OPTICAL PROPERTIES OF TROPOSPHERIC AND ATOMOSPHERIC AEROSOLS

MULTIFREQUENCY SOUNDING OF ATMOSPHERIC AEROSOL

A. P. Ivanov, A. P. Chaikovskii, N. P. Vorobei, F. P. Osipenko, and V. N. Shcherbakov

We report here the results of studies aimed at developing procedures and apparatus for multifrequency laser sounding of the atmosphere, performed at the Physics Institute of the Belorussian Academy of Sciences. The purpose of these studies was to develop methods of measuring, in a wide spectral interval, the optical characteristics of the atmosphere and the microstructure and spatial stratification of the aerosol.

So far, most investigations on laser sounding are carried out at not more than three or four frequencies. This is insufficient for a number of problems. The most favorable in this sense are dye lasers which make it possible to carry out the measurement in a wide range of the radiation spectrum.

The systems "Lidar-2" and "Gloriya," with working interval ~ ~ 0.36 • 1.07 ~m [i], were were developed at the Physics Institute with Special Design and Technological Office and with Observation Point, of the Belorussian Academy of Sciences. The "Lidar-2" emitter is tunable and has a spectral line width 0.1-400 A (depending on the selecting element in the cavity). The "Gloriya" system makes possible sounding at seven frequencies in a time of the order of 20 sec, an important factor under conditions of unstable atmosphere. The receiving parts of both Lidars were based on a mirror objective of 300-mm diameter and make it possible also to measure the degree of polarization of the reflected echo signal. Numerous tests of the apparatus under laboratory and field conditions have shown it to be sufficiently reliable and effective.

Using these systems, a cycle of experiments aimed at measuring the optical character- istics of the boundary layer of the atmosphere was performed in the city of Minsk and in the region of the high meteorological tower of the Institute of Experimental Meteorology (IEM). The investigations at the IEM were carried out in collaboration with N. P. Romanov, V. S. Shuklin, and V. A. Korshunov, who carried out simultaneous laser sounding of the atmosphere at four wavelengths ~ = 0.347, 0.530, 0.694, and 1.06 ~m. Provision was also made for measurement of the meteorological parameters in the lower 300-m layer. The backscattering coefficient was reconstructed from the measured echo signal in the section 80-300 m, i.e., in the near zone of the lidar. In this case the attenuation of the radiation on the propaga-

tion path could be neglected and ~ was determined uniquely.

The obtained spectral dependences ~.(~), after subtracting the molecular backscattering

coefficients, were used to determine the effective value of the refractive index m=~-~ and to reconstruct the density distribution function with respect to size ~(~) of the atmospheric

aerosol. The algorithm of the calculation [2] was based on the regularization method of Tikhonov [3, 4], and takes into account prior knowledge of the singularities of the aerosol as the object of investigation.

It is known that the reconstruction of ~(Z) from the optical characteristics of an ele-

mentary volume entails the solution of a Fredholm equation of the first kind, which is an in-

correctly posed problem. Since, by virtue of technical difficulties, ~e(~) is measured for

a finite (on the order of I0) number of values of A , to reconstruct the distribution density function of the aerosol from the dimensions, it is usually insufficient to resort to such prior information as the fact that the solution belongs to the ensemble of smooth func-

tions or that the norm and the derivatives of ~(t) are bounded. The functions obtained

Translated from II Vsesoyuznoe Soveshchanie po Rasprostraneniyu Lazernogo Izlucheniya v Dispersnoi Srede, Tezisy Dokladov (Second All-Union Conference on Propagation of Laser Radia- tion in Dispersive Medium, Summary of Proceedings), Part I, pp. 3-6, 1982.

0270/2010/84/0501-0001508.50 �9 1984 Plenum Publishing Corporation i

il \I i ><

I J. - h, ? 0 0 m \ a-/t ~.',0 m

0 0,5 ]0 2 pm

Fig. i

�9 ~ - k = 3 b 0 m

l

' \ / - a I

I i I V , , =

0 0.5 {0 % ~m

F i g . 2

as solutions in these cases contain negative values whose optical contribution cannot be neglected.

The solutions turn out to he more acceptahle if it is recognized that, as shown by data

from natural measurements, ~(~) is close to certain model typical-of-aerosol functions,

~C~,~)(~ is the vector of the parameters) having a specified analytic form, but does not

necessarily coincide with them. This circumstance is taken into account in the inversion al- gorithm in the following manner.

On the grid of the parameters ~, the set of functions ~) that minimize the function- al

was obtained, where K(~,~ is the effectiveness factor of the backscattering~ ~ is a stabil-

izing functional, and cZ is a regularization parameter. From among the obtained set, the

solution was chosen to be that value of ~(~) , which satisfies the condition ~(~)~0 and mini-

mizes the variance 92 �9

As a result of the reduction of the data obtained by multifrequency laser sounding with the systems "Lidar-2" and "Gloriya," we obtained the aerosol distribution density functions

of the height ~ of the location of the layer, of the time of the day, and of the meteor-

ological situation. Figures 1 and 2 show by way of example ~(~) for the summer anticyclone

(relative humidity ~5#L, obtained values ~ = 1.41, ~ = 0.02) and for the cyclone ( ~=80%,

= 1.35, 2= 0.002).

In conclusion, we note the following main features: 1 -- the apparatus and procedures developed make it possible to obtain information on aerosol particles with size up to several microns; 2 -- organization is necessary of comprehensive experiments using contact measurement methods for the dimensions of the aerosol particles in order to determine the limit of applic- ability of the method of multifrequency sounding; 3 -- further work is needed to optimize the algorithms of the data reduction so as to reduce the time of computation and analysis of the lidar-measurement data.

i,

2.

.

4.

LITERATURE CITED

A. P. Ivanov, et al., "Lidars for the investigation of the structure of the atmosphere and water," Preprint No. 151, Inst. Phys. Belorussian Acad. Sci., Minsk (1978), p. 27. A. P. Ivanov, et al., "Reconstruction of the parameters of atmospheric aerosol from data on spectral measurements of the backscattering index," in: 6th All-Union Symp. on Propagation of Laser Radiation in the Atmosphere, Part I, Tomsk (1981), pp. 12-15. A. N. Tikhonov and V. Ya. Arsenin, Methods of Solving Incorrectly Formulated Problems [in Russian], Nauka, Moscow (1979), p. 285. I. ~. Naats, Theory of Multiparticle Laser Sounding of the Atmosphere [in Russian], Nauka, Novosibirsk (1980), p. 157.

CONSTRUCTION OF REGIONAL SEMIEMPIRICAL MODELS

OF OPTICAL CHARACTERISTICS OF THE ATMOSPHERE

V. P. Ivanov, A. S. Makarov, and V. L. Filippov

By now, several models were developed capable of describing the spectral variation of the optical thickness of the atmosphere in the transparency windows [1-4]. A common short- coming of these models, however, is that they are static. They reflect certain "average" conditions and do not take into account the real fluctuations of the optical density of the atmosphere, which are connected with variability of the synoptic processes.

A different approach to the construction of the model of the change of the optical thick- ness in the transparency windows was developed by the authors of the present communication. The proposed model of the optical state of the air layer next to the earth is based on a syn- thesis of the results of generalization of natural optical experiments performed under vari- ous weather conditions, and is free to a considerable degree of the shortcomings indicated above. This makes it possible to describe real changes of the transmission of the atmosphere in the transparency "windows."

When constructing a block model for the aerosol, the main stress should be on determin- ing the optimum ensemble of criteria for identification of the optical state of the aerosol. The basic characteristics of the ensemble of the input parameters of the aerosol block are

Translated from II Vsesoyuznoe Soveshchanie po Rasprostraneniyu Lazernogo Izlucheniya v Dispersnoi Srede, Tezisy Dokladov (Second All-Union Conference on Propagation of Laser Radia- tion in Dispersive Medium, Summary of Proceedings), Part I, pp. 7-10, 1982.

0270/2010/84/0501-0003508.50 �9 1984 Plenum Publishing Corporation 3