the temperate zone i n summer (Pig.2).

I

0 0-2 0.4 0,6 Cc

Fig.2. Xeasurements versus calcula- t i o n s of o p t i c a l thickness f o r t r o p i c s and temperate zone of the Northern Hemi- sphere. '

Estimates of aerosol a t t e n u a t i o n based on known a e r o s o l models have a l s o shown t h a t

i t i s small under these conditions.

With a low water content i n the tem- p e r a t e zone i n w i n t e r ( E0 1 0.05) tbe r a t i o

ye/

s u b s t a n t i a l l y i n c r e a s e s suggesting the n e c e s s i t y of introducing an

a e r o s o l mechanism f o r the a t t e n u a t i o n of r a d i a t i o n i n winter. Besides, t h e r e i s a s i g n i f i c a n t a e r o s o l a t t e n u a t i o n i n summer under the conditions of a t u r b i d a,$mosphere.

i The experimental f a c t s

-

a nonlinear dependence of absorption on water content,

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10

in the

8-93?

region

.

The results of the analysis of field and laboratory experiments allow the follo-

11

wing concluaiona :

The water vapour continuum absorption must be taken into consideration when cal- culating the C02-laser radiation attenuation by the atmosphere.

For SM&5-10 km the complete attenua-

tion of radiation by the atmosphere in the windows at 10-13,~ is almost entirely de-

termined by the mentioned continuum. Esti- mates of the attenuation can be successfully obtained by making use of approximate expressions obtained in the laboratory,

In

winter (low water content) and un-

der the turbid conditions (SM<

5

km) the

relative contribution of aerosol to the observed attenuations may become appreci- able.

The 8-10/r range observations perfor-

med show that

T.

exceeds distinctly

due to the atmospheric minor constituents, selective absorption.

An

additional contribution of the li-

ne spectra of minor atmospheric components to the absorption ahould be taken into ac-

count in the 8-10p windows.

References

1.

R.

Bignell

,

I?. Saidy

,

P. A. Sheppard

wJ.Opt. Soc.Americsw

,

z,

466, (1963).

2. S.S.Penner, P.Varanasi, S.Chou

"JQsRTw,

2,

1537,

(1968).

3.

K. Bignell ?Quart. J.Roy.Met.Soc. lV

96,

390,

(1970)

4. D.E.Burch. Investigation of the

abaorption in infrared radiation by atmo- epheric gaees. Semi-annual technical re-

port U-4784 under contract mP 19628-69-

C-0263 (1970).

5. J.L.McCoy, D, B.Rensch, R.K.Long

"Appl0Opt.", €3, 1471 (1969).

6. N.N.Moscalenko, O.V.Zotov, V.P.

Dugin. "Jurnal Prokladnoy Spek troskopii1;

,

17,

881, (1972). '

7. G.L.Trusty, S.H.Koozekanani, R.K.

Long. vJ.Opt.Soc.Americall, 6J, 491 (1973).

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Gontcharov, V.I.Dianov-Klokov. "Pribory i

teknika eksperirnentaw, No 1, 198-201, (1974).

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s,

2085 (1976).

10. Aref l ev V.N., Dianov-Klokov

V.

I.

"Optika i spektroskopiyal', Q, No 5, 849-

855

(1977).

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