NO2 Lidar Comparison: Fluorescence vs Backscattered Differential Absorption

Jerry Gelbwachs

Aerospace Corporation, P.O. Box 92957, Los Angeles, California 90009. Received 25 June 1973. Mapping atmospheric pollutants by lidar (laser radar)

has received wide attention in recent years. Analytical studies have evaluated differential absorption and scatter­ing (DAS) and fluorescence techniques for remotely moni­toring nitrogen dioxide (NO2).1,2 These treatments dem­onstrated that the DAS method is superior to fluorescence detection for daytime NO2 determinations in an environ­ment containing aerosols. In this Letter the inherent dif­ference between the two modes of detection is examined. This difference suggests the techniques as applied to the singular case of NO2 are complimentary rather than com­petitive and that earlier comparisons may have unfairly penalized the fluorescence method by improperly choosing parameter values. The atmospheric conditions for which fluorescence monitoring affords the greater sensitivity for NO2 detection are mentioned.

Briefly, the DAS method consists of a pulsed laser that transmits two wavelengths, one that coincides with a peak in the absorption curve for NO2 molecules and the other at an absorption minimum. Within a range cell the ener­gy at each wavelength is differentially absorbed by the NO2 molecules and elastically scattered back toward the receiver by the molecules and aerosols within the cell. Detection is performed in a narrow optical band centered about the transmitted wavelengths. A photomultiplier tube (PMT) operating in the photon-counting mode records the signals. A comparison of signals received from the range cell with those from the preceding range cell allows the quantitative determination of the NO2 content.

The photoelectronic counts C1,i due to backscattering at λ1 from the ith range interval at the distance Ri for the case of low loss to the transmitted beam as it propagates to and from the range cell is expressed as

where E1 = energy transmitted at λ1

h = Planck's constant; c = speed of light;

β1 = atmospheric elastic backscattering at λ1; L = length of range cell; n = NO2 concentration;

α1 = NO2 absorption coefficient at λ1; A = receiver area; K = efficiency of optical detection system.

A related expression can be written for C2,i. For a small uniform concentration of NO2 within the range cell it can be shown approximately that

where Act is the differential absorption α1 - α2. This equation is similar to Ahmed's Eq. (7) for nΔαL « l.3 It has been assumed that the wavelength separation between λ1 and λ2 is small so that the backscattering at each wavelength is effectively the same. Recent data support this assumption and provide a value for ∆α. It has been found that a suitable pair of wavelengths exists in the blue

2812 APPLIED OPTICS / Vol. 12, No. 12 / December 1973

portion of the visible spectrum that differ by about 1 nm.4

Since detection is performed at the laser wavelengths, filters with narrow optical bandwidths can be employed to discriminate effectively against background sources of radiation. DAS can therefore be characterized as a ratio method for which the sensitivity improves with decreased visibility, i.e., large atmospheric backscattering, and is signal shot-noise limited, i.e., independent of background conditions.

Lidar fluorescence determination of ambient NO2 is ac­complished with a pulsed laser at a wavelength corre­sponding to a peak absorption of NO2. The wavelength can be identical to λ1 of the DAS scheme. The NO2 mol­ecules then fluoresce over a broad spectral region extending from the laser wavelength to the near infrared. The signal that reaches the PMT consists of NO2 fluorescence and radiation from background sources. The background level can be determined with little uncertainty by opening the range channel in the absence of a transmitted laser pulse and collecting photoelectronic counts during time inter­vals much longer than 2L/c. In the presence of fairly uni­form background spectral radiance the continuum nature of the NO2 fluorescence5 permits greater precision for flu­orescence detection to be achieved with optical filters that are wide band in contrast to narrow band filters, which are preferable for DAS. Optical bandwidths of 100 nm are appropriate and should be sufficiently displaced from the laser wavelength to avoid interference from Raman scattering from the major molecular constituents of the atmosphere, i.e., nitrogen, oxygen, carbon dioxide, and water vapor. Consequently, the fluorescence technique is a direct method strongly influenced by background condi­tions and relatively independent of visibility. For large background levels fluorescence detection is background limited, while dark skies permit signal shot-noise limited operation.

Broadband fluorescence of ambient atmospheric aerosols has recently been recorded.6 If a component of aerosol fluorescence falls within the NO2 fluorescence monitoring band it may reduce the sensitivity of this method. Our computations of minimum detectable NO2 concentrations determined by fluorescence lidar have assumed no inter­ference from aerosol fluorescence. By virtue of measuring signals at the laser wavelength, the DAS scheme is im­mune to aerosol fluorescence interference.

The lidar equation for fluorescence detection trans­formed to yield photoelectronic counts rather than re­ceived power at the PMT1-2 takes the form

where CF are the photoelectronic counts from NO2 fluo­rescence, Q is the factor by which the NO2 fluorescence is quenched at 1 atm,5 and ΔB/B is the fraction of fluores­cence that falls within the detection band AB. The back­ground radiation CB in the detection band is given by

where N is the background spectral radiance, Ω is the re­ceiver field of view, and λp is the wavelength at the center of the fluorescence detection band.

In the determination of sensitivity, the only terms not common to both techniques are atmospheric backscatter­ing, background spectral radiance and optical bandwidth for fluorescence detection. A meaningful comparison of the two methods is therefore possible. In Table I, the

Table I. NO2 Concentrations Determined by DAS and Fluorescence Lidar for SNR = 3 as a Function of Range

concentration of NO2 corresponding to a signal-to-noise ratio of 3 has been calculated as a function of range for both methods of detection. Typical values for NO2 lidar parameters are as follows: E1 = E2 = E/2 = 0.1 J; λ1 ≈ λ2 = 0.45 μm; β1 = β2 = 4.4 × 1 0 - 5 m _ 1 for clear skies and 5.8 × 10- 4 m - 1 for 10-km visibility1, L = 100 m; α = α1 = 2α2 = 10-3 m - 1 p p m - 1 (Ref. 4); A = 0.1 m2; Ω = 3 × 1 0 - 8

sr; K = 0.1; λF = 0.65 μm; ∆B = 100 nm; ∆B/B = 0.2; Q = 2 × 10-5 (Ref. 5); and Nλ = 10 W/m 2 ∙ s r ∙μm during the day and 1 0 - 2 W/m 2 ∙ s r ∙μm at night. From Table I it is apparent that neither technique is superior under all conditions. It is clear that, on the one hand, DAS pro­vides greater sensitivity under daylight conditions with reduced visibility. On the other hand, fluorescence lidar may be better suited for determining NO2 levels on clear nights.

Maximum sensitivity for NO2 detection could be at­tained for all atmospheric conditions with a single lidar. When conditions favor fluorescence viewing, the DAS system could be readily converted by simply transmitting pulses at λ1 only and replacing the narrow band filter in front of the PMT by a wide optical band filter centered at λF. Of course, an alteration in the processing of the re­ceived signals would be required. For ease of operation and full utilization of both elastic backscattering and flu­orescence, might not future NO2 lidars come equipped with two photodetectors?

References 1. H. Kildal and R. L. Byer, Proc. IEEE 59, 1644 (1971). 2. R. M. Measures and G. Pilon, Opto-Electron. 4, 141 (1972). 3. S. A. Ahmed, Appl. Opt. 12, 901 (1973). 4. T. Igarashi. Fifth Conference on Laser Radar Studies of the

Atmosphere, 4-6 June 1973, Williamsburg, Virginia. 5. J. A. Gelbwachs, M. Birnbaum, A. W. Tucker, and C. L.

Fincher, Opto-Electron. 4, 155 (1972). 6. J. Gelbwachs and M. Birnbaum, Appl. Opt. 12, 2442 (1973).

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