R= relates the visual range R with o Extinction coefficient consist from absorption coefficient a and scattering coefficient b: o-= a+b.

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1 Use of spectral integrating nephelometer for investigations of the scattering coefficient l.a. Razenkov, A.P. Rostov, N.A. Shefer Institute ofatmospheric Optics SB RAS 1, Akademicheskii av, Tomsk, , Russia ABSTRACT The results of measurements with an automatic integrating nephelometer intended for recording of molecular and aerosol scattering coefficients are discussed. The diffraction grid was used as a dispersive element. The frequency range of recording of molecular and aerosol scattering coefficients was from 300 to 820 nm. The spectral calibration of the device with the help of gases with known scattering coefficients is discussed, the evaluation of sensitivity of the nephelometer with use of the calibrated source of photons is considered. The measurements of dynamic range of the nephelometer are presented. It is shown that the device has dynamic range about 5 orders. The measurements of scattering coefficient in room conditions and outdoors are considered. The use of the nephelometer for continuous extended in situ observations of the scattering coefficient is discussed. High frequency of data recording permits us to use the nephelometer for estimation of turbulent aerosol flows, if the measurements of components of a wind velocity will be carried out synchronously with use of the nephelometer. Keywords: integrating nephelometer, aerosol scattering coefficient, turbulent flows 1. THEORY OF DEVICE The intensity of a parallel beam oflight projected through a uniform turbid atmosphere is attenuated exponentially. That is, the intensity at the distance L follows an equation such as I = I exp( ol). The quantity is called the extinction coefficient. Koschmieder showed that in daylight a simple expression, R= relates the visual range R with o Extinction coefficient consist from absorption coefficient a and scattering coefficient b: o-= a+b. In the atmosphere an absorption usually small compared with scattering, so that the nephelometer,which estimates b, usually gives an useful estimate of and hence of R. In scattering theory basic equation for scattering coefficient is b=27rffl(ø)sinqjø, where,8(ø) is directed scattering coefficient at the angle q5. Nephelometer is the device for measuring scattering coefficient. The term "integrating nephelometer" implies the device in which light scattered by particles is measured as the cosine-weighted angular integral representing to a component of attenuation, stipulated by scattering both on particles, and on molecules of gas. First such device had Ninth Joint International Symposium on Atmospheric and Ocean Optics/Atmospheric Physics: Part II, G. G. Matvienko, V. P. Lukin, Editors, Proceedings of SPIE Vol (2003) 2003 SPIE X/03/$

2 developed by R. Beuttell in Originally this device was intended for measurement of meteorological distance of visibility, which is determined as a distance on which absolute black object almost disappears (merges with a hum noise), when it consider near to a line of horizon in daylight. Beuttell used a human eye as the sensitive element and he obtained a coefficient ofscattering up to 5*104 m'. The "signal/noise" ratio in this case was close to one. Figure 1 showes a diagram illustrating the principle of the integrating nephelometer. A diffusive screen located at L, is illuminated from above by a lamp. The photomultiplier receives light from this source which has been scattered through an angle 0 by particles present at V; as q$ varies along the volume seen by the photomultiplier and defined by diaphragms A and B, the instruments integrates light scattered over a range of angles. V Figure 1: Illustration of scatterinheory. One can show that the luminous flux scattered towards the photomultiplier is F = Qo)s Tflø sin, rh 0mm where s is the area of the opening in diaphragm A, Qthe total luminous energy emmited by the diffusive Lambert's screen (opal glass), h the distance of the diffusing screen from the axis defined by diaphragms A and B and Co is the solid angle subtended by the opening in B at A. Comparing previous equations we see that F= to a good approximation since Sfl 0 vanishes at 0 In the instrument described these angles are about 5 and 175 respectively. 0 and 7i, and 0mm and ømax are close to these limits. 2. DESIGN OF THE NEPHELOMETER Main idea of the spectral integrating nephelometer is the using of the diffractive grid. Figure 2 shows the schematic optical diagram. Common view of the nephelometer developed in our laboratory showed Chopper 8 Proc. of SPIE Vol. 5027

3 3. SPECTRAL CALIBRATION Parameters of grating: grating grooves per mm; brilliance angle; nm maximum for 1-st order Parameters of monochromator: nm spectral resolution for 1mm diameter fiber; nm/step mechanical resolution; deg/step mechanical angle resolution. Figure 4 shows dependence wavelength from the grating position calculated by equation: where,t' is incident ray, is reflected ray, d600 mm'. = d(sinyi+sinço), Figure 3: Common view of the nephelometer. Values of angles iu and ço depends from position of step-motor which rotate the grating: = o.226[deg/ step] * GratPos[step], ço = 2 i,t', = Calibration data at the graph Figure 4 has a good linear approximation: WaveLength = [GratPos - GratPos(600)], where GratPos(600) is the grating position when Wavelength600 nm. For absolute spectral calibration we use "red" semiconductor diode with maximum spectrum equal 630 nm (Figure 5). Proc. of SPIE Vol

4 In reality calibration making by software SIN program (Spectral Integrating Nephelometer) for Windows. After choosing <Spectrum>, <C(alibration)>; switching diodes ("green" and "red"); <Start>; program SIN automatically found position of the maximum of spectra. 900 WaveLength= *[GratPosGratPos(6OO)] E C 0) C Cu I'.',- ouu s-.-.i 500 iç 30C Grating position - Position (600nm), steps Figure 4: Calibration graph for stepping rotation system of nephelometer. For finishing the calibration user must press the button <Apply>. After that will calculated value GratPos(600) and it will recorded to mi-file "SIN.ini": fspectrum calibrationj GratPos60039 For checking and fixing received result it's possible to make a spectrum scan and to be sure that light from diode has a maximum at 2 630nm > 1. U) I 5000,vuvu nnn p1 / ;...r \ Red dioj K Wavelength, nm Figure 5: "Red" emitted diode used for spectral calibration of nephelometer. 10 Proc. of SPIE Vol. 5027

5 4. SENSITIVITY OF NEPHELOMETER Sensitivity of the nephelometer was checked by using calibrated source of photons (#A125, X,565nm, 1.9*10") photons/sec). Calibrated source was installed to the wall on the light trap. Distance from input diaphragm (diameter 3 mm) was equal 650 mm. It is possible to estimate how many photons coming to the detector in second: N D-3mm =1.9*1010 =1.9*1010 5OOOOHz 2irR2 2* , spectral interval ( 0. 1, loses in optics ( 0.5 and We have to account quantum efficiency of PMT ( grating reflecting coefficient ( 0.5. Atthe result coming photons from source at the?=565nm should be N*D=3mm 0.1* 0.1* Hz Result of the testing showed on the Figure 6. We can see difference when source was installed and was absent equal about 50. It's good result if to account that some of parameters we could not to know exactly. NI Co c,) Cr) : i::-1 : [ibrated source Jj i: Time, mm Figure 6: Result oftesting device sensitivity by using calibration light source. Measured signal is in Hz. 5. DYNAMIC RANGE Acquisition system of nephelometer works in photon counting regime. So, that is why dynamics of device is a big. For illustrating that point is Figure 7. At the beginning was typical indoor data and value was about 100 M/m (1 M/m=106 md). After that had appear smoke from cigarette near nephelometer. It was only 2 times pushed smoke. Value was about 1000 M/m. Signal increased about 10 times. Few minutes later three times smoke was pushed inside nephelometer and signal increased more then M/m. It was three order up from previous room condition. After smoke was pushed CO2 gas inside device and scattering coefficient decreased to the typical molecular value for that gas and started equal to 40 M/m. That result Proc. of SPIE Vol

6 illustarated that the nephelometer has a dynamic range about 5 orders. Note, that a signal in maximum was more then photons/sec, i. e. higher then 1 MHz. E ( 0 ::. E Smoke inside ir Smoke_outside IL.... I). o ".,. c 10.,. 11:00 11:30 12:00 12:30 13:00 13:30 14:00 I I Local time, Hour L11J Figure 7: Time series of scattering coefficient data. Logarithmic scale showed dynamic range about 5 order. 6. CALIBRATION Closed type of chamber to allow make an absolute calibration by using gases with famous scattering coefficients:.2 a (T, P) = a (STP)273 T where STP is the standard conditions P = mBar; T = C. Calibration gases: Table 1. Scattering coefficients of span gases for three wavelengths Gas X450nm 2550nm X=700nm Clean air CO Figure 8 shows how signal depends from the used gases. Instead of air was used gas N2. Calibration curve showed at the Figure REFERENCES 1. E. J. McCartney, Optics of the Atmosphere, Mir, Moscow, M. Born, E.Wolf, Principles of Optics, Nauka, Moscow, A.V. Pavlov, Opto-electronic devices, Moscow, R. Hadson, Infrared systems, Mir, Moscow, P 12 Proc. of SPIE Vol. 5027

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