May 1982 / Vol. 7, No. 5 / OPTICS LETTERS 221

Remote measurement of atmospheric mercury using differentialabsorption lidar

M. Ald6n, H. Edner, and S. Svanberg

Department of Physics, Lund Institute of Technology, P.O. Box 725, S-220 07 Lund, Sweden

Received January 4, 1982

Atomic mercury in the atmosphere was detected by the differential-absorption lidar technique. Laser light at themercury resonance wavelength of 253.65 nm was generated by anti-Stokes shifting in H2 of the frequency-doubled

output from a Nd:YAG-pumped dye laser. The measurements demonstrate that a concentration of 4 ng/m 3 , corre-

sponding to a typical background value, can be detected if a path length of 2 X 1 km is used.

Introduction

The first reported lidar measurements on atomic mer-cury in the atmosphere are given here. A differential-absorption lidar system was used on and close to theresonance transition of mercury at 253.65 nm (6s21So-6s6p 3P1). The remote-sensing measurementswere supported by laboratory absorption studies, andestimates of sensitivity and detection limits aregiven.

During recent years the sources, mechanisms oftransport, transformations, and sinks of mercury in theenvironment have received considerable attention (see,e.g., Ref. 1). The toxic properties of mercury and itscompounds are well known, and the expected increasein environmental mercury resulting from a more wide-spread use of coal for heating and power generation isa matter of increasing concern. Mercury is the onlypollutant present as a free atom in the lower atmo-sphere, which makes it special with regard to detectionby optical techniques. The background concentrationfo atomic mercury is about 2-6 ng/m3 and correspondsto 0.25-0.75 ppt by volume (1 ppt = 1:1012). Atomicvapor normally dominates over water-soluble mercurycompounds. The latter are deposited in the water, inwhich methyl mercury is an important compound. Inthe global mercury cycle it is believed that mercury isreemitted into the atmosphere as atomic vapor.2 In-creased concentrations of atmospheric mercury areobtained locally from industrial activities, particularlychloralkali plant operation. Clearly, powerful mea-suring methods for mercury are of great interest in as-sessing the impact of anthropogenic mercury and forinvestigations of the global mercury cycle. Remote-sensing techniques offer unique possibilities for atmo-spheric monitoring, and in this Letter we show howthese can be extended to atomic mercury.

Laser-radar techniques have been used for remotesensing of a large number of molecular atmosphericpollutants, especially NO2 , SO2, and 03 (for reviews see,e.g., Refs. 3 and 4). The technology in this field in somecases is approaching operational status.5'6 The pre-ferred technique is the differential-absorption lidarapproach (DIAL), in which the change in range-resolved

particle backscattering is monitored for on- and off-resonance wavelengths. Laser-induced fluorescenceis less suitable for tropospheric use because of the strongcollisional quenching of the fluorescence but is quiteattractive for probing hydrospheric conditions.3' 4 Onthe other hand, fluorescence is the best method formonitoring low concentrations of stratospheric atoms,7 ' 8

as quenching is absent at such altitudes. The fewemission lines of atoms as compared with moleculesfavor atomic over molecular fluorescence monitoringbecause of easier background rejection in narrow-banddetection systems. In our experiments on mercury,absorption as well as fluorescence was observed. Be-cause of the few electronic states in atoms comparedwith the multitude of rotational-vibrational levels inmolecules, the sensitivity for atomic detection is favoredby several orders of magnitude, of which we criticallytake advantage for monitoring of the extremely lowatmospheric-mercury number densities.

Experimental Arrangement

A scheme of the setup used in the mercury experimentsis shown in Fig. 1. We have used a frequency-doubledQuanta-Ray DCR-1A Nd:YAG laser, which produced230-mJ pulses at 532 nm at a repetition rate of 10 Hz.The green beam pumped a Quanta-Ray PDL-1 tunabledye laser, yielding pulse energies of about 100 mJ withthe dye rhodamine 6G. The linewidth was about 0.3cm-1 but could be narrowed to about 0.07 cm- 1 by usingan intracavity 6talon.

In order to reach the mercury absorption line at253.65 nm, we tuned the dye laser to 567.06 nm and useda KD*P crystal to frequency-double the visible light toUV radiation at 283.54 nm. The angle tuning of thedoubling crystal was servo controlled, which made itpossible to make continuous UV-frequency scans.9' 10

The tunable-laser output at 283.53 nm with a pulseenergy of 25 mJ was directed into a Quanta-Ray RS-1Raman shifter, operating with 8 atm of H2. With aRaman shift in H2 of 4155 cm-, the desired mercurywavelength, i.e., 253.65 nm, was reached by anti-Stokesshifting. The pulse energy in this first anti-Stokes

0146-9592/82/050221-03$1.00/0 © 1982, Optical Society of America

222 OPTICS LETTERS / Vol. 7, No. 5 / May 1982

Fig. 1. Schematic diagram of the measurement system.

beam was typically 0.7 mJ. After the Raman shifter aquartz plate was inserted in the beam to split off a smallpart of the light for reference purposes. The main partof this light was directed into a power meter for settinga constant laser power, whereas a smaller part was di-rected through a reference cell containing atmo-spheric-pressure air and a droplet of mercury. Theabsorption from mercury in the reference beam servedas our reference signal in all the subsequent measure-ments. The reference signal was detected with an EMI6256 photomultiplier tube (PMT) and was processedwith a PARC Model 162 boxcar integrator equippedwith Model 165 gated integrators.

The main beam from the Raman shifter was sent intothe atmosphere by using a Newtonian telescope with a25-cm mirror and a focal length of 100 cm. The back-scattered light was detected with an EMI 9558 QA PMTthrough an interference filter. No optimal filter wasavailable, and the experiments reported here wereperformed with a filter of 20-nm full half-width and atransmission of 3%. The signal from the PMT was fedto a fast transient digitizer, Biomation 8100, which wasused to convert the signal to time-resolved digital data.The transient digitizer was interfaced to a speciallyconstructed computer, which averaged a large numberof lidar curves. Further details of the lidar optical andelectronic systems have been published.1 In the fre-quency-scanning experiments the PMT signal was fedinto one channel of the boxcar integrator, as indicatedby the dashed line in Fig. 1, while the reference signalfrom the power meter was fed to the second channel.

Measurements

Measurements were made on absorption cells withvarious path lengths and different mercury-vaporpressures. The experiments yielded an absorptioncoefficient of 2.7[(mg/m 3)m]- 1 with the laser linewidthachieved with the 6talon in the dye laser. Vapor-pressure data from Ref. 12 were used. With an esti-mated detection limit of 2% absorption this indicates

a sensitivity of about 8(gg/m 3)m, or an average con-centration of 4 ng/m 3 over a path length of 2 X 1 km.Whereas this is a typical background value, more than10-times higher concentrations could occur close tochloralkali plants.

Remote detection of atomic mercury was made on alarger open chamber at a distance of about 100 m. Thesample chamber had a path length of 2 m along the lidardirection and was open at each end. Three small vesselswith mercury inside the chamber provided the mercuryvapor. The openings could be closed, thus allowing thevapor pressure to increase before the measurements.The laser beam was directed through the chamber andbackscattered light detected from either a target or theatmosphere behind the chamber. Figure 2 shows thesignal versus wavelength when the boxcar integrator wasgated to the echo from a board placed just behind thechamber. The relative absorption at the mercury linedepended a great deal on how soon after the opening themeasurement took place. A better stability of themercury concentration could be achieved by using theclosed back door as target.

The lidar approach was demonstrated to be quitefeasible in this wavelength region. With only 0.2 mJ ofoutput from the telescope a good atmospheric back-scatter signal was registered out to 1-2 km with signalaveraging during good atmospheric conditions. Therange was, however, sensitive to haze and dust in thepath. Figure 3 shows that the lidar curves in a DIALmeasurement on the distant open-ended cell averagedover 50 laser shots. The on- and off-resonance wave-lengths were 253.65 and 253.68 nm, respectively. Theattenuation that is due to absorption in the chamber isclearly noticeable in the on-resonance curve and in theratio curve. In this divided representation of the signal,the slope of the curve corresponds to the mercury con-tent. The on-resonance curve also shows a peak beforethe attenuation, which is caused by resonance fluores-cence from the mercury atoms. The peak at the be-ginning of each of the two curves comes from scatteredlight when the laser pulse emerges from the telescope,and it serves as a zero-time reference. The data in Figs.2 and 3 were taken without the 6talon, and simultaneousreference measurements in the laboratory indicate amean concentration of 0.1 mg/m 3 in the chamber. Thisis, as expected with the open chamber used, much lower

2516 253.7 2536dWA~sflEA^GrTfl'm/

Fig. 2. Intensity of backscattered light versus wavelengthfrom a fixed target behind a remotely positioned samplechamber.

May 1982 / Vol. 7, No. 5 / OPTICS LETTERS 223

as,

- ON RESONANCE

----- OFF RESONANCE

J00

ON/OFF

100 200 J60RANGE 1.

Fig. 3. A DIAL measurement of the mercury-vapor contentin the distant chamber. The two upper curves show the in-tensity of atmospheric backscattering on and off mercuryresonance, respectively; the lower curve displays the dividedDIAL signal.

than the 4 mg/M3 concentration calculated from a sat-urated-vapor pressure at the ambient temperature.With a better interference filter in the detection systemand with high-reflectance mirrors in the beam-handlingsystem, the signal intensity could be increased by afactor of 20.

Discussion

We have demonstrated the feasibility of measuring at-mospheric mercury with the lidar technique. A de-tection limit of 8 (,ug/m3)m was achieved with a laserlinewidth of about 0.15 cm-1 . The remote measure-ments were, however, performed without an 6talon sincethe 6talon reduced the pulse energies at 254 nm by afactor of 4. There are other methods for reaching themercury absorption line than by using Raman shifting.One possibility is to double a coumarin-dye output di-rectly, but the doubling has a poor efficiency in thiswavelength region and we did not consider this ap-proach. Yet another option would be to use frequencymixing of a doubled DCM-dye output and the residual1.06-jum beam in another KD*P crystal. This approachwould probably not yield substantially higher pulseenergies than we achieved and would also result in alarger linewidth without line narrowing of the Nd:YAGlaser.

Remote monitoring of mercury using atmosphericbackscattering is quite applicable to measurements ofplumes, as simulated in our experiments. Typical levelsof mercury at the outlet of a coal-fueled power-plantstack should be about 10 (pug/m3 )m,13 resulting in alidar-curve attenuation of about 2.5%. Correspondingvalues for refuse-destruction plants could be about 10

times higher. The range-resolved detection of lowconcentrations distributed over a longer path could behampered by the occurrence of resonance fluorescenceand has to be investigated more thoroughly. In thisapplication the employment of a topographic target foraverage-concentration measurements should be a moreuseful method. We intend to do more specific fieldtests using the mobile lidar system,5 when comparisonswith point monitors for mercury and its compounds willbe made.

The authors are grateful to B. Nilsson and P. Norr-man for assistance in the measurements.

Interesting communications with C. Brosset and B.Galle at the Swedish Water and Air Pollution ResearchInstitute, Gbteborg, are acknowledged.

This work was supported by the Swedish Board forSpace Activities.

References

1. J. 0. Nriagu, ed., The Biogeochemistry of Mercury in theEnvironment (Elsevier/North-Holland, Amsterdam,1979).

2. C. Brosset, "Investigation of mercury in air and naturalwaters," Tech. Rep. KHM 05 (Swedish State PowerBoard, Vdllingby, Sweden, 1981) (in Swedish).

3. R. M. Measures, "Analytical use of lasers in remotesensing," in Analytical Laser Spectroscopy, N. Omenetto,ed. (Wiley, New York, 1979).

4. S. Svanberg, "Lasers as probes for air and sea," Contemp.Phys. 21, 541 (1980).

5. K. Fredriksson, B. Galle, K. Nystr6m, and S. Svanberg,"Mobile lidar system for environmental probing," Appl.Opt. 20,4181 (1981).

6. J. G. Hawley, G. F. Wallace, and L. D. Fletcher, "A mobiledifferential absorption lidar (DIAL) for range resolvedmeasurements of S02, 03, and NO2 ," presented at theNineteenth Meeting of the American Institute of Aero-nautics and Astronautics. Aerospace Sciences on Mea-surement and Modeling of Atmospheric Trace Species inthe Troposphere, in St. Louis, Mo., 1981.

7. M. R. Bowman, A. J. Gibson, and M. C. Sandford, "At-mospheric sodium measured by a tuned laser radar,"Nature 221, 456 (1969).

8. J. P. Jegou, P. Juramy, G. Megie, and M. L. Chanin,"Dynamics and photochemistry at the mesopause levelfrom lidar sounding of alkali atoms," presented at Inter-national Laser Radar Conference, Silver Spring, Md.,1980.

9. G. C. Bjorklund and R. H. Storz, "Servo tuning and sta-bilization of nonlinear optical crystals," IEEE J. QuantumElectron. QE-15, 228 (1979).

10. B. Nilsson and P. Norrman, "Construction and testingof an automatic frequency-doubling system," Lund Reps.on Atomic Physics LRAP-10 (Lund Institute of Tech-nology, Lund, Sweden, 1981) (in Swedish).

11. K. Fredriksson, B. Galle, K. Nystr6m, and S. Svanberg,"Lidar system applied in atmospheric pollution moni-toring," Appl. Opt. 18, 2998 (1979).

12. Handbook of Chemistry and Physics, 54th ed. (CRCPress, Cleveland, Ohio, 1973).

13. C. Brosset, Swedish Water and Air Pollution ResearchInstitute, Gbteborg, Sweden (personal communica-tion).