METROLOGICAL SUPPORT TO RSS LIDARS IN ATMOSPHERIC POLLUTION MONITORING

E. K. Ivanov, V. A. Kolbenkov, L. A. Konopel'ko and V.V. Rastoskuev

UDC 389.14:543.27

RSS lidars are promising for monitoring atmospheric pollution, these being optical locators based on Raman-scattering spectroscopy. High-power lasers and current optoelec- tronic equipment enable one to implement the advantages of Raman scattering: the scope for remote monitoring of numerous pollutants with a single laser giving limits of detection of about 1"10 -4 at ranges of i00 m. This makes RSS particularly promising for monitoring the atmospheric pollution from factory chimneys, on highways, etc. [i, 2].

The principle of RSS sounding is that one records the radiation scattered by molecular components of the atmosphere from a laser pulse. The synchronization of the laser pulse with the received scattered radiation enables one to record this radiation and relate it to particular gas layers in the atmosphere at various distances from the source, The received radiation power is given by the lidar equation [i]

Pr( '~r,R)-Po(vo)KIR -~ % N(R) ARA rT(')o)T(vr), ( i )

where Pr(vr, R) is the received RSS power in W, P0(v0) is the radiated laser pulse power in W, K~ is an apparatus constant that incorporates the characteristics of the receiving and transmitting optical systems, R is distance in m, o n is the scattering cross section in cm 2, N(R) is the component concentration in cm -a, ~R is the spatial resolution in cm, A r is the effective detector aperture in m 2, T(vr), T(v 0) are the transmission coefficients of the atmosphere at the laser frequency v 0 and at the RSS frequency Vr, respectively.

The overall error in concentration measurements by RSS is such that the main contribution comes from the systematic error, since the random components can be reduced by increasing the number of pulses (probing time). On differentiating (i) with respect to all variables and using the method of [3], one gets an expression for the limit to the residual systematic error:

o . 40R+O~=HOAR+OA+OTo+OTr (2 )

where 0 i a r e t h e e r r o r c o m p o n e n t s . On t h e a d d i t i o n o f t h e s e c o m p o n e n t s , a l l t h e i n f l u e n c e c o e f f i c i e n t s a r e one , a p a r t f rom t h e d i s t a n c e i n f l u e n c e c o e f f i c i e n t , wh ich i s two.

The l a r g e s t c o n t r i b u t i o n s t o t h e o v e r - a l l e r r o r comes f rom t h e c om pone n t s r e l a t e d t o d e - t e r m i n i n g t h e r e l a t i v e t r a n s m i s s i o n ( t r a n s p a r e n c y ) o f t h e a t m o s p h e r e a t t h e l a s e r and RSS f r e - q u e n c i e s (gTo and @Tr). Thesa components arise from uncontrolled changes in the transparency along the working line, so one cannot estimate them correctly without a priori assumptions about the atmospheric aerosol, which influences the transparency of the lower atmospheric layers.

Here we estimate the error components from the following a priori information on the aerosol:

i) a model for smoke [I] with a metrological visibility range MVR of I0 km in the range of wavelengths % from 0.31 to 25.31 ~m;

2) the optical location model for continental aerosol [4] for an MVR in excess of i0 km in wavelength range from 0.347 to 1.06 pm;

3) the model of MacClatchky et al. [4] for an MVR of 5 km in the wavelength range from 0.347 to 1.06 pm;

4) an empirical formula relating the attenuation coefficient s(%) to the meteorological visibility range V [5],

Translated from Izmeritel'naya Tekhnika, No. 5, pp. 56-57, May, 1985.

0543-1972/85/2805-0467509.50 �9 1985 Plenum Publishing Corporation 467

e(t)=(3,91/V)(O.55/1)~ (3)

5) a r e f i n e d e m p i r i c a l f o r m u l a r e l a t i n g ~(1) and V f o r t h e w a v e l e n g t h r a n g e from 0 .24 t o 0.40 gm [61:

(X) = (0.538--X) / (0.028V) +[0,08 / (X--0.18) ]3. ( 4 )

The e r r o r was e s t i m a t e d f o r t h r e e w a v e l e n g t h s : t = 0 .266 gm ( t h e f o u r t h ha rmonic o f an y t t r i u m - y t t r i u m - g a r n e t l a s e r YAG), t = 0 .532 ~m ( t h e second ha rmonic o f t h e YAG l a s e r ) , and h = 0 .694 gm ( t h e f u n d a m e n t a l o f a ruby l a s e r ) f o r MVR from 5 t o 20 km w i t h a l l o w a n c e f o r t h e c o n t r i b u t i o n o f t h e m o l e c u l a r s c a t t e r i n g t o t h e a t t e n u a t i o n . The s y s t e m a t i c - e r r o r components r e l a t e d t o d e t e r m i n i n g t h e t r a n s p a r e n c y were e s t i m a t e d on t h e a s s u m p t i o n o f h o r i z o n t a l homo- g e n e i t y in t h e a t m o s p h e r e [4 ] - The e s t i m a t e s show t h a t t h e s e components can v a r y w i d e l y , by up t o • f o r example in a s s e s s i n g SO 2.

Such l a r g e v a l u e s o f t h e e r r o r make i t u n p r o m i s i n g t o use t e s t gas m i x t u r e s ( i n combina- t i o n with short cells) for calibration in order to simulate a polluted working line. The numerical values thereby obtained for the transformation coefficients would apply only for strictly defined atmospheric transmission characteristics, and the additional errors would apply only for strictly defined atmospheric transmission characteristics, and the additional errors would greatly exceed the basic error.

The optimum way of calibrating RSS lidars is a method based on using atmospheric nitrogen or oxygen. The calibration is based on the similarity between the Raman-scattering processes for all the compounds constituting the atmosphere, as well as the constancy of the nitrogen and oxygen partial pressures. On this basis, we draw up equations (i) for nitrogen and a test component to get the following equation [i, 2]:

P~(,~,R)% (5) N (R) =K~ Pra (~ra,R)a~.a "Ha (R),

where N(R) is the test component concentration, Na(R) is the nitrogen conecentration, and K 2 is a constant that incorporates the transmission of the detector system at the RSS frequen- cies for the test component and nitrogen and the difference in the atmospheric attenuation coefficients at these frequencies.

The bounds to the residual systematic error in that case can be represented as

ON~I'I V 0#' J_O ~ ~_n~ . n ~- ~0 ~ P r a T P r ~ K 7 - ~ [ ~ r . a ~. Na ' (6)

where 0~i:~-- is the relative error in determining the ratio of the Raman-scattering RS cross sections for the test component and nitrogen, while 0Na is the relative error in determining the nitrogen concentration.

Comparison of (2) and (6) shows that if one can ensure that the systematic error com- ponents OK, ON~ and O~I~ are small, the overall error will be much less in the second case.

The main contribution to O K arises from the indefiniteness of the atmospheric attenuation coefficients at the RS wavelengths for nitrogen and the test component, since the detector spectral characteristics can be determined quite accurately. We estimated this component on the basis of the above optical models for the atmospheric aerosol. We found that using probe lines of 0.1-0.5 km, the maximum errors (not exceeding 10-15%) occurred on probing at 0.266 ~m. This error was determined firstly by the uncertainty in the a priori information on the atmospheric aerosol [4-6] and secondly by the absence of information on the MVR assumed in that case.

When one determines the concentration from (5), the errors due to these factors partly balance out because of the similarity in the RS frequencies for nitrogen and the major pollu- tants. The error can be reduced further to 3-5% if the RSS lidar is used in a method of mea- suring the atmospheric transparency at the nitrogen RS wavelength [I].

We estimated the error component related to the uncertainty in the ratio of the RSS cross sections for nitrogen and the test component from published data and special measure- ments. The values of the relative normalized differential RS cross sections given in [7] were considered as independent observations and were processed in accordance with the recom-

468

TABLE 1

C o tl].-

ponent

CO

NO

NO=

H2S

SO=

N Ha

Av, cm -I

2143

1877

1285

2611

1151

3354

0

%

8

25

15

7

18

14

087

0,32

2,02

6.83

4,63

5,76

TABLE 2

~ . / ~ - , a t ~,, p m oo,~/on a , %, a t ~,, l i r a

Component 0,2(% 0,266

CO

NO

II2S

SOx

NH3

0, 48B 0, ,382

0,70 0,9

0,48

6,6 6,8

5,6

5,4 5,3

1,05

0,3(/

8,4

80

7

O, 488 O, 532

4 G

10

10 10

0

4 4

5

20

12

50

I1

mendations of [3]. Table 1 gives the results from processing the published data for certain atmospheric pollutants, where Av is the frequency shift.

The relative RS cross section were measured on exciting RS at 0.488 and 0.266 um. The exciting source was an argon laser of output 1-2 W and a YAG-neodymium laser of mean power 1.6 W at the second harmonic and 0.15 W an the fourth. The RS spectra were recorded with a DFS-24 spectrometer, with the glass optical components replaced by quartz components. The scat- tered line was corrected at 90 ~ to the exciting radiation with a depolarizer in front of the entrance slit to eliminate errors associated with the polarizing action of the monochromator. A phase plate set the polarization of the laser beam perpendicular to the observation direc- tion. The illumination scheme was equivalent to that for light collection in back-scattering as realized in laser probing of the atmosphere. The spectrometer was calibrated in the visible region from the emission from an SI8-200u standard lamp and the luminescence of a solution of quinine bisulfate, while in the UV range it was calibrated with a DND-90 deuterium-neon lamp and the luminescence of benzene in hexane. Table 2 gives the measurements on the relative cross sections or/eva for three wavelengths; we also give the errors character- izing the random component.

Table 1 and 2 show that our results and the published data agree within the error limits on exciting RS in the visible region. When the spectrum is recorded in the UV range, the measured relative RS cross sections increase somewhat (up to 80% for SO2). As the published data on the RS cross sections in the UV region are at present inadequate for statistical ana- lysis, we estimated the error component by taking the experimental values. On the whole, the relative RS cross sections indicate that the component 0~/~d does not usually exceed i0- 20%.

The other error components in (6) are small and mainly do not exceed 5%, so we conclude that the systematic errors in measuring the concentrations of atmospheric components by RSS lidar do not exceed 25%.

Thus the main task in calibrating RSS lidars is to set up a data bank on RS cross sec- tions for major atmospheric pollutants. The calibration can be based not on the RS cross sections themselves but the ratios of them to those of major atmospheric components. Deriv- ing information on pollutant concentrations from (5) represents indirect measurement, so RSS lidars may be checked by an elementwise method using existing standard measurement facilities for other physical quantities.

469

LITERATURE CITED

i. Laser Monitoring of the Atmosphere [Russian translation], Mir, Moscow (1979). 2. Analytical Laser Spectroscopy [Russian translation], Mir, Moscow (1982). 3. All-Union State Standard 8.207-76: The State System of Measurements: Direct Measurements

with Multiple Observations: Methods of Processing the Observational Results: Basic Con- cepts [in Russian].

4. G. M. Krekov and R. F. Rakhimov, An Optical Location Model for the Continental Aerosol [in Russian], Nauka, Novosibirsk (1982).

5. M. Bertolotti, L. Muzil, and D. Sette, Appl. Opt., 8, 117 (1969). 6. W. Baum and L. Dunkelman, J. Opt. Soc. Am. 45, 166 (1955). 7. Gas and Liquid Raman-Scattering Spectroscopy [Russian translation], Mir, Moscow (1982).

470