Diapositiva 1

DIfferential Absorption Lidar: DIAL

Lidar Training Course. Cayo Coco, April, 6-8 2015. DIAL Technique. Elian WolframElian WolframCEILAP, Argentina1OutlookDial MotivationReview of Lidar equationDeduction of DIAL equationDetection limit and Error propagationDIAL for ozone measurement (inst. Description)Example of real ozone measurementWater vapor Raman lidarSignal correction Examples

22DIAL MotivationDIAL systems in both the ultraviolet (UV) and infrared (IR) spectral regions were developed and fielded for ozone and industrial emissions including SO2, NO2, NH3 , HCl, CO etc.The DIAL technique uses the idea of differential-absorption measurement.Two light pulses of different wavelengths are launched along the same path into the atmosphere, and two corresponding backscattered profiles are simultaneously measured. The DIAL wavelengths are selected so that the light at the one wavelength, lon, is strongly absorbed by the absorbing species under investigation, whereas the light at the second wavelength, loff, is absorbed not at all or at least much less

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Basic Setup of the lidar instrument44Overlap FactorBackscatter Lidar Telescope Field of View 1 10 mrad Laser Divergence 0.1 1 mrad Overlap height accordable 300 m 2km Laser0 37 nm en el dominio ultravioleta de estudio(Inn y Tanaka, 1953). Wavelength Pair Selection99DIAL equation (1)

TransmissionExtinctionMolecular and Particle Scattering(Rayleght Mie Scatt.)Molecular and particle Absorption1010DIAL equation (2)

Number density of gases of interestMolecules/unit volume1111DIAL equation (3)

==1212DIAL equation (4)

If :

The elimination of the instrument parameters is only possible if the ratio of the lidar constants and the transmitted powers at the two wavelengths are range-independent1313

DIAL equation (5)

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45 kmDIAL Measurement

1515DIAL equation (6)

DIAL equation is self-calibrating measurement techniques!!!!!No instrumental constants appear in final equation

With the assumption thatBackscattering absorption At two wavelength are identical

Simplified case: Extinction due only to absorbing gas 1616The foregoing analysis assumes that there are no range-dependent differencesin the lidar responses at the two wavelengths. Fredriksson and Hertz provided an extensive summary of experimental problems that could cause systematic differences at the two wavelengths and consequenterrors in the measured value of N,

Ismail and Browell presenteda thorough analysis of the sensitivity of DIAL measurements toboth random signal errors and to differential cross section errors. Thelatter type of error arises from both atmospheric and system effects, including - temperature and pressure sensitivities of the trace gas spectrum,Doppler broadening of the Rayleigh return, the non-zero widthpossible shift of the laser line, -uncertainties in its spectral purity and center wavelength.K.A. Fredriksson, H.M. Hertz: Appl. Opt. 23, 1403 (1984)S. Ismail, E.V. Browell: Appl. Opt. 28, 3603 (1989)

A word of caution is in order of self calibrating:

1717DIAL equation (7)

In practice, lidar signals are not recorded or analyzed as continuousfunctions, but rather as values in discrete range bins.

A real lidar system will have some limit with which it can resolvethe term in parentheseslidars limit of detection NLD for the gas of interest.R.T.H. Collis, P.B. Russell: Lidar Measurement of Particles and Gases byElastic Backscattering and Differential Absorption. In Laser Monitoring of theAtmosphere, E.D. Hinkley, ed. (Springer-Verlag, NewYork 1976), p. 102

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Error propagationAssuming for simplicity that the error inthe off signal can be ignored in comparison with that in the on signal, one canobtain a simple formula for the relative error of the chemical species concentration.

where DtA,dif is the differential optical depth, that is, the difference between the optical depths tA,on and tA,off over the rangeWhen DR is small, the quantities Pon(R) and Pon(R + DR) may be highly correlated;therefore, the covariance term of the signals, COV(PR, PR+DR) Is includedthe range element DR in DIAL measurements must be long enough toprovide acceptable accuracy in the retrieved chemical species concentration.Thus the local differential absorption optical depth is the most important factorthat influences accuracy of the measured data.1919DIAL equation (8)

In the general case, the atmospheric backscatter coefficient is not the same at the two DIAL wavelengths and there is differential extinction due to air molecules, aerosols, and interfering gasses, in addition to the gas of interest.

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For most molecular species such as H2O, SO2,NO2, and NOthe wavelength separation between the ON and OFF wavelengthscan be smaller than 1 cm-1. If this is the case, thedifferences in the scattering properties of the atmosphereand the differential extinction due to aerosol and interferinggases can be neglected and the gas numberdensity is given simply by

For the particular species mentioned above great care should be takento avoid systematic errors or random deviations due to largelaser linewidth or wavelength instability of the lidar transmitter.This problem is not essential for species like ozone or chlorinethat has broad absorption features.

21DIAL measurement of different species 21The determination of gas concentration profileswith DIAL must include the following operations (Browell, 1985):

(1) Measurement of the elastic lidar signals at the on and off wavelengths.An additional lidar signal measurement may also be made at a referencewavelength, lref, that allows determination of the backscatteringand extinction corrections.(2) Calculation of the first raw estimate of the absorbing gas concentrationprofile N(R) .This makes it possible to estimate the dataquality and the achieved measurement range.(3) Calculation of the particulate extinction coefficient profile at the referencewavelength and determination of the backscatter and extinctioncorrections for the ozone concentration.(4) Calculation of the final absorbing gas concentration profile by using thebackscatter and extinction corrections. Note that the backscatter andextinction corrections can be made either after taking the derivative ofthe signal ratio logarithm or before this operation. One can avoid additionalnumerical differentiation when determining the backscatter correctionterm, making the corrections before the ozone concentration isextracted (Kovalev and McElroy, 1994; Kovalev et al., 1996).2222Lidar Retrieval for O3 MeasurementsExample: DIfferential Absorption Lidar technique for stratospheric ozone measurementsWavelength selected pair (308 nm & 355 nm)High, middle & low stratosphere (powerful laser source & not too strongly absorbed )Statistical error minimization in the high stratosphere ()correction term is dominated by the aerosol backscatter differential effectnot useful in large aerosol loading conditionsUse of Raman signals (onR, offR) from first Stokes of N2differential absorption cross section

number of detected photons at iRbackground radiation at iRcorrection term* Suppression of the backscatter term* Test of Rayleigh signals linearity* Retrieval measurements in the lowermost Stratosphere2323Corrective o complementary term:Statistical Error:differential atmospheric scattering (Rayleigh & Mie)

differential Rayleigh & Mie extinction coefficients differential extinction by other atmospheric constituents

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telescope areafinal vertical resolutionemitted laser powerduration of the measurement

DIfferential Absorption Lidar: DIALLidar Retrieval for O3 Measurements2424First lidar measurements of ozoneFirst lidar measurements of ozone vertical distribution using dye lasers Mgie G., J.Y Allain, M.L. Chanin, J.E. Blamont Nature270, 329 - 331 (1977) | doi:10.1038/270329a0.Optimisation of lidar ozone measurements Minimisation of statistical error following Shotland, J Appl Meteor 13 (1974), for water vapor measurements Pelon J. and G. Mgie, Nature, 1982First measurements with an excimer laser Possibility to reach the high stratosphere Werner J, K. W. Rothe and H. Walter: Appl. Phys. B, 1983

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2626Ozone Raman Lidar

Ozone absorption cross section and wavelengths applied in the ozone RamanDIAL technique for upper tropospheric and stratospheric measurements (>300 nm)and for boundary-layer measurements in the solar-blind region (