Long-path differential optical absorption spectros-copy (DOAS) systems have been employed since themid-1970s for measuring tropospheric trace gases.1 Aclassical long-path DOAS instrument uses one New-ton telescope to emit a parallel light beam from anartificial light source (usually a Xe-arc lamp), whichis collected at a distance of several hundred meters bya second Newton telescope and dispersed by aspectrometer.2–5 The collected light carries the ab-sorption structures of trace gases that are presentalong the light path; these structures can be detectedwith high sensitivity. Usually optical densities of theorder of 10�3–10�2 are detected. The spectral absorp-tions of several trace gases are measured simulta-neously, and column densities corresponding toconcentrations averaged along the light path are de-rived with DOAS analysis algorithms.
A variety of trace gases have been measured by thistechnique, and even long-term measurements have
been performed (e.g., Ref. 6). Today there are severalcompanies around the world that construct theselong-path DOAS devices (e.g., Hoffmann Messtech-nik GmbH).
The most important species measured in the visiblerange �400–700 nm� are NO2, IO, O4, H2O, and NO3.In the near UV �300–400 nm� the focus is on NO2,SO2, O3, H2CO, HONO, and BrO. The advantage ofusing artificial light sources—in contrast to the solaror passive DOAS technique—is that measurementscan also be performed at shorter wavelengths. Hencearomatic compounds such as benzene and toluene aremeasured between 240 and 290 nm, and even OHradicals are measured at 308 nm with a frequency-doubled red dye laser as the light source.7 The re-quired path lengths depend on location as well as onthe typical concentrations of the species. High aerosolcontents have negative effects on the measurement,first by weakening the light tremendously by extinc-tion processes and second by scattering solar lightinto the telescope. Therefore shorter light paths ofapproximately 300–1000 nm may be chosen, whichcan be sufficient for highly polluted areas and tracegases such as NO2, SO2, O3, H2CO, and HONO. Forthe trace gases studied mostly in remote areas(namely, IO, NO3, and BrO), usually very longpaths—as long as 10 km—are required.
During many measurement campaigns, long-pathDOAS instruments have been used to study thechemistry in remote areas as well as in urban areas.
The authors are with the Institute of Environmental Physics,University of Heidelberg, Germany, Im Neuenheimer Feld 229,69120 Heidelberg, Germany. I. Pundt’s e-mail address is [email protected].
Received 29 July 2004; revised manuscript received 7 January2004; accepted 14 February 2005.
Intercomparisons have been performed to improvethe measurement technique (e.g., Ref. 8). However,single-path integrating (as well as single-point) mea-surements are not sufficient if small-scale variationsof emissions, chemistry, or transport become impor-tant. Long-path DOAS tomography measurementswhen multiple light paths are used, which provideconcentration maps, may close this gap.
A. Coaxial Telescope and Retroreflector Technique
Figure 1 shows the design of the classical long-pathDOAS coaxial Newton telescope designed by Axels-son et al.9 This device uses one coaxial unit of a trans-mitting and receiving telescope. The principle is asfollows: From the artificial light source a light beamis directed toward corner-cube retroreflectors, whichare located at a distance of 200–10,000 m from thesource. The advantage of the reflectors is that theysend back light exactly in the direction of incidence;therefore they partly eliminate the effect of atmo-spheric turbulence on the light beam. After travelingback along this path, the light is collected by the sametelescope and fed into a spectrograph.
In detail, an outer ring of the telescope’s concavemain mirror is used for the emitting light beam,which is coming from the light source through thein-coupling plane mirror. Near the telescope the crosssection of the emitted beam resembles a ring; butafter the beam travels a short distance, its center isilluminated as well because of light divergence andatmospheric turbulence. In addition the retroreflec-tor array mirrors the light parallel to the originallight beam with a lateral offset; therefore a fraction ofthe returning light reaches the inner part of the tele-scope’s main mirror. The reflected light coming fromthe inner part of the mirror is then reflected by anoutcoupling plane mirror onto an optical fiber or di-rectly onto the entrance slit of a spectrometer.
B. Long-Path DOAS Tomography
Long-path DOAS tomography combines multipleDOAS measurements with tomographic inversiontechniques.10,11 The basic setup consists of a combi-nation of several coaxial telescopes and multiple re-troreflectors, building up together 10–30 or evenmore light paths. Overlapping10 or nonoverlapping12
light beams can be used to probe the concentrationfield from different directions. Using tomographic in-
version techniques, one can derive two- or three-dimensional trace gas distributions.10–13
The first tomographic arrangement was setup10,11 in the framework of the motorway emissioncampaign BAB II in April–May 2001.14 By usingtwo conventional coaxial Newton telescopes andeight retroreflector arrays, 16 light paths were set up.Two-dimensional maps of the NO2 emission plumeperpendicular to the motorway were gained. How-ever, the DOAS telescopes scanned the retro-reflectors successively; the number of spectra andinformation was therefore limited. The time resolu-tion can be enhanced by at least a factor of 10–100, ifthe light paths are probed simultaneously, becausenot only the recording time of the spectra but also thetime for the stepper motor movement from one ret-roreflector to the next and the optical adjustment ofthe instrument are time consuming.
2. Multibeam Instrument
The multibeam instrument is a modification of thesystem described in Subsection 1.A. The aim is thesimultaneous emission and collection of several lightbeams. Figure 2 shows the complete setup.
Telescope A holds one main mirror of 1500 mmfocal length and 300 mm diameter. The main changeis done inside the lamp housing, B. Several additionalsmall mirrors are positioned inside the lamp housing,creating four virtual light sources in the focal plane ofthe main mirror. Each virtual light source emits onelight beam into one specific direction, depending onthe position and the orientation of the lamp housingmirror. The tower, G, in front of the telescope holdsfour large plane mirrors (at a distance of approxi-mately 15 m, mirror size, 360 mm � 320 mm), theso-called tower mirrors. These mirrors redirect thelight beams in the desired directions, for example, tothe retroreflector arrays, I (only one array is shown),at different distances to the telescope or to a rotating
Fig. 1. Coaxial Newton telescope design of Axelsson et al.,9 fre-quently employed nowadays, is the basis of the multibeam tele-scope (A, main mirror; B, outcoupling mirror; C, incouplingmirror).
Fig. 2. Instrumental setup of the entire multibeam system (in-cluding, A, telescope; B, lamp housing; C, mode mixer; D, spectro-graph; E, CCD detector; F, stepper motor controller; G, mirrortower with four mirror units; H, rotating disk retroreflector; I,retroreflector array at a distance between 500 and 5000 m). Thedistance between the telescope and the mirror tower is usually10–15 m. The distance between the rotating retroreflector and thetower is also 10–15 m. The rotating disk can be placed close to thetelescope.
corner-cube–retroreflector array, H. Each tower mir-ror can be turned around two axes by using two step-per motors. These mirrors can be turned �30° in thehorizontal direction and �15° in the vertical direc-tion. After reflection at the retroreflectors the light isredirected through the tower mirrors to the telescope.The received light is focused on the entrance of thequartz fibers, which pass a mode mixer, C.15 The lightfrom the different light beams is then transmittedthrough these fibers to an imaging spectrograph, D,and is spectrally analyzed on a two-dimensional CCDarray, E.
Figure 3(a) is a photograph taken from the front ofthe telescope. Two virtual light sources can be seen.Figures 3(b) and 3(c) show the mirror tower holdingfour mirrors as well as the retroreflector disk.
A. Telescope
The emitting and receiving parts of the telescope areturned to each other by 90° (Fig. 4, upper panel). Asan example, Fig. 4, middle panel, shows a sketch fortwo emitted light beams resulting from mirrors L1and L2 inside the lamp housing. The two plane mir-rors, B and C, inside the telescope have the samefunction as those described in Subsection 1.A. Their
size is slightly enlarged to capture the light from allvirtual light sources. Because of the shadow effects ofmirrors B and C, any two different outgoing (withrespect to the direction of emission) and incominglight beams have cross-sectional shapes of differentintensity. For long light paths the differences in theoutgoing beam shapes are compensated by atmo-spheric turbulence.
Figure 4, lower panel, shows two light beams re-flected from different retroreflectors. The light is cap-tured by the different inner parts of the telescopemain mirror. After reflection on mirrors B and D thelight beams are focused on two optical fibers, whichare placed in the focal plane of the collecting part ofthe telescope.
B. Spectrometer and Detector Unit
The collected light is transmitted through optical fi-bers through a mode mixer used to reduce the resid-ual structures on the detector.15 The light thenreaches the entrance slit of a Czerny–Turner spec-trometer with enhanced imaging quality (Acton Spec-tra Pro 500i). The focal length of the spectrograph is500 mm; its aperture is 1:6.5. The dispersion of thegrating �600 grooves�mm� used during the field mea-surements (Section 4) is 3.366 nm�mm correspond-ing to 0.04377 nm�pixel for the employed CCD. Thefibers from the different light paths are aligned alongthe entrance slit with sufficient space between eachfiber. To separate the different spectra, a two-dimensional CCD (Roper Scientific, Spec-10:2kBUV)is used as a detector. The light is dispersed in the xdirection, and the different spectra are separated inthe y direction (with each spectrum covering approx-imately 14–17 lines of the CCD array). Hence it ispossible to measure the different spectra simulta-neously. An example of a two-dimensional image onthe CCD is displayed in Fig. 2. The instrumentaltransmission function is similar for different lightpaths but shifted slightly by wavelength.
C. Shortcut System and Lamp-Reference Spectrum
For DOAS measurements, usually a lamp-referenceor correction spectrum is required, which accounts forthe shape of the lamp spectrum and the optical struc-tures caused by the instrument. The reference spec-trum is taken with a specific shortcut optic, whichguides the light directly from the lamp to the en-trance of the optical fiber respecting the entranceangles of the fiber and the spectrometer. Differentalternatives for the setup of this optic are currently inuse (e.g., Ref. 15). However, for the multibeam instru-ment this kind of configuration requires one specificoptic for each optical fiber, which would demand agreat technical effort. With the mirror tower placed infront of the telescope, the problem can be solved: Forthe shortcut spectrum the light beams are directedtoward the tower mirrors (in exactly the same way asfor the long-path spectrum) but then reflected to-gether onto one special retroreflector array. This ar-ray consists of three retroreflectors, which aremounted on a rotating disk of 300 mm diameter in
Fig. 3. (a) View from the front of the telescope. Two virtual lightsources can be seen on the main mirror of the telescope. (b) Mirrortower with four mirror units. (c) Retroreflector disk used for theshortcut system.
Fig. 4. Optical principle of the multibeam telescope. In contrast tothe conventional setup, additional small mirrors are placed insidethe lamp housing (L1, L2). They generate several virtual lamps onthe focal plane of the main concave mirror, allowing the emissionof multiple light beams. The light path for two light beams isshown: middle panel, emission part of the telescope (plane x–y);lower panel, receiving part of the telescope (plane y–z).
order to laterally distribute the reflecting surface ashomogeneously as possible. The frequency of the ro-tation is 1380 turns�min. Figure 3(c) depicts a short-cut retroarrangement. To measure a shortcutspectrum, the tower mirrors can be turned automat-ically in the direction of the shortcut retroarray. Theoptics, which are traversed by different light beams,are equivalent to those of the long path.
D. Stepper Motors and Computer Control
The measurement process and data storage are com-pletely controlled by a PC. For automation a newsoftware, named Camera Operation Software, wasdeveloped. The software controls the amplitude of thesignal by optimizing the integration time per scanand readjusts the mirrors to direct the light beams tothe different retroreflector arrays. Together the fol-lowing motors are used for the multibeam system:
Y two stepper motors mounted at each tower mir-ror to redirect the light beams in both the horizontaland the vertical direction toward the different retro-reflector arrays or the shortcut retroarray;
Y one stepper motor in front of the optical fiberthat carries a filter in order to remove light at other(longer or shorter) wavelengths, thus reducing straylight in the spectrograph;
Y one motor for a baffle in front of the lamp torecord background spectra.
3. Instrumental Characteristics
Several effects that have consequences on the qualityof the spectra may result from the specific arrange-ment of the multibeam instrument. In Table 1 theseeffects are listed. They are described and quantifiedin the following subsections.
A. Telescope Light Transmission
As pointed out above, light transmission through theinstrument is reduced because of shadow effects andthe limited surface of mirrors B and C inside thetelescope. Therefore the emitted light intensity as a
function of beam direction is an interesting charac-teristic of the instrument. The intensity, dependingon the direction of the light beams, has been calcu-lated.
The light source emits a photon flux �0 (photons�s),which can be assumed to be homogeneous for thedirections of interest. Therefore the photons flux�1��, �� at the main mirror can be written as
�1(�, �) � �0
14�
AE(�, �)
f 2 , (1)
where � is the azimuth angle (the angle in the hori-zontal direction away from the optical axis) and � isthe elevation angle (the angle in the vertical directionaway from the optical axis) of the emitted light beam.AE is the area illuminated on the main mirror, whichdepends on the position of the virtual lamp corre-sponding to a specific direction of the emitted lightbeam; f is the focal length of the telescope.
For the received beam, independent of the beamdirection, a constant transmission � through the at-mosphere including the reflection at the retroreflec-tors is assumed. The intensity distribution ispresumed to be homogeneous, and the reflectivity Rof the mirrors is the same for different beam direc-tions. The received photon flux �2 is given by
�2(�, �) � �0
14�
AE(�, �)
f 2 AR(�, �)
�r2 R7, (2)
where AR is the illuminated area of the main mirrorof the received light beam and r is the radius of thereflected light beam entering the telescope.
The fluxes of the different light beams are dividedby those of the center beam corresponding to a virtuallight source located on the focal point of the mainmirror ��, � � 0�. The relative intensity quotient Qthen depends only on areas AE and AR, which havebeen calculated by ray tracing:
Table 1. Important Contributions to the Residual Structure and Their Resulting Detection Limits
Note: Important contributions to the residual structures owing to the different instrumental components of the ensemble (the multibeamsystem, spectrometer, and detector) and the 2 detection limit resulting from structures for measurements in the wavelength regionbetween 295 and 365 nm and a total light path of 5 km. For the calculations, spectra containing 100 coadded scans and seven coadded lineson the CCD chip, saturated by 70%, were used.
aParts in 109 by volume.bWhite noise used to calculate the detection limit.cDepending on the intensity of the considered light beam compared with the other light beams (see also Subsection 3.B.4).dWhite noise, smoothed to resemble the residuum structure, is used to calculate the detection limit; see also Subsection 3.B.3.eStructures from the lamp, the grating of the spectrometer, and the detector sensitivity.
In Fig. 5 the relative intensity as a function of theelevation and azimuth angles is shown. Further neg-ative azimuth angles as shown are not possible be-cause the light beams are blocked by the lamp. Therelative intensity is not symmetric with respect to theazimuth angle, because in contrast to the single-beam telescope shown in Fig. 1 the center of mirror Cis not placed in the optical axis of the telescope. It canbe seen that the transmission is best for atmosphericmeasurements between a �0.4° and 1° azimuth an-gle and between a �1.6° and 1.6° elevation angle.
However, for horizontal and vertical separation an-gles of 2° the light intensity is still sufficient (e.g.,four light beams through the edges of the squarelocated between �0.3° and �1.7° in the horizontaldirection and between �1° and �1° in the verticaldirection). In this case the distance between the tele-scope main mirror and the tower mirrors has to be atleast 12 m in order to separate the two light beamsfrom each other. (The width of the tower mirrormounts is 0.5 m.) For larger distances the separationangles can be decreased; therefore the light intensityincreases. Based on the size of the tower mirrormounts and the angle between the different lightbeams, the tower mirrors are usually placed 12–20 mfrom the telescope.
B. Spectral Structures and Offsets
Small structures from the lamp source, the optics andelectronics of the system, contribute to the measure-ment spectra. Since DOAS is intended to observeoptical densities down to less than 10�3, it is neces-sary to avoid these structures or correct for themduring the analysis procedure. Some structureschange with time; therefore correction spectra haveto be taken at regular intervals during the measure-ment cycles. At the end of the analysis procedure the
remaining so-called residual structures are mostlyresponsible for the detection limit of the measure-ments.
In this section the different sources for these struc-tures are discussed and quantified. Some sources arenot attributed to the multibeam parts of the instru-ment but to the characteristics of the detector. How-ever, they have to be characterized in order to assessthe measurement capabilities of the whole ensemble.
1. Random Noise (Electronics, Photons)The random noise in the spectra consists of two parts:first, the electronic noise due to the resolution of thedigitalization, i.e., the noise of the analog-to-digitalconverter and other electronic components; second,the photon noise that is the square root of the numberof photons according to the Poisson statistical theory.For a DOAS measurement it is important to know theamount of noise of each component in order to decidewhether the spectra should be saturated: If the pho-ton noise dominates, only the total number of countsplays a role in the reduction of the relative noise-to-signal ratio. If the electronic noise dominates, thespectra should be as saturated as possible (by 70% ifpossible; if more, there is risk of overflow, whichshould be avoided), because the amount of electronicnoise is similar for each spectrum (scan). Hence therelative noise-to-signal ratio decreases for an en-larged number of photoelectrons per spectrum. In ourcase the light from different light beams is measuredsimultaneously on the same CCD. Facing the differ-ent absolute intensities of the different light beams,the question above is even more important.
The spectral noise has been derived from a series ofspectra taken with different illuminations. The CCDparameter of gain was set to 2, and the binning modewas set to 4 pixels. The electronic noise was found tobe less than 4 counts�scan pixel (one sigma). Thetypical signal is 30,000–45,000 counts�pixel.
Adding up both noise sources, we get a total noise-to-signal ratio of approximately 4.8 � 10�3 for onepixel with 30% saturation or approximately 3� 10�3 for one spectrum with 70% saturation.
2. Electronic Offset and Dark CurrentThe electronic signals resulting from the CCD chipare overlaid as additive signal to each spectrum andtherefore have to be subtracted. The dark currentdepends on the temperature of the CCD; it is lessthan 1 count�s at �60 °C. For typical intensities of200,000 counts�s this signal plays a minor role. Theoffset level for one scan is 100–150 counts�pixel,which is approximately 3‰ of the measured signal(30,000–45,000 counts�pixel if 4 pixels are binned).Structures resulting from an incomplete removal ofthe offset signal can be neglected.
3. Scattered Light from Other Light Beams in theTelescopeLight from one beam could be scattered into the op-tical fiber belonging to another light beam. This effecthas been investigated by directing one light beam, A,
Fig. 5. Calculated relative intensity as a function of the elevationand azimuth angles. (The elevation is defined as the vertical angleabove the horizontal of the optical axis and the azimuth as theangle in the horizontal direction away from the optical axis.)
toward a retroreflector array located at a distance of500 m. A second beam, B, was also emitted but notreflected by any retroreflector array. Two fibers, A=and B=, were placed at the position in the focal planeof the receiving telescope where the two light beamswould be focused.
As a result the signal for any pixel on the CCD usedto record a spectrum with fiber B= was less than1 count�s, whereas the average signal of the otherbeam was 30,000 counts�s. This measurement wasrepeated for different positions of the two fibers. Thestray light�intensity ratio was found to be less than3 � 10�5 and therefore negligible, even if one fiberreceives only 10% of the intensity of three other fi-bers.
4. Spectrometer Stray Light and Limited ImagingQualityAnother possible error source is the limited imagingquality of the spectrometer and the stray light insidethe spectrometer. Figure 6 shows an image recordedwith the CCD chip. The four white strips are illumi-nated directly by the light from four different fibers.The areas between these strips are not directly illu-minated; however, light can be detected on them.
It was found that in the region of the chip used fordata evaluation the signal is 0.3% of the light inten-sity of the neighboring light beams. On the left- andthe right-side border of the chip the relative lightintensity increases to as much as 0.7%. For simulta-neous measurements with four illuminated lightbeams the stray light is 1% in the region of interest,but, if one fiber receives only 10% of the intensity ofthe other fibers, the relative stray light is ten timeshigher (10%), which is too much. To avoid this situ-ation, the intensities of the fibers should be kept sim-ilar. An automatic adjustment routine for all lightbeams is essential for the measurement. If the lightintensity of one beam is lower than 50% of any otherbeam, even after optimum adjustment, the strongersignal needs to be decreased. This can be donethrough a slight misalignment of the stronger lightbeam under the use of the respective rotatable towermirror.
Assuming a relative stray light offset of 1% and aone sigma structure of 3% inside the offset (resultingfrom the absorption structures present in the neigh-boring light beams), this results in a multiplicativeone sigma structure of 1% � 3% � 0.3‰. In addition
a constant intensity offset of 1% also leads to anunderestimation of the species concentrations by 1%,which should not be neglected.
5. Angle of Incidence Dependencies of the TowerMirror StructuresThe angle of incidence dependencies of the tower mir-ror structures were measured with a halogen lamp asthe light source. For different angles of incidenceshortcut spectra of the same light beam were re-corded. The spectra were then divided by one an-other, but no structures related to the mirror positioncould be observed.
6. Optical StructuresIn this subsection four different optical effects from(a) the grating of the spectrometer, (b) the lamp, (c)the detector sensitivity, and (d) the incomplete re-moval of other trace-gas absorptions are consideredtogether.
(a) The response function of the spectral gratingdepends on the homogeneity of its illumination. Notonly the spectral resolution but also the reflectivity atdifferent wavelengths varies to some extent if differ-ent parts of the grating are illuminated. Dividing twospectra, which have been obtained at different times,may therefore lead to multiplicative high-frequencystructures, which can add up to 10% of the lightintensity (peak to peak) for a very inhomogeneousillumination of the grating. Also the division of thelong-path by the short-path spectra leads to struc-tures, even if measured nearby, because the path-ways through the instruments are not exactly thesame; therefore the grating is illuminated differentlyfor the two paths.
(b) In addition to this effect there are structuresresulting from incomplete removal of the lamp spec-trum. The long atmospheric light path and the shortreference light path differ slightly from each other;therefore the image of the lamp on the optical fiber isslightly different for the two spectra. Imaging a smallarea of the lamp arc in one case and a larger area inthe second case leads to different lamp structuresholding different strengths of the xenon emissionlines of the light source.
(c) The detector sensitivity is slightly differentfrom one pixel to the next. The resulting structureshould in theory cancel out when the short path isused. However, small particles on the protecting sur-face can lead to shadow effects on the detector, whichalso differ if the light comes from different areas ofthe spectrometer grating.
(d) The DOAS measurement technique consists ofthe measurement of several trace-gas absorptions inthe same wavelength range. Because of interferencebetween the various absorption structures, the in-complete removal of one trace-gas absorption canlead to structures remaining in the residual of the fit.
It is difficult to distinguish these four componentsfrom one another. In general, they are responsible for
Fig. 6. Spectra from four light beams on the two-dimensionalCCD array. The dispersion direction is in the horizontal. For eachfiber ending at the entrance slit a complete and separate spectrumis generated on the CCD.
the remaining residual structures. They were there-fore estimated from the atmospheric measurements,subtracting the effects from the other sources. Theone sigma signal of the optical structure was found tovary from 0.01% to 0.1% of the intensity.
C. Detection Limits
The main characteristics of the instrument and theimplication for the detection limits of different tracegases are summarized in Table 1. As an example, thedetection limit was calculated for trace gases NO2,SO2, O3, and H2CO. A typical light path of 5 km waschosen, and 100 spectra were coadded, correspondingto a total integration time of typically 1–1.5 min (or45 s in the best case). Therefore a large amount(2000–3000) of artificial random noise spectra wasgenerated, and the trace-gas absorption cross sec-tions were fitted to those spectra when the DOASalgorithms described in Section 4 were applied. Thisprocedure was applied to random noise sources suchas electronic and photonic noise. For the other errorsources (scattered light, optical structures) the ran-dom noise spectra were slightly smoothed (when atriangular convolution, which adds up ½ of the pixelvalue with ¼ of the two neighbor pixel values, wasapplied 20 times) in order to resemble the respectivestructures.
It can be seen that the largest error source is givenby the optical structures followed by the scatteredlight and the photon noise. The derived detectionlimits of the trace-gas concentrations are similar tothose of the conventional long-path system: 0.2–1parts in 109 by volume (ppbv) for NO2, 0.07–0.33 ppbfor SO2, 2–11 ppbv for O3, 0.3–1.5 ppbv for H2CO.Note that the error due to the optical structures de-creases if longer light paths are used. On the otherhand, for longer light paths the errors due to therandom noise increase because of intensity loss.
4. Atmospheric Measurements
First atmospheric measurements with the entire sys-tem were performed at the Bresso airport in north-west Milan, Italy, in the framework of the secondEuropean FORMAT (formaldehyde as a tracer of pho-tooxidation in the troposphere) campaign (e.g., Ref.16). During this campaign, three to four light beamswere used simultaneously. A computer-controlled au-tomatic measurement routine was used that includedthe assessment of one atmospheric spectrum and onebackground spectrum by turns. After every 10 spec-tra the telescope adjustment was checked and, ifnecessary, optimized. The routine adjusted the ex-ternal mirrors so that all light beams provided thesame signal on the CCD with a minimum of10,000 counts�s. Lamp reference spectra were takenevery 12–24 h.
A. Stability of the Optical Alignment
The stability of the optical alignment of the telescopedepends mainly on the mechanical stability of theexternal mirrors and the distance between these mir-rors and the retroreflector array.
During the first test measurements in Heidelberg,when two wind-protected tower mirrors were used todirect one light beam to a retroreflector at a distanceof 2 km (one way), automated measurements wereachieved for more than 24 h. Under these conditionsan automatic readjustment of the light beam tookless then 30 s for misalignment of the light beam to asgreat as 500 rad.
During the field measurements, complete windprotection of the mirror tower could not be achieved;therefore the stability of the optical alignment de-creased more rapidly with time. Under usual windconditions (wind speed, �5 m�s) and a light path aslong as 1.5 km (one way) the system operated auto-matically for 4–12 h. The total automatic adjustmentduring that time was performed within 1 min for fourlight beams after every ten spectra. However, after-ward the adjustment had to be done by hand. Betterwind protection and stability of the mirror tower leadto increased stability for the future.
B. DOAS Analysis
The spectra were analyzed by using the DOAS anal-ysis software MFC.17 Absorption cross sections ofNO2, SO2, O3, H2CO, HONO, and O4 were taken fromthe literature and adapted to the resolution of thespectrometer by convolution with the spectrometer’sslit function measured with a mercury lamp. The slitfunctions derived for the different light beams couldvary slightly from each other and must be considered.
The references for the absorption cross sections arelisted in Table 2. After division by a lamp-referencespectrum, which was recorded every 12–24 h, the at-mospheric spectra were high-pass filtered. The samefilter was applied to the absorption cross sections.Then the absorption cross sections, a high-pass-filtered lamp-correction spectrum, a high-pass-filtered solar Fraunhofer-correction spectrum, alamp-correction spectrum, and a polynomial of fifthdegree were fitted together in a wavelength rangebetween 295 and 365 nm. It was found that both ahigh-pass filter and polynomial fifth degree are nec-essary to eliminate the structures resulting from theincomplete removal of the lamp structure with thelamp-reference spectra. Figure 7 shows the results ofthe analysis for a spectrum taken on 7 October 2003.
Table 2. Specifications and References for the Trace-Gas AbsorptionCross Sections Used in This Study
Figure 8 shows the NO2 average mixing ratio resultsof an intercalibration measurement performed withthree light beams directed to the same retroreflectorarray for 18 h on 7 October 2003. The complete lightpath length was 3250 m. The average mixing ratiosvary from 15 to 63 ppbv during this period, and thetime series of the three light beams show similar timedependences. For further comparison the mixing ra-tios of each beam are displayed versus the averagemixing ratios of the three light beams ([Figs. 9(a)–9(c)]. The differences between the three linear fitslopes and the axis intercepts are smaller than 5%and 2.8 ppbv, respectively.
These NO2 fit parameters are summarized withthose of SO2, O3, and H2CO in Table 3. In addition theaverage one sigma value of the residual structure andthe corresponding two sigma detection limit are listed
for each light beam and each species. Note that somevalues are different from those of Table 1 because ofthe shorter path length. For O3 and H2CO the linear
Table 3. Overview of Fit Parameters Calculated by a Linear Fitbetween the Trace-Gas Slant Column Densities of One Light Beam and
the Average Values Derived with Three Light Beams along the SameLight Path during the Intercalibration Measurement
Fig. 7. Example of the analysis for a spectrum with beam 3 re-corded at 4:17 p.m. central European summer time (CEST) on 7October 2003: full curve, atmospheric absorption; dotted curve,laboratory cross sections. From top to bottom, left, residual struc-ture, NO2, H2CO, O3; right, total fit, SO2, HONO, lamp, solarspectra.
Fig. 8. NO2 mixing ratios measured during 18 h between 0 a.m.and 6 p.m., 7 October 2003 CEST. All three light beams weredirected to the same retroreflector. Except for a time period withstrong winds (between 9:00 and 12:00 a.m.) simultaneous mea-surements were performed continually with all three light beams.
Fig. 9. Comparison of NO2 mixing ratios presented in Fig. 8. Thevalues of each light beam are plotted against the average values ofthe three light beams. The result of a linear fit is shown as a fullline: upper panel, beam 1; middle panel, beam 2; lower panel, beam3.
fit slopes differ by less than 5%; for SO2 they differ byas much as 10% maximum. The differences in theaxis intercepts of SO2, O3, and H2CO are smaller than0.06, 4, and 0.1 ppbv, respectively.
For all trace gases and all light beams the axisintercepts are of the order of the calculated detectionlimits. However, these offsets can change with time;therefore regular intercalibration measurements areuseful in reducing the errors in the different lightbeams.
5. Conclusions
We have presented a new DOAS instrument thatallows the emission and reception of at least fourlight beams and therefore simultaneous measure-ment along multiple light paths, i.e., the multibeamlong-path DOAS instrument. With this technique,multiple light beams can be directed from one tele-scope toward different retroreflectors and reflectedback into the same telescope. The light from eachlight path is collected into one specific optical fibereach and measured together with the light from theother light paths on a single CCD chip by using animaging spectrometer.
The scattered light from one beam to another in-side the spectrometer is less than 1% of the absoluteintensity (presuming similar intensities for all lightbeams). The derived detection limits of the trace-gasconcentrations are similar to those of the conven-tional long-path DOAS system: 0.2–1 ppbv for NO2,0.07–0.33 ppb for SO2, 2–11 ppbv for O3, and0.3–1.5 ppbv for H2CO for a total light path of 5 kmand an integration time of 45 s to 1.5 min.
The first successful atmospheric measurementswith the entire system were performed in the Milanarea during the FORMAT II campaign (October2003). Intercalibration measurements between threedifferent light beams along the same light pathshowed very good correlations for NO2, SO2, H2CO,and O3.
If measurements are performed with four lightpaths, the measurement information corresponds tothat of four single-beam instruments placed at thesame location. Compared with a single instrument,with four light beams it is possible to derive spatialinformation of the trace-gas concentrations. Com-pared with four single instruments the productionand the consumption costs are smaller, and handlingis much easier.
In summary the multibeam instrument has a va-riety of advantages compared with the earlier instru-ments although the detection limits of the trace gasesare the same. The new multibeam technique is suit-able for long-path tomography DOAS, which providestwo- or three-dimensional trace-gas concentrationfields from measurements along 20–50 different lightpaths.10
We thank Zhou Bin, Liu Wen-Qing, Pinhua Xie (An-hui Institute of Optics and Fine Mechanics, China),and Alexander Hoffmann (Hoffmann MesstechnikGmbH) for cooperation. The German Ministry of Re-
search and Education (BMBF) is gratefully acknowl-edged for funding through project 07 ATC-03 (YoungResearchers Fellowship Program for Research Groups,AFO 2000-C) and the European Commission for fund-ing through project EVK2-CT-2001-00120 (FORMAT).
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