Atmospheric atomic mercury monitoring using differential absorption lidar techniques

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<ul><li><p>Atmospheric atomic mercury monitoring using differentialabsorption lidar techniques</p><p>Hans Edner, Gregory W. Faris, Anders Sunesson, and Sune Svanberg</p><p>Three-dimensional mapping of atmospheric atomic mercury has been performed with lidar techniques, to ourknowledge, for the first time. Industrial pollution monitoring, as well as measurements of backgroundconcentrations, is reported. High-efficiency frequency doubling of narrowband pulsed dye laser radiationwas employed to generate intense radiation at the mercury UV resonance line. Field measurements weresupplemented with extensive laboratory investigations of absorption cross sections and interfering lines ofmolecular oxygen.</p><p>1. IntroductionRange-resolved monitoring of atmospheric atomic</p><p>mercury pollution employing the differential absorp-tion lidar technique is reported, we believe, for the firsttime. A mobile lidar system that was equipped with anarrowband tunable laser transmitter able to generatepulses of adequate power at the mercury resonanceline at around 254 nm was employed. Interfering ab-sorption lines due to molecular oxygen were studied indetail to allow mercury measurements with a sensitiv-ity down to typical background levels, 2 ng/m 3 .</p><p>Atomic mercury is an atmospheric'pollutant that isdirectly generated from chlorine-alkali plants, coal-fired power plants and refuse-incineration plants.However, anthropogenic mercury enters the environ-ment also in the aquatic phase as water-soluble mercu-ry compounds, e.g., CH 3HgCl and HgCl2 . Again, in-dustrial activities as well as inadequate wastemanagement are responsible for such emissions. Acomplex and not fully understood interaction betweenthe water and atmospheric phases occurs in the envi-ronmental mercury cycle.1'2 Atomic mercury is alsoan interesting geophysical tracer gas associated withcertain ore deposits,3 -6 as well as geothermal, 7 8 seis-mic,9 and volcanic' 0 activities.</p><p>Typical background concentrations of atomic mer-cury are a few ng/m3 .11-13 Low concentrations of Hgare normally measured by point monitors employing</p><p>When this work was done all authors were with Lund Institute ofTechnology, Physics Department, P.O. Box 118, S-221 00 Lund,Sweden; G. W. Faris is now with SRI International, Molecular Phys-ics Laboratory, Menlo Park, California 94025, and A. Sunesson isnow with RIVM, Laboratory for Air Research, P.O. Box 1, 3720 BABilthoven, The Netherlands.</p><p>Received 17 March 1988.0003-6935/89/050921-10$02.00/0. 1989 Optical Society of America.</p><p>gold amalgamation techniques combined with flame-less atomic absorption spectroscopy. 4"15 Direct opti-cal absorption or Zeeman absorption spectrometerscan also be utilized.16 Remote sensing techniques,such as lidar (light detection and ranging), and inparticular the differential absorption lidar (DIAL)17,18provide important advantages over point monitoring.Typical Hg concentrations, which are exceedingly lowby DIAL standards, can still be measured by thattechnique only since the oscillator strength of the elec-tronic transition at 254 nm is concentrated in an atom-ic line rather than spread over the large number ofrotational-vibrational molecular transitions normallyencountered. Actually, mercury is the only pollutantthat is present in the troposphere in elemental form.Because of the sharpness of the mercury absorptionline a narrowband laser transmitter is necessary toattain maximum absorption when the laser is tuned toresonance with the line. For range-resolved lidar mea-surements that rely on atmospheric backscattering, asubstantial laser pulse energy is necessary to achieve auseful range of -1 km.</p><p>Early attempts to measure mercury with the DIALtechnique are reported in Ref. 19. Sensitivity andrange were severely limited by the large linewidth ofthe laser used (0.015 nm) and low output pulse energy(0.5 mJ) obtained by stimulated Raman scattering inH2 of frequency-doubled dye laser radiation (567 nm).To exploit a broad laser, the gas correlation lidar tech-nique20 was introduced and demonstrated for Hg. Byrecording on- and off-resonance signals simultaneous-ly, a greater immunity to atmospheric turbulence isobtained, but the sensitivity is reduced. In a paralleldevelopment we have also explored the potential ofpath-averaged measurements of mercury2l with theDOAS (differential optical absorption spectroscopy)method.22 23 Although this method is quite useful, it isvery difficult to achieve high spectral resolution to</p><p>1 March 1989 / Vol. 28, No. 5 / APPLIED OPTICS 921</p></li><li><p>obtain optimal sensitivity and freedom from the influ-ence of interfering lines.</p><p>Three-dimensional, highly sensitive mapping of Hg,reported in this paper, had to await the availability ofhigh-power narrowband pulsed laser sources at 254nm. The measurements were performed with our newmobile lidar system24 which, for this measurement,was equipped with a Quantel Datachrome system witha dual wavelength option and a linewidth of 0.001 nmin the UV. Using a betabarium borate (BBO) crystal,pulse energies up to 5 mJ could be generated by directfrequency doubling.</p><p>In the next section the mobile lidar system arrange-ments for the field work are described as well as thesetup for laboratory measurements of the Hg crosssection and interfering 02 lines. Laboratory measure-ments are described in Sec. III.A while the field work isreported in Sec. III.B. Results for plumes from achlorine-alkali plant are presented as well as measure-ments on background concentrations of Hg. In a sepa-rate section the lidar signal contributions due to Hgresonance fluorescence are discussed. Fluorescence isnormally of little importance in tropospheric lidarwork on molecules but is utilized in lidar monitoring ofstratospheric atomic layers.2526 For Hg, in atomicform, fluorescence occurs even at tropospheric pres-sures and can contribute substantially to the signalwhen polarization techniques are employed. Finally,conclusions are drawn in the last section.II. Experimental Arrangement</p><p>The system that was used has been described exten-sively in Ref. 24. It is housed in a mobile truck with alaboratory area of 6.0 X 2.3 M2 . Power is supplied by a20-kVA diesel generator installed in a trailer towed bythe truck.</p><p>A Nd:YAG-pumped dye laser is tuned alternately totwo close-lying wavelengths, one that is on the mercuryresonance line and one that is off the line. The laserbeam passes a 6X beam expander and is transmittedinto the atmosphere using quartz prisms and a largeplane mirror housed in a dome construction that ishoisted up through a trapdoor in the roof of the truckduring measurements. The large mirror can be rotat-ed around both the horizontal and the vertical axes,thus allowing the beam to be aimed in the desireddirection. Backscattered radiation is collected withthe same mirror and directed into a Newtonian tele-scope with a 400-mm diameter. After the telescope,an interference filter selects the appropriate wave-length range for the detection. Detection is per-formed with an EMI 9816QA photomultiplier tube.The photomultiplier is gain-modulated to reduce thedynamic range of the signal and the mean power dissi-pation in the tube.</p><p>The electronic signal is A-D converted in one of thetwo channels of a LeCroy transient recorder with 10-nstime resolution. A GPIB interface transfers the digi-tized signal to an IBM AT-compatible computer wheredata are averaged and stored on floppy disks. Duringa measurement the system is controlled by the com-puter. It handles laser wavelength switching, mea-</p><p>surement direction setting, and control of data sam-pling and data averaging. Up to three plume scanswith fifteen directions each can be performed withoutinterference from the operator. Both horizontal andvertical sweeps are possible.</p><p>For this measurement the system was equipped witha new Quantel YG 581C Nd:YAG laser and a TDL 50dye laser. The pump laser delivers pulse energies of500 and 200 mJ at 532 and 355 nm, respectively, with10-Hz pulse repetition rate. The dye laser can deliverup to 200 mJ at 560 nm with a linewidth of 0.08 cm-'.To generate the 254-nm radiation the dye laser wasoperated with coumarin 500 at 507 nm and the outputwas frequency doubled with a betabarium borate(BBO) crystal, supplied by CSK Co., Los Angeles.Betabarium borate is a relatively new material fornonlinear optical processes. It has a high damagethreshold, good thermal stability, and it allows effi-cient doubling down to -200 nm. It was possible togenerate pulse energies of up to -5 mJ at 254 nm froma dye laser pulse energy of 25 mJ at 507 nm. Thelinewidth of the frequency-doubled laser beam was0.001 nm. To tune the dye laser on and off resonance adual-wavelength feature supplied by Quantel was uti-lized. In the dye laser oscillator the rear end of thecavity is split into two parts. The two parallel beamsare, after reflection from the single grating, tuned bytwo individual mirrors. The two cavities are tunedtogether by the laser wavelength tuning mechanismand the wavelength of the second cavity can be offsetrelative to that of the first one. A small chopper thatblocks the two cavities alternately as it rotates ismounted in the intracavity space and a computer-controlled stepper motor rotates the chopper, thusalternately allowing the laser to oscillate on one of thetwo wavelengths. This is advantageous comparedwith moving the tuning mechanism of the laser at 5 Hz,which can cause undesirable vibrations and wave-length instability.</p><p>To calibrate the dye laser wavelength 10% of theoutgoing beam is split off with a beam splitter anddirected into a calibration unit. After passagethrough neutral-density filters it is split by a 50% beamsplitter, and one part of it is passed through a thinquartz cell containing a drop of mercury in air whilethe other part is used as a reference beam. The inten-sities of the two beams are measured using photodi-odes. The signals are A-D converted in the transientrecorder, which is run as a boxcar averager by thecomputer. Measurements of the mercury differentialabsorption cross section were performed with the sameunit.</p><p>The setup shown in Fig. 1 was used for the laboratorymeasurements, where the oxygen absorption spectrumaround the mercury line was studied. The laser sys-tem and the data acquisition system in the mobile lidarsystem were used. The laser beam was directed fromthe truck, which is docked to the laboratory when it isnot used in a field campaign, to an optical table where amirror directed it through a lens and a diaphragm to,the input window of a White multipass cell that wasconstructed in our laboratory. 2 7 The cell length is 2 m</p><p>922 APPLIED OPTICS / Vol. 28, No. 5 / 1 March 1989</p></li><li><p>and its body is a Pyrex tube with end flanges of stain-less steel that, together with Invar rods, make up theframe of the device. The mirrors are Al and MgF2coated for maximum UV reflectance. Path lengths upto 200 m with lamps and 500 m with lasers have beenachieved. A He-Ne laser was used to align the cell.After the cell, the beam passed through one more dia-phragm and a lens and reached the detector, and EMI9558 QA photomultiplier tube. The signal could beviewed directly on an oscilloscope, while to collect thedata the transient recorder and the computer wereutilized. To provide a marker of the mercury lineposition the part of the beam that was split off forcalibration was passed through a thick mercury cell inthe calibration unit and was detected by a photodiode.This signal was also digitized in the transient recorderand stored together with the spectra.Ill. Measurements</p><p>A. Laboratory MeasurementsLaboratory measurements were performed to estab-</p><p>lish the possible interference from oxygen absorptionin a measurement of mercury vapor in the atmosphere.The setup illustrated in Fig. 1 was utilized, where theWhite multipass cell was filled with 1 atm of pureoxygen. The path length through the cell was adjust-ed by observing the signal from the PMT at the output,either directly on an oscilloscope or after digitizationon the transient recorder. A read out from the latter isshown in Fig. 2. The sharp peak originates from thetransmitted laser light, while the signal at shorter de-lay times/distances is due to scattered light in sequen-tial reflections within the cell. The absorption wasmeasured by integrating the channels containing thesignal from the transmitted laser beam and normaliz-ing to a few channels for the first reflections. Thisconstituted an easy way of compensating for fluctuat-ing laser power during a wavelength scan, withoutinserting a beam splitter and an additional photode-tector. Thus, possible problems with fringes and dif-ferent characteristics of two photodetectors wereavoided. During an absorption measurement thenoise was lowered by using a small capacitor on thePMT, which distributed the signal due to the trans-mitted light over more channels in the transient re-corder. The path length used during the experimentswas 340 m.</p><p>Figure 3(a) shows the measured oxygen absorptionin a region close to the mercury resonance line. Arecorded Hg spectrum is inserted as a dotted line. Anenlargement of the Hg cell spectrum is shown in Fig.3(b). The comparatively broad structure of the Hgline is due to the different isotopic and hyperfine struc-ture lines present, as indicated in the figure.28 Thetypical linewidth of these lines at atmospheric pres-sure, together with the laser linewidth used, is alsoinserted. As can be seen from Fig. 3(a) two weakoxygen lines are very close to the mercury line. Thesetwo lines lie on the long-wavelength side of the v = 0 -v = 7 band of the Herzberg I system but can neither beidentified with Herzberg's listings nor with later list-</p><p>MOBILE LIDAR SYSTEM </p><p>_ m</p><p>LABORATORYOsilloscop</p><p>Fig. 1. Experimental arrangement.</p><p>z</p><p>0 200 400PATHLENGTH ()</p><p>Fig. 2. Temporally resolved White cell signal.</p><p>ings of the Herzberg I, II, and III systems, to ourknowledge.29-33 The interference from these lines in aDIAL measurement of low background values of Hgover long atmospheric paths can be minimized if thetwo wavelengths are chosen carefully. With the laserlinewidth of 0.001 nm used here, one wavelength can beplaced on the Hg absorption peak, while the referencewavelength must avoid the oxygen lines. In thepresent measurement we used a wavelength differenceof 0.008 nm with the reference on the long-wavelengthside.</p><p>The relevant Hg absorption cross section for thepresent system was established by measuring the ab-sorption in several small Hg cells of known sizes andtemperature. The results are plotted in Fig. 4, yield-ing a cross section of 3.3 X 10-18 m2 or 9.8 X 10-6 m2/ng.</p><p>B. Field MeasurementsDuring a one-week period the mobile lidar system</p><p>was used in a field measurement of Hg emission from achlorine-alkali plant, which uses mercury in the pro-cess. The system was placed -550 m from the largestHg emission source, which was the outlet from a 60-mlong clerestory 13 m above the ground. Measurements</p><p>1 March 1989 / Vol. 28, No. 5 / APPLIED OPTICS 923</p></li><li><p>0.75</p><p>z</p><p>0 .01...</p></li></ul>