differential optical absorption spectroscopy (doas) system for urban atmospheric pollution...

Differential optical absorption spectroscopy(DOAS) system for urban atmospheric pollutionmonitoring

Hans Edner, Par Ragnarson, Stefan Spannare, and Sune Svanberg

We describe a fully computer-controlled differential optical absorption spectroscopy system for atmo-spheric air pollution monitoring. A receiving optical telescope can sequentially tune in to light beamsfrom a number of distant high-pressure Xe lamp light sources to cover the area of a medium-sized city. Abeam-finding servosystem and automatic gain control permit unattended long-time monitoring. Usingan astronomical code, we can also search and track celestial sources. Selected wavelength regions arerapidly and repetitively swept by a monochromator to sensitively record the atmospheric absorptionspectrum while avoiding the detrimental effects of atmospheric turbulence. By computer fitting tostored laboratory spectra, we can evaluate the path-averaged concentration of a number of importantpollutants such as NO2, SO2, and 03. A measurement of NH3 and NO close to the UV limit is alsodemonstrated.

1. IntroductionWith the growing awareness of the serious environ-ment impact of industrial and automotive operationsthe need for the development of powerful measure-ment techniques for atmospheric air pollutants isincreasing. Optical remote sensing techniques thatuse lidar (light detection and ranging) and long-pathabsorption methodsl-3 are particularly advantageousbecause they permit large-area monitoring and avoidsample preparation difficulties characteristic of manypoint monitoring techniques. The differential ab-sorption lidar (DIAL) technique, which uses powerfulpulsed laser sources, permits a three-dimensionalmapping of the atmospheric pollutants, while cwlaser sources provide high spectral resolution andsensitivity in long-path absorption applications.Classical light sources combined with optical spectros-copy in differential optical absorption spectroscopy(DOAS) systems do not have the high performance oflaser-based systems but, on the other hand, they canprovide realistic, automated, and enduring measure-ment capabilities for multispecies monitoring, asdemonstrated in this paper.

The authors are with the Department of Physics, Lund Instituteof Technology, P.O. Box 118, S-221 00 Lund, Sweden.

Received 18 June 1991.0003-6935/93/030327-07$05.00/0.o 1993 Optical Society of America.

The DOAS technique was pioneered by Platt etal.- 6 A number of species have been measured (see,e.g., Ref. 6). Our own work with the DOAS tech-nique started in 1983, and includes the developmentof a specialized system for atmospheric Hg measure-ments7 and spectroscopic investigations for 03 (seeRef. 8) and NH3 (see Ref. 9) monitoring. The possi-bility of measuring aromatic hydrocarbons has re-cently been investigated. 10 Here we describe a DOASmonitoring system that permits automated monitor-ing of the air mass above a medium-sized city andwith some specialized research capabilities. Thehardware description of the multipath atmosphericspectroscopy system is given in Subsection 2.A, whilemeasurement routines and evaluation procedures aredescribed in Subsections 2.B and 2.C, respectively.Atmospheric air pollution spectroscopy performedwith the system and practical concentration evalua-tion are illustrated in Section 3 for NO2, SO2, NH3,and NO. Examples of data collected with the systemover a period of time are given in Section 4. Finally,the potential and limitations of the DOAS techniqueare discussed.

2. System Description

A. Hardware

A schematic diagram describing the DOAS system isshown in Fig. 1. The light source normally used forDOAS is a high-pressure Xe short-arc lamp. Thelamp is placed at the focus of a spherical mirror to

20 January 1993 / Vol. 32, No. 3 / APPLIED OPTICS 327

Fig. 1. Setup of the DOAS station.

obtain a well-collimated light beam. Maximum lumi-nance is located around the cathode, and decreasestowards the anode. The divergence of the beamtransmitted over long distances makes it difficult tocollect all light reflected by the mirror and, therefore,high maximum luminance rather than high power isrequired from the lamp. The Xe lamp has a broad,smooth emission spectrum except for some regionswith strong emission lines in the blue and near-infrared regions. The lifetime of these lamps is ofthe order of six months to one year, with a tendencytoward slightly more rapid intensity decay in the UVregion.

The vertically positioned Newtonian receiving tele-scope has a primary mirror of 30-cm diameter. Infront of this there is a large planar mirror, which canbe rotated horizontally and vertically by steppingmotors (0.47 and 0.63 mrad/step, respectively), en-abling the telescope to look in different directions.To improve the angular resolution of the telescopeorientation the secondary mirror is controlled bystepping motors. This unit has a resolution of 0.025mrad/step. The dispersive instrument is a Spex 500M spectrometer (f/4) with a 1200-groove/mm gratingwith blaze at 300 nm. A slotted disk scans thespectrum in 10 ms. The disk has 20 slits 100 gmwide and rotates at 5 revolutions/s. A mask in thefocal plane limits the scan to cover approximately 40nm. A trigger signal, generated when the slit passesan IR light barrier close to the edge of the mask, isused to synchronize the wavelength scale of eachindividual scan. The entrance slit of the spectrome-ter is normally 100 plm wide, which gives an optimumspectral resolution of 0.23 nm. The detector is anEMI 9558 QA photomultiplier tube (PMT) with a5-cm-diameter photocathode to capture the light pass-ing the scanning exit slit. After amplification thesignal is stored in a custom-made (MCA) multichan-nel analyzer plug-in card in an IBM-compatible ATcomputer. The MCA is equipped with a 12-bit ana-log-to-digital converter (ADC), which divides eachscan into 1024 channels and can add as many as60,000 scans before the result is transferred to thecomputer.

Currently the system is arranged with three mea-surement paths overlooking the city of Lund, Sweden(population 50,000). Three individual Xe lamps, A,B, and C, are placed 2000, 1600, and 400 m respec-tively, from the receiver, as shown in Fig. 2. Theoptimum path length varies for different species.A trade-off has to be made between higher sensitivityfor longer paths and higher transmission for shorterpaths, which are less likely to be blocked by fog. Thelight paths are located at a height of 10 to 30 m abovethe ground. The diameters of the lamp telescopemirrors are 30, 15, and 30 cm, and the lamps haveoutput powers of 500, 75, and 150 W. Normally, thelamp telescopes are sealed from the atmosphere by aquartz window for protection.

B. Measurement RoutinesIn a measurement sequence the telescope is firstturned to the specified light source and the grating isset for the appropriate wavelength region for the firstspecies to be measured. The voltage of the PMTtube is set to an empirical value, depending on thedistance to the light source as well as its spectralpower, the transmission efficiency of the atmosphere,and the spectroscopic equipment in this wavelengthregion. Because of thermal effects and backlash inthe stepping motor machinery, the viewing directionof the telescope is optimized by adjusting the second-ary mirror with stepping motors. The PMT voltageis then regulated in such a way that the signal fromthe interesting part of the spectrum corresponds to

75% of the maximum level for the ADC. Aftereach measurement meteorological data (wind velocityand direction) from a small weather station aremeasured and stored. While the MCA card is accu-mulating scans, the computer is free to evaluate theprevious measurement, print the results, and storethem in the computer on floppy disks. The resultsand all relevant data are stored in files in a formatsuitable for use in graphic presentation programs.After the measurement of a particular wavelengthregion is completed, a new region can be chosen forthe study of different pollutants. Other light sourcescan then be selected.

Several celestial objects can be searched and trackedby using an astronomical code for steering the tele-scope.11 12 The use of the sun, moon, and scatteredsky light, together with fixed light sources, permitmeasurements of both the tropospheric and strato-spheric concentrations of many species,13 -16 but thisoption is not demonstrated here.

C. EvaluationAfter a light source has been selected and dataacquisition has been performed according to Subsec-tion 2.B, the atmospheric spectrum is evaluatedaccording to the algorithms described by Platt andPerner.6 The routine starts by subtractingthe back-ground from the recorded spectrum. The triggersignal starts the measurement before the exit slitenters the unmasked area and continues to measure a

328 APPLIED OPTICS / Vol. 32, No. 3 / 20 January 1993

Fig. 2. Measurement paths overlooking Lund, Sweden. In the insshown.

short while after the slit is masked again. An aver-age of these parts of the scan is considered to be thebackground that is due mainly to PMT dark currentand offset in the preamplifier. A portion of thespectrum is selected, and a polynomial of degree 5 or 6is calculated to fit this part of the spectrum. Theatmospheric spectrum is then divided by the polyno-mial and normalized. The result is a high-passfiltering that enhances the differential structure andeliminates many instrument-specific features, suchas the broad structure in the lamp spectrum, as wellas differences in the spectroscopic transmission and

sets, photographs of the receiving station and the lamp transmitters are

detector sensitivity. Since the evaluation relies onthe Beer-Lambert law, a logarithmic transformationof the normalized spectrum is made. The meanconcentration of the species in the light path is thenevaluated by correlation with as many as six differentreference spectra recorded in the laboratory. Natu-rally the reference spectra must be recorded in thesame way as the atmospheric spectrum, using thesame width of the entrance slit, etc., and evaluatedwith the same parameters. The standard deviationof the difference between the atmospheric spectrumand the correlation spectra indicates the accuracy of


the measurement. Because of temperature effectsand backlash in the spectrometer grating drive, theatmospheric spectrum may be offset a few channelswith respect to the laboratory spectrum, and thecorrelation routine, therefore, optimizes the correla-tion by shifting the atmospheric spectrum within±20 channels. The offset value and the correlationcoefficient are printed and stored and can be used asan indication of the correctness of the correlatedvalues. The integrated intensity value of the rawspectrum is also printed and stored and this indicateswhether the measurement has been performed undersuitable conditions. A routine of rejecting data frommeasurements during low-light-level conditions canbe implemented. All details of the evaluation are setin an evaluation sequence menu and an evaluationparameter menu that can be stored in the computer.In this way, we have created a directory of predefinedmeasurement cycles.

3. Atmospheric Air Pollution Spectroscopy

In Fig. 3 the evaluation of a spectrum, according tothe routines described in Subsection 2.C, is shown.Figure 3(a) shows a raw spectrum obtained by adding30,000 scans made over 5min. The sharp edges atboth ends of the spectrum are due to the maskingprocedure described above. In Fig. 3(a) some sharpfeatures from the Xe lamp are evident in the spectrum.NO2 has the strongest absorption bands in a regionwhere the Xe lamp has strong emission lines. Therelative intensity in these lamp structures is timedependent because of the aging of the lamp, so theregion of evaluation that is free from these featuresmust be chosen. This region, marked in Fig. 3(a), isshown in Fig. 3(b) together with the fitted polynomial.The differential structure is enhanced by normaliza-tion to the polynomial (dotted curve), and the finalresult after logarithmic transformation is shown inFig. 3(c) together with the reference spectrum (dottedcurve) to give the best fit; 15.2-tig/M3 NO2 over the2000-m atmospheric path. Figure 3(d) shows theresidual spectrum when the reference spectrum hasbeen subtracted from the atmospheric spectrum.The larger structures in the residual spectrum mightbe related to other species present in the atmosphereor due to artifacts from the normalization to thepolynomial fit. The noise in the spectrum indicatesa minimum detectable absorption of 5 x 10-4 for thesystem, which, for the 2-km path, results in a detec-tion limit of - 0.4 [ig/m 3 .

Figure 4 shows an example of a multispecies mea-surement at short UV wavelengths. An additionalmeasurement path of 350-m was used, and the receiv-ing telescope was optimized for the short UV region.In Fig. 4(a) the atmospheric spectrum (solid curve) isshown together with a reference spectrum of SO 2scaled to represent 5.0 ,ug/m3 over the 350-m path(dotted curve). The atmospheric spectrum after sub-traction of the SO 2 reference spectrum is shown inFig. 4(b) together with an NH 3 reference spectrum of0.7 g/M 3. The residual after subtraction of the

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NH3 reference is shown in Fig. 4(c) together with thereference spectrum of NO scaled to equal 2.8 pg/M3.The final residual is shown in Fig. 4(d). As in themeasurement presented in Fig. 3, the larger struc-tures might be the result of the evaluation procedure.The noise in this measurement is, of course, greaterbecause of the smaller light flux in this wavelengthregion. Multispecies spectra are normally evaluatedwith a simultaneous correlation of the referencespectra, as described in Subsection 2.C. Figure 4shows the contribution from the separate species, anda sequential evaluation can be used if the correlationbetween the different reference spectra is small.

4. Examples of ResultsData for NO2 in three directions recorded over 24 hare shown in Fig. 5. The influence of the morning


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Fig. 6. NO2 and 03 correlations measured over 24 h (400-m pathin Fig. 2).

traffic can be clearly seen. Figure 6 shows theconcentration of 03 and NO2 for one day. In themorning the NO2 concentration increases stronglybecause of traffic. The anticorrelation of the 03 andNO2 concentrations due to atmospheric chemistry isclearly seen. NO2 data for one week are shown inFig. 7. Diurnal variations related to traffic are seen.It should be noted that the period covers the Easterholidays, 1990, with 13 April (Friday) being the firstfree day and 17 April (Tuesday) being, again, aregular working day. Figure 8 shows the averageconcentration of SO 2 as a function of wind direction,measured between 29 May and 14 July 1990. Theimpact of winds from the south and southeast isclearly indicated.

5. DiscussionDetection techniques based on optical spectroscopyhave many advantages compared with conventionalwet chemistry instrumentation. Primarily, a meanvalue over a long distance is often more representa-tive than a value from a point monitor. A large areacan be monitored from one station, as shown in thisstudy. Optical instruments can often provide rapidmeasurements that give values with high temporalresolution. A further advantage is the low fre-quency of calibration necessary. Spectroscopic in-struments are also easy to computerize and DOAS

70 NO2 CONCENTRATION

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Fig. 7. NO2 data for one week (2000-m path in Fig. 2).


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SFig. 8. Average SO2 concentrations as a function of the winddirection, 29 May to 14 July 1990 (2000-m path).

systems can run unattended for long periods, commu-nicating with the operator through a modem on thetelephone network for automated data reports orreprogramming. An alternative approach to the onedescribed here is to replace the lamp telescopes withretroreflectors and to construct the transmittinglamp and receiving telescope as one unit.' 7 Thepractical and economical benefits of replacing activelamp telescopes with passive retroreflectors encour-age the use of a larger number of retroreflectors,which would ensure optimum path lengths for allmeteorological conditions. Rain does normally notaffect the measurements if the windows are shielded,but fog can block the light if the path is long. Hazecan occasionally scatter sunlight into the receivingtelescope but this effect can be compensated for in theretroreflector concept; the lamp is blocked while thescattered sunlight is measured and this signal canthen be subtracted from the measurements. Theatmospheric turbulence makes it necessary to mea-sure all wavelengths simultaneously or to freeze theatmosphere by measuring the spectrum quicklyenough.18 The first approach is implemented withphotodiode or CCD arrays, and has the big advantageof good light economy. Disadvantages are the sensi-tivity variations between different pixels and alsowithin each pixel. These variations can change intime, and frequent calibration is, therefore, oftennecessary. Array detectors can also suffer from ta-lon effects originating from surface coatings and fromsubstantial readout noise. To reduce detector noise,array detectors are normally cooled and thereforeflushed with dry air. All this could, in principle, behandled, and these devices are used in many scientificinstruments. 92 0 Historically, array detectors havebeen expensive, delicate, and not appropriate for

unattended long-time measurements; then the slot-ted disk was developed to tackle the turbulenceproblem the other way. Here the exit slit in thespectrometer has been replaced by a rotating slotteddisk. The disadvantage of this concept is the poorlight economy and the fact that the exit slit is verticalin only the middle of the scan. The spectral resolu-tion then varies along the spectrum, in our case from0.23 nm in the middle of the spectrum to 0.53 nm atboth ends. This also means that the spectral resolu-tion changes with a vertical translation of the spec-trum in the focal plane. The practical advantageswith rapid scanning devices still make them competi-tive in the development of new systems. 21 22

The authors gratefully acknowledge valuable helpfrom B. Galle and H. Axelsson of the Swedish Environ-mental Research Institute, Goteborg, and the loan ofa xenon light source from OPSIS AB. This work wassupported by the Swedish Environmental ProtectionBoard (SNV) within the EUROTRAC framework,subproject TOPAS, and by the Swedish Board forSpace Activities (DFR).

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