Calibrated remote measurements of SO2 and O3 using atmospheric backscatter

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  • Calibrated remote measurements of SO2 and O3 using atmospheric backscatterW. B. Grant and R. D. Hake Jr. Citation: Journal of Applied Physics 46, 3019 (1975); doi: 10.1063/1.321992 View online: View Table of Contents: Published by the AIP Publishing Articles you may be interested in Rate coefficient measurements for SO2+O=SO+O2 J. Chem. Phys. 73, 987 (1980); 10.1063/1.440748 Measurement of electron affinities of O3, SO2, and SO3 by collisional ionization J. Chem. Phys. 62, 3829 (1975); 10.1063/1.430941 Calibrated remote measurement of NO2 using the differentialabsorption backscatter technique Appl. Phys. Lett. 24, 550 (1974); 10.1063/1.1655049 Use of O2 for ESR Calibration for Quantitative Measurement of Gas Concentrations J. Chem. Phys. 44, 1715 (1966); 10.1063/1.1726918 Upper Atmosphere Temperatures from Remote Sound Measurements Am. J. Phys. 16, 465 (1948); 10.1119/1.1991145

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  • Calibrated remote measurements of 802 and 0 3 using atmospheric backscatter

    W. B. Grant and R. D. Hake, Jr.

    Stanford Research Institute, Menlo Park, California 94025 (Received 3 March 1975)

    Remote measurements of calibrated samples of S02 and 0 3 have been achieved with a lidar using ultraviolet signals produced by a tunable dye laser and a nonlinear crystal. The operating wavelengths for these measurements were 292.3 and 293.3 nm for S02 and 292.3 and 294.0 nm for 0 3, The atmosphere in front of and behind the chamber acted as a distributed reflector to send laser light back through the chamber to a receiver near the laser. The laser measurements agreed well with in situ measurements. Integration of eight laser pulses at each of two wavelengths allowed the determination of S02 concentration with an uncertainty equivalent to 0.6 ppm in 100 m for low concentrations. For 0 3, the corresponding uncertainty limit was 1.2 ppm in 100 m. The measurement errors are primarily attributable to variations in atmospheric backscattering intensity during the experiment, since the different wavelengths were radiated sequentially rather than simultaneously. The sensitivity of a system transmitting more favorable wavelengths at intervals separated by less than 1 min is estimated to be near O.I ppm in 100 m for both S02 and 0 3,

    PACS numbers: S7.60.P, 89.60., 42.6O.Q, 42.68.M

    Recent experimental results have shown that it is possible to make remote measurements of N02 at con-centrations typical of urban environments. 1,2 The mea-surements in Refs. 1 and 2 were made USing mono static pulsed-laser radars employing the differential-absorp-tion-Udar (DIAL) technique. This technique uses two wavelengths for which the gas of interest has differing absorption coefficients. The measured difference in at-tenuation for the two wavelengths, determined by mak-ing time-resolved observations of the energy backscat-tered by the atmosphere, can be combined with the known difference in absorption coefficients to provide a range-resolved measurement of the absorbing species.

    The N02 measurements were made using pulsed dye lasers operating in the visible near 450 nm. It has been suggested that a similar device operating in the uv could be used to monitor S02. 3,4 Total COlumn-content mea-surements of S02 have, in fact, been made using a retroreflector of uv energy to provide the return sig-nal. 5 This paper reports results of active remote mea-surements of S02 and 0 3 using atmospheric backscat-ter; it demonstrates the feasibility of using the DIAL

    r - - --

    5.3 x BEAM

    technique for range-resolved measurement of S02 and 0 3 at uv wavelengths with an eye-safe lidar.

    The equipment arrangement for this experiment is similar to that for N02 in Ref. 2. The principal changes are the use of a nonlinear crystal [ammonium dihydro-gen arsenate (ADA)]6 after the dye laser to generate uv wavelengths by nonlinear doubling, and the replacement of the Pyrex windows in the sample chamber with com-mercial-grade quartz windows (see Fig. 1). Other de-tails of the laser and receiver are given in Table L The ADA cr'ystal is temperature tuned to achieve 900 phase matching at the different wavelengths. The wavelengths employed were selected to give the highest available differential in absorption coefficients for S02 and 0 3 that could be achieved with operation of the ADA crystal in a reasonable temperature range. Use of a single ADA crystal prevented taking the measurements alternately at the two wavelengths at 8-s intervals, as was the case for N02 in Ref. 2. Instead, data were accumulated at one wavelength for about ~-h, with at least a i-h delay between different wavelengths required for changing and stabiliZing the crystal temperature. Thus, there was


    r=~ ~ fr~~ ) FIG,!. Block diagram of the ex-perimental apparatus used in this experiment. The equipment en-closed in the dashed line was not used in these measurements but could be added to provide nearly simultaneous transmission at two wavelengths (see text for discussion) .


    MAGNETIC ,-----2.5 m----j TELESCOPE 12~




    Journal of Applied Physics, Vol. 46, No.7, July 1975 Copyright 1975 American Institute of Physics 3019

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  • TABLE I. Experiment parameters.

    Laser (Refs. 7 and 8)

    Energy Dye Beam diameter Beam divergence (far field) Pulse length Grating Linewidth Repetition rate

    ""15 mJ Rhodamine 6G (3.5 Xl(l"5 Mil in MeOR) ""2 mm (3 dB) 0.7 mrad (3 dB)

    250 ns (3 dB) 316 l/mm, used in ninth order "'II,. 0.017Hz

    Second-harmonic-generating crystal (Ref. 6)

    Type Ammonium dihydrogen arsenate Length 2 cm Index matching fluid Fluorocarbon 77 Conversion efficien-cy for 15-mJ funda- 5% mental Pulse length '" 2 00 ns (3 dB)



    PMT ~tical efficiency from ADA crystal through PMT



    PMT Accuracy

    Wavelength (rIm)

    292.3 293.3 294.0


    0.056 m2

    293 nm, 13-nm half-width, 27% peak transmission RCA 7200



    Deuterium 285.7 nm, 3-nm half-width, 17% peak transmission RCA IP 28 7 ppm

    Spectral data

    Absorption coefficient (cm-1 atm-1) (base e)

    S02 0 3 (Ref. 9) (Ref. 10)

    26 28 14


    Atmospheric conditions

    Time Visibility Molecular scattering coefficient (calcu-lated) Aerosol scattering (extinction) coeffi-cient (observed) Ambient S02' N02, and 0 3 concentra-tions (Ref. 11)

    19 Dec. 1974, midnight to 5:00 a. m. 10-20 km

    "" 0.14 km-1 atm-!

    "" O. 5-0. 7 km-!

    < 0.03 ppm

    at least a l-h delay between collection of comparable data at different wavelengths.

    The current range-resolution limit is 70 m, deter-mined by the amplifier bandpass that was fixed at 2. 5 MHz to oversample these test returns. The resolution of a field system would be limited to 35 m by the 200-ns uv pulse duration.

    3020 J. Appl. Phys., Vol. 46, No.7, July 1975

    The 802 was injected by syringe and had a l/e resi-dence time of about 14 min. The 0 3 was generated using a Welsbach ozonator. 12 It could fill the O. 9_m3 sample chamber with 30 ppm of 0 3 in 2 min. The gas content of the chamber was monitored by a dual-beam transmissometer.

    Typical return signals with air and with 105 ppm 802 in the sample chamber are shown for 292.3 and 293.3 nm in Fig. 2. Each signal point is an average of eight consecutive return pulses. The initial peak near 100 m arises from the coaxial nature of the transmit/ receive geometry. The effect of the shadow created by the central obscuration decreases with distance and is negligible by about 150 ro. The peak near 300 m is the return signal from the quartz windows due to scat-tering caused by bubbles and surface reflections. Both peaks are artifically flattened by the data-acquisition system to extend the dynamic range of the 8-bit digitizer to low signal levels. Lidar data similar to those shown in Fig. 2 were obtained during four nights of operation. Minor system improvements were made between the various runs.

    In order to make a comparison of the lidar data with the transmissometer data, a number must be extracted from the lidar return sign