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The vertical distribution of aerosol concentration and size distribution function over the tropics and their role in radiation transfer

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Physica Scripta. Vol. 36, 358-361, 1987

The Vertical Distribution of Aerosol Concentration and Size Distribution Function Over the Tropics and Their Role in Radiation Transfer* A. Jayaraman, B. H. Subbaraya and Y. B. Acharya Physical Research Laboratory, Ahmedabad 380 009, India Received Seprember 1 , 1986; accepted January 27, I987

Abstract By measuring the atmospheric attenuation of the direct solar radiation and the angular distribution of the scattered radiation at different altitudes, the altitude profile of the aerosol concentration and size distribution function can be obtained. This principle has been employed to study the vertical distribution of aerosols over the tropical site, Thumba (8.5"N, 76.9"E), using rocket-borne photometers upto an altitude of 22 km in February 1980. Later, a suntracking multichannel photometer was developed and flown on a balloon platform from Hyderabad (17.5"N, 78.6"E) on April 18, 1984 and aerosol measurements were made upto an altitude of 32 km. Comparison of the two sets of data shows that the aerosols measured over Thumba are influenced by the near equatorial eruption of Sierra Negra volcano which occurred three months earlier. Mie scattering coefficients are computed for different wavelengths for the aerosol concentrations and size distribution functions measured over Thumba and Hyderabad and the spectral variations are compared.

1. Introduction Need for atmospheric aerosol study is manyfold. Most importantly, they influence the radiation balance of the earth's atmosphere [ 11 and thus play a role in climatology and atmospheric dynamics [2, 31. Aerosols are also found to play a role in atmospheric neutral [4] and ion [5] chemistry. The aerosol particles exhibit high spatial and temporal variations [6] and the short time scale climatic fluctuations are associated with the volcanic aerosols [7, 81. The tropical lower stratosphere and tropical tropopause are more sensitive to perturbation in radiative heating rates than at other latitudes [9, IO]. Furthermore, both observations and model calculations [11] show that the radiative effect of volcanic aerosols can induce a substantial warming in the tropical lower stratosphere. Since the tropical tropopause temperature plays a dominant role in determining stratospheric H, 0 concentration and can influence the tropospheric climate [ 121 the study of the sensitivity of tropical thermal structure to trace gases and aerosol is important. A programme of in situ aerosol measurement is being undertaken in India using rockets and balloons over the tropical sites, Thumba (8.5"N, 76.9"E) and Hyderabad (17.5'N, 78.6'E) respectively. This programme is aimed at the construction of a data base for the study of main features of the vertical distribution of the background aerosols over the Indian tropical region and their temporal variations, both long term and short term, at different altitudes and the possible influence of these variations in the equatorial atmospheric energy budget. A total of three rockets and three

balloon-borne aerosol experiments have been conducted so far. The results of the rocket measurements made over Thumba were reported earlier [ 131. Out of the three balloon experiments conducted over Hyderabad, the first one was a failure. The second balloon experiment conducted on April 18, 1984 was successful and the results are reported here. The data of the third balloon experiment conducted on October 22, 1985 is being analysed and the results are not yet available. The results of the Hyderabad measurements are also compared with the Thumba results. 2. Balloon-borne suntracking multichannel photometer

Figure 1 shows the block diagram of the balloon-borne suntracking multichannel photometer. The instrument essentially consists of three parts, viz., ( I ) the sensor assembly containing four photometers, (2) the tracking mechanisms for zenith orientation of the sensor assembly and (3) a motor, slip-ring assembly for the continuous rotation of the sensor part for direct as well as scattered radiation measurements. The four photometers are operated at 280nm, 310nm, 450 nm and 800 nm using interference filters, each having a bandwidth (FWHM) of 10 nm. Baffles are used to restrict the field of view of the photometers to 9". Logrithmic amplifiers having dynamic response of 6 orders of magnitude are used to amplify the photometer current. The tracking of the sun in elevation is achieved using two photodiodes mounted perpendicular to each other in the vertical direction. The difference in the output signals of the photodiodes is fed to a servo motor which orients the sensor assembly towards the sun.

LOOOUTPUT 3 70 TELEMETRY( LOO WTPUT 4

TELL COYMIND TELE coMYbnn

*

This paper was contributed to the "SCOSTEP Sixth Quadrennial International Solar-Terrestrial Physics Symposium", Toulouse, 1986, and will be included in part I1 of the Conference proceedings. (Editors: B. Hultqvist, D. Rees and U. von Zahn).

Physica Scripta 36

I

2

Fig. 1. Block diagram of the balloon-borne suntracking multichannel photometer.

The Vertical Distribution of Aerosol Concentration and Size Distribution Function Over the Tropics Also a calibrated solar sensor is used to monitor the sun's zenith angle during the flight and a magnetic aspect sensor to monitor the orientation of the gondola. A more detailed description of the instrument is given elsewhere [14].

The suntracking multichannel photometer was flown on a balloon from Balloon Facility, Hyderabad for the first time on October 20. 1983. The Hydrogen inflated balloon was expected to reach an altitude of 32 km. But, the balloon burst at the tropopuase level. The telemetered data was also found to be noisy and even the tropospheric data could not be usefully analysed. However, the instrument could be recovered in good condition and was successfully reflown on April 18. 1984 at 06: 10 Hrs. The data was recorded on paper chart for quick look and on magnetic tape for detailed analysis. The 800 nm data is analysed for the aerosol concentration and size distribution function. The data analysis mainly involves the estimation of the attenuation of the incoming solar radiation at each altitude and deriving the angular distribution of the aerosol scattering function from the scattered radiation measurement. From the ratio ( 4 ) of the aerosol scattering function to the Rayleigh scattering function, the aerosol number density N,, can be obtained [15].

E

x

v

7 -

30y 35

-

3. Experiment and results

359

25 I3

5a

20

NO DATA

~

2.5

3.0

3.5

40

Y

Fig. 3. Altitude variations of the slope ( v ) of the power law size distribution curve obtained over Hyderabad on April 18, 1984.

of the three aerosol concentration profiles obtained over Thumba during February 1980 are also shown. The aerosol concentrations over Hyderabad are found to lie in the range of 40 to 20 particles per cm3in the 10 to 15 km altitude range and few particles per cm3 above 21 km upto an altitude of 32 km. The error bar shown in the concentration profile is where oR is the Rayleigh scattering cross section, 6 is the mainly due to the uncertainty of & 0.2 in determining v. This scattering angle, q is the Mie angular function, go and gz uncertainty in curve fitting exists at all altitude levels. The are the air density at ground level and at the altitude z other error is in determining the scattering angle 6. In this respectively. r , , the lowest particle size is taken as 0.04 pm. v. particular experiment due to the large undulation of the the slope of the power law size distribution curve is deter- balloon platform in the altitude region of 17 to 20 km (near mined by comparing the experimentally observed angular the tropopause) the uncertainty in determining the scattering variation of the aerosol scattering function with that of the angle was found to exceed the total field of view of the theoretical estimation for different v values. A detailed des- photometer and the data could not be analysed. cription of this procedure can be found in reference [16]. Figure 3 shows the altitude variation of the slope, v of the Figure 2 shows the height distribution of aerosol number power law size distribution. The uncertainty in determining density obtained over Hyderabad. For comparison the mean its value is i.O.2 and is nearly same at all altitudes. At the tropospheric altitudes, the v values are found to be around 3.0. The lowest v value, 2.7 is found at 21 km. v increases with 3 ? l r , , , , , , , altitude, above 21 km, and reaches a maximum value of 3.6 at 32 km. The low v values in the 21-25 km altitude range show that the region is characterised by a relatively large fraction of bigger particles. Similar results have been reported earlier (for example, Ref. [17]). Comparison of the obtained v values over Hyberabad with that obtained over Thumba in 1980 shows that the Thumba values are much smaller than the 1984 Hyderabad values. In general, the v values over Thumba are found to vary between 2.4 and 2.9 in the tropospheric altitudes (below 17 km) and between 2.1 and 2.4 in the 17 to 22km altitude region. This shows that the particles found over Thumba are relatively bigger than the particles observed over Hyderabad at all altitudes. Comparison of the 5 1 computed aerosol scattering coefficient over Thumba with the results of satellite-borne stratospheric Aerosol Gas 0 Experiment (SAGE) of NASA clearly shows (following I 10 100 1000 section) that the aerosol characteristics over the equatorial AEROSOL NUMBER DENSITY ( c ~ n )- ~ region are influenced by the eruption of Sierra Negra volFig. 2 . Aerosol concentrations measured over Hyderabad on April 18, 1984 cano. This explains the observation of bigger particles, shown (thick line) is compared with the mean of the three rocket measurements v value, over Thumba during February, 1980. by low (thin line) made over Thumba during February 1980. 1

1

Physica Scripta 36

360

A . Jayaraman, B. H . Subbaraya and Y . B. Acharya

4. Estimation of mie scattering coefficients

-c

Computations of aerosol scattering coefficients (PA) from the measured aerosol concentration and size distribution function is important to study the role of aerosol scattering in attenuating the incoming solar radiation and the outgoing earth’s radiation. The scattering coefficient can be defined as the sum of radiation scattered into all 471 directions. Following Mie scattering theory, the aerosol scattering function can be written as

PA

= 271

1; (?/a’)i(a,

8) sin 8 d8

n

1s dN(r)(r’/a*)i(a, 8) sin 8 d8

‘2

= 271

Q

3.0 2.0

1

PA can be rewritten

F M A M J J A S O N D J 1979

F M A M J J A S O N 1980

Fig. 5 . Comparison of the aerosol scattering coefficient obtained over

(3) Thumba with SAGE results for I pm and for an altitude of 21 km. The

rl*

For power law size distribution

-

4.0

‘E 1

SAGE DATA

THUMBA ROCKET DATA

(2)

when there are dN(r) particles per cubic centimeter, all particles with radii r will contribute to scattering coefficient and hence

PA

-

-

@

as [18],

SAGE results are the monthly mean zonal values for the equatorial region of &looN.

(4) measured over Thumba and Hyderabad respectively. In order to delineate the general features, the mean profile is PA/Cvalues are computed for v values from 2.0 to 4.0 in steps shown for Thumba. While the aerosol scattering coefficients of 0.5. For intermediate v values, the coefficients are obtained over Hyderabad are found to vary between 4.8 x (km-’) at 32 km, the values by interpolation of log (PA/C).C, the constant in the power (km-’) at 10 km and 4.4 x obtained over Thumba are found higher, varying from law size distribution function is calculated from the measured 5 km to 2.85 x (km-’) at 5.85 x (km-I) at 23 km. aerosol number density Nz using the relation The higher values of aerosol scattering coefficient obtained Nz over Thumba are due to the low v values. Recently, the results c = of the satellite-borne SAGE measurements of the global distribution of aerosol extinction for 1.O pm and 0.45 pm have The altitude profile of the aerosol scattering coefficient is been reported [19, 201. The monthly mean zonal extinction obtained by multiplying PA/Cwith C value corresponding to values for the equatorial region of O-lO’N for 1pm and for a typical altitude of 21 km is plotted in Fig. 5 for a period of the aerosol number density at that particular altitude. Figure 4 shows the calculated PA values for a wavelength February 1979 to November 1980. The computed aerosol of 1 pm using the aerosol number densities and v values scattering coefficient for the same wavelength and altitude for the Thumba aerosol measurements is also plotted for comparison. It can be clearly seen that the extinction value over the equatorial region is increased by a factor of about 3.5 during February and March 1980. It is due to the eruption of 3 5 1 Sierra Negra volcano (O.X0S,91.2OW) in the equatorial region 30 on November 13, 1979. A period of 3 to 4 months for the ejecta to disperse around the globe at stratospheric altitudes is considered typical [6]. The relatively low scattering values 25 over Hyderabad correspond to a volcanically quiet period of 1984. It should be noted that the ejecta of the El Chichon E Y volcano erupted in April 1982 was seen by Mauna Loa Lidar w 20over the equatorial region (Scientific Event Alert Network, n I3 SEAN, Monthly Bulletins of the Smithsonian Institution). As c 5 our measurement being the first one over Hyderabad it is 15 difficult to conclude whether the results represent the normal background aerosol or not. However the aerosol scattering coefficient values are about a factor of six lower than that 10obtained over Thumba.

-

5. Spectral dependence of aerosol scattering Aerosol scattering coefficients are also calculated for other wavelengths ranging from 400 nm to 1000nm for the two sets AEROSOL SCATTERING COEFFICIENT [ K m ’ ) of aerosol data obtained over Thumba and Hyderabad. Fig. 4. Aerosol scattering coefficients obtained over Hyderabad (thick line) In Fig. 6 the spectral variation of PA over Thumba and are compared with the mean of the three rocket measurements (thin line) made over Thumba during February 1980. Hyderabad are shown along with that of Rayleigh scattering 10-6

Physica Scripta 36

1 6 ~

10‘~

The Vertical Distribution of Aerosol Concentration and Size Distribution Function Over the Tropics

361

eruption. Aerosol scattering coefficients are calculated and their spectral variations are compared.

'E Y

1

'\\

Acknowledgements

400

I

I

I

600

800

1000

WAVELENGTH

I

(nm)

Fig. 6. Spectral variations of aerosol scattering coefficients obtained over Hyderabad (thick line) and Thumba (thin line) are compared with that of Rayleigh scattering coefficient for a typical altitude of 22 km.

coefficient for a typical altitude of 22km. It is seen that the importance of aerosol scattering over Rayleigh scattering increases with wavelength. It is because as wavelength increases the Rayleigh scattering decreases in accordance with l/A4 law. The aerosol scattering is less wavelength dependent and is proportional to 1 / % - 2[18]. To estimate the importance of aerosol scattering over Rayleigh scattering both aerosol number density and size distribution function and refractive index are important. The difference in the absolute values of BA and its spectral variation for Thumba and Hyderabad are due to the presence of bigger particles over Thumba, found to be the influence of Sierra Negra volcanic eruption.

6. Summary A sum-tracking multichannel photometer is developed and flown on a balloon platform over Hyderabad on April 18, 1984. The aerosol concentration and the size distribution function are derived independently from the direct and the scattered radiation measurement aloft. The results are compared with the results of the earlier measurements made over Thumba using rockets in February 1980. Relatively bigger particles are detected over Thumba compared to Hyderabad and are found to be the influence of Sierra Negra volcanic

The programme on Aerosol studies at Physical Research Laboratory was initiated by the encouragement and support of Prof. S . P. Pandya, Director, Physical Research Laboratory. The balloon experiments were undertaken as part of the Indian Middle Atmosphere Programme (IMAP). Special mention is made of the efforts of Dr B. V. Krishnamurthy of VSSC, Trivandrum, Coordinator of the Aerosol Campaign of IMAP. Acknowledgementsare due to the staff of the Hyderabad Balloon Facility for the successful balloon launch, in particular to Mr. R. T. Redkar and Mr. R. U. Kundapurkar. Messrs A. J. Shroff, J. T. Vinchhi and S. M. Shukla have contributed significantly to the development of the instrument. Mr. K. S. Pate1 has helped in data reading and analysis.

References 1. Harshvardhan, J., Atmos. Sci. 36, 1274 (1979). 2. Hansen, J., Johnson, D., Lacis, A., Lebedeff, S . , Lee, P., Rind, D. and Russell, G., Science 213, 957 (1981). 3. Turco, R. P., R. C. Whitten and 0. B. Toon, Rev. Geophys. Space Phys. 20, 233 (1982). 4. Whitten, R. C., 0. B. Toon and R. P. Turco, PAGEOPH 118, 86 (1980). 5. Amold, F., Henschen, G . and Ferguson, E. E., Planet Space Sci. 29, 185 (1981). 6. Kent, G. S . and McCormick, M. P., J. Geophys. Res. 89, 5303 (1984). 7. Pollack, J. B., Toon, 0. B., Sagan, C., Summers. A., Baldwin, B., Camp, W. A., J. Geophys. Res. 81, 1071 (1976). 8. Howard, R., Bull. Amer. Meteor. Soc. 62, 241 (1981). 9. Dickinson, R. E., Liu, S . C. and Donahue, T. M., J. Atmos. Sci. 35, 2142 (1978). 10. Fels, S. B., Mahlman, J. D., Schwarzkopf and Sinclair, R. W., J. Atmos. Sci. 37, 2265 (1980). 11. Hansen, J. E., Wang, W. C., Lacia, A. A., Science 199, 1065 (1978). 12. Ramanathan, V., Proceedings of Indo-US Workshop on Global Ozone Problem, NPL., New Delhi (1983). 13. Subbaraya, B. H. and Jayaraman, A., PAGEOPH 120, 407 (1982). 14. Acharya, Y. B., Jayaraman, A. and Subbaraya, B. H., Adv. Space Res. 5, 65 (1985). 15. DeBary, E. and Rossler, J. Geophys. Res. 71, 1011 (1966). 16. Jayaraman, A., Measurement of Atmospheric Minor Constituents using Rocket-borne Techniques, PhD. Thesis, Gujarat Univ., Ahmedabad, India (1985). 17. Hofmann, D. J. and Rosen, J. M., Geophys. Res. Lett. 10, 313 (1983). 18. Bullrich, K., Adv. Geophys. 10, 99 (1964). 19. McCormick, M. P., SAGE Aerosol Measurements, Vol. I, NASA-RP1144 (1986). 20. McCormick, M. P., SAGE Aerosol Measurements, Vol. 11, NASARP-1149 (1986).

Physica Scripta 36