Measurements of aerosols at Tenerife J.P. Diaz", F.J. Exposito\ A. Diaz", F. Herrera\

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1 Measurements of aerosols at Tenerife J.P. Diaz", F.J. Exposito\ A. Diaz", F. Herrera\ J.C. Guerra" ^Atmospheric Physic and Environment Group, Department of Physic, University of La Laguna, Canary Islands, Spain ^Communications and Remote Sensing Group, Department of Physic, University of La Laguna, Canary Islands, Spain Abstract The improvements done in an Optronic OL-752 spectroradiometer to measure direct solar radiation and compute the aerosol optical depth are presented. The features of the collimating tube and the optical fiber bundle that connects it to the monocromator are shown The equipment was installed at the Izana GAW (Global Atmospheric Watch) station to calculate the extraterrestrial signal to 630 nm (Langley's method). Finally, the instrument was placed at Nautical School (at sea level) in Santa Cruz. A correlation about 85% between the Optronic and the NOAA satellites data was obtained. Our implementation of the King's method to retrieval the aerosol size distribution, considering a Maritime distribution for a set of wavelength was checked: the best agree was in the region between 0.1 and 5 ^tm. Introduction The study of atmospheric aerosol has a special interest in a wide set of important topics, radiative mechanics in the Earth-Atmosphere system, urban pollution, correction of satellite images, measurements of atmospheric gases: ozone, NOx,...(Hansen[l], Stowe[2]). Our aim is to design a system (hardware and software) that permits us to study the atmospheric aerosols in urban condition. The city that we want study, Santa Cruz de Tenerife, has a big rate of traffic and a petrol factory inside the urban center. This pattern describes a situation where the aerosols play an important role In general, it is necessary to know three parameters in order to describe adequately the aerosols extinction in the atmosphere: the complex refraction index of the particles, the size distributions and the particle shapes. A

2 402 Pollution Control and Monitoring knowledge of the first two quantities is sufficient if it can be assumed that the particles are spherical and the Mie theory is applicable. We will present the improvements done in an Optronic OL-752 spectroradiometer. Initially, this equipment was thought as a global radiation measuring instrument We developed it adding a collimating tube in order to measure direct solar radiation. This tube was designed following the rules dictated by World Meteorological Organization (WMO[3J). In a second step, and to get a best tracking of the sun, we connected the output of the tube to the input port of the monocromator, via optical fiber bundle The instrumentation described above permits us to measure the aerosol optical depths. This is a magnitude that gives idea about the attenuation power of these particles. This instrument was placed at Izana GAW (Global Atmospheric Watch) station to measure the extraterrestrial constant to the wavelength of 630 nm The correlation factor in the fit was 98% that suggests the good features of the collimating tube and the realization of Langley calibration. We will present the results of comparing the optical depths obtained with the OL-752, and the measurements done with the NOAA satellites over the coast line of Santa Cruz de Tenerife. In order to know the size distribution and make use of the possibility that we have of measuring the optical depths, we implemented the non-parametric King's method (King[4]), to resolve the inverse problem. Our first interest was to determinate the error in the solution. So we computed the theoretical optical depths for Maritime aerosol distribution and using this values we run the program to calculate the size distribution. We show the result of this comparison Retrievals of aerosol optical depth Optronic algorithm The incident solar beam transmitted directly by the Atmosphere can be written as (Lambert-Beer-Bouguer law): where L^(X) and L(X) are the monochromatic intensities of the incident and transmitted radiation, respectively, 7% is scattering Rayleigh coefficient, T^. is the coefficient of absorption, T^ is the aerosol optical depths and m is the air mass. In order to use the eqn. (1), it is necessary to measure direct solar radiation, so we change the initial design of the Optronic (originally developed to calculate

3 Pollution Control and Monitoring 403 absolute solar global radiation) We build a collimating tube following the WMO rules Infig. (1) we show the diagram of this dispositive, which has the following features: a=r/r=l, b=l/r=64.7 and Zo=tair* (a/b)= f 5 L 50 N , 1 so! so i 75 ;. [ ij W+R= :=5._ [ I! ik ^r 3523 ^ ^^ Figure 1: Collimating tube diagram (distances are in mm). The coefficients T^ and m were calculated using the next expressions (WMO[6], Iqbal[5]): o +0.15( <,, (2) and = X (3) To correct the measures did at the GAW station (altitude: 2360 m), we use the factor: P(mb)/ (Iqbal[5]), and the ozone value for this day (300 DU), which was obtained by a Brewer. The fig (2) shows the Langley's calibration to the wavelengths: 310.1, 313.5, 316.8, 320.1, and nm (Diaz[7]). The fit to nm data gave an extraterrestrial constant of 3.51 SE- TA. The design of the bundle used to connect the monocromator to the collimating tube was negotiated with Polymicro Technologies, Inc., trying to resolve the handicap of the high attenuation and the low solar radiance in the UV region. The principals characteristics are: Total length: 2m, diameter: 1cm. Silica core (for one opticalfiber).200±8^m, doped silica cladding: /xm, buffer coating: 240±5/im. Numeric aperture:0.22 ±0.02. Step refraction index. Temperature range: -50 to 400 C.

4 404 Pollution Control and Monitoring v!b f nm I nm <> nm j Q 3168nm f nm A nm Air Mass Figure 2: Langley's calibrations. TeraScan algorithm To obtain the aerosol optical depths by the NOAA satellites, we used a TeraScan system that can capture and process this kind of data. The subroutine employed begins with the radiative transfer equation, which will be applied to every clear pixel of the image (Liou[8], Kaufman[9]): dr =A(r,ft)- I4T,Q')P(G,Q')dO'- 4 IT * (4) 47T where L is the diffuse intensity, T the optical depth, o^ the single scattering albedo, ju, the cos0 (6 = satellite zenith angle), ^ the COS#Q (0Q = sun zenith angle), 0 the solid angle (0,0) (0 = azimuth angle), P(Q,Q0) the phase function (maritime, because we have these environmental conditions) and 7rE0 the incoming radiative solar flux. A cloudless pixel (or clear pixel) must not have any of the next conditions, satellite zenith angle > 50, solar zenith angle > 50, brightness temperature CH4 < 0, CH1/CH2 ratio < 1.5, CH2 albedo >3 and the CH2 albedo variation of a pixel with his neighbours > The maritime air verifies several conditions, and one of the more important is that the single

5 Pollution Control and Monitoring 405 scattering dominates the multiple scattering. So, we can write the solution of the above equation as: If we suppose an optically thin atmosphere, i.e., (5) become: Due to the fact that the sea contribution is negligible for the signal detected by the satellite, to the wavelength of the CHI and CH2, we obtain that (DurkeeflO], Frostfll]): (7) where L^ is the radiance due to aerosols, L* the radiance detected by the satellite and L% the radiance due to scattering Rayleigh. Comparison between both algorithms The instrument placed over ground was installed at sea level at the Nautical School inside the city of Santa Cruz. We use a half band width of 2 nm and an Aerosol Optical Depth (TeraScan -Maritime-) TeraScan (Maritime) vs Optronic OL-752 y=(a+bx) r=85% a= b= o o/o 0.3 ^/ // o 02 y^ Aerosol Optical Depth (OL-752) Figure 3: TeraScan data vs. OL-752 data.

6 406 Pollution Control and Monitoring ozone value of 300 DU. The NOAA channel 1 is centered on 630 nm and has a wide band width, about 100 nm The data taken was obtained over the sea, very close to the coast line and in a cloudless area. The results are showed in the The aerosol size distribution retrieval With the improvement done in the OL-752 we have the possibility of measuring the optical depth at different wavelengths. The particle optical depth r^ is calculated by T = dr (8) where dn^(r)/dr is the column particles number r the radius of the spherical particles Qga is the efficiency factor n the refraction index We have used the constrained linear inversion technique of King et al [4] to resolve eqn. (8) for dn^(r)/dr. Our first intention is to calculate the error of our implementation of the King's method, thus, we computed the aerosol optical depths for the sets of wavelengths in table 1, assuming a maritime distribution (WMO[12]). X(nm) Table 1. Aerosol Optical Depths (AOD) TA A(nm) , f.a The fig. 4 shows a maritime distribution (continues curve) and the fit (dashed line) obtained by King's method. Input data for this curve are the AOD for the next wavelengths: 400.0, 488.0, 515.0, 694.0, , , , nm

7 Discussion Pollution Control and Monitoring 407 The good conditions at Izana GAW station for the days 8& September and 1 8* November 1993, permitted us to get very high correlations in all Langley's plot calibration (about 99%), such as it's shown in the figure 2. We have used a value of 300 DU to eliminate of the measurements over ground the absorption due to the ozone. Although the recorded aerosol optical data are not so much, and the correlation between them is not high (r=85%), they can give us an idea, in a first approximation, of the behavior of the employed instruments. So, we can see that the TeraScan has a shift about 0. 18, which could be explained by different factors, like the wide bandwidth in the AVHRR channels and the several approximations done to resolve the radiative transfer equation (eqn. (4)). On the other hand, the data are included a wide range of optical depths, which permit us to check the response of the instruments for a complete set of the different conditions. The fit shown in the figure (4) was obtained dividing the abscise axis into eight intervals, with the same length in a logarithmic scale. Using our King's method implementation, we obtained a good approximation (dashed line) to the Log-normal theoretical curve (continuous line), for the particles whose radii are including between 0. 1 and 5 /xm <H 1 t 10' o Q r(fjm) Figure 4: Number of particles vs. radius for the analytical solution and for the implemented software solution.

8 408 Pollution Control and Monitoring Aknowledgements The authors gratefully thank the Institute Nacional de Meteorologia, and specially to Emilio Cuevas (Chief of the Izana Observatory) and all the observers of this station. Many thanks also go to the Centro Superior de Nautica y Estudios del Mar for use of the installation References 1. Hansen J., A Lacis, R. Ruedy & M. Sato. Potential climate impact of Mount Pinatubo eruption. Geophys. Res. Letters, 1992, 19(2), Stowe L.L., R.M. Carey & P.P. Pellegrino. Monitoring the Mt Pinatubo aerosol layer with NO A A-11 AVHRR data Geophys. Res. Letters, 1992, 19(2), World Meteorological Organization. Measurements of radiation and sunshine. Guide of meteorological instrument and observing practice. 4^ edition. WMO No.8, T.P.3. Geneva, Switzerland King. M.D., Dale MB, Benjamin M.H. and John A R. Aerosol size distribution by inversion of spectral optical depth measurements. Journal of Atmospheric Science. 1978, 35, Iqbal M. An introduction to solar radiation. Academic Press, Canada World Meteorological Organization. Report of second expert meeting on turbidity measurements. October 24-27, Boulder, Colorado, U.S.A Diaz J.P., F.J. Exposito and A. Diaz. Espesor optico de aerosoles a la longitud de onda de 500 nm desde la estacion BAPMoN de Izana. Rev. Acad. Canar. Cienc IV (nums. 1 y 2), Liou K.N. An introduction to atmospheric radiation. Academic Press, New York, NY, US A Kaufman Y J Aerosol optical thickness and atmospheric path radiance. J. of Off%?/?y.s. 7f?j (D2), Durkee. PA, D.R Jensen, E.E. Hindman and T.H. Yonder Haar. The relationship between marine aerosol particles and satellite-detected radiance../ o/ofo#/?y.s. /&% (D3), Frost EM Global scale estimates of aerosol particle characteristic. Master's Thesis. Naval Postgraduate School. Monterey California. U.S.A World Meteorological Organization. A preliminary cloudless standard atmosphere for radiation computation. WCP-112, WMO/TD-No