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Optical monitoring of volcanic sulphur dioxide emissions—comparison between four different remote-sensing spectroscopic techniques
Optics and Lasers in Engineering 37 (2002) 267–284
Optical monitoring of volcanic sulphur dioxide emissionsFcomparison between four different remote-sensing spectroscopic techniques P. Weibringa, J. Swartlinga, H. Ednera, S. Svanberga, T. Caltabianob, D. Condarellib, G. Cecchic, L. Pantanic b
a Department of Physics, Lund Institute of Technology, P.O. Box 118, SE-22100 Lund, Sweden Istituto Nationale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma 2, I-95123 Catania, Italy c Consiglio Nazionale delle Ricerche, Istituto Ricerca sulle Onde Elettromagnetiche-Nello Carrara, Via Panciatichi 64, I-50127 Firenze, Italy
Received 12 July 2001; accepted 28 July 2001
Abstract The emissions of sulphur dioxide from the Italian volcanoes Mt. Etna and Stromboli were studied in ship-borne underpasses of their plumes. Four different optical spectroscopy techniques were used and inter-compared. All techniques utilise the absorption signature of the gas in the wavelength region of around 300 nm. A differential absorption lidar was employed in active gas concentration assessment. In parallel, a differential optical absorption spectroscopy system (DOAS) provided spectrally resolved absorption spectra. In one configuration the DOAS used a vertically looking telescope and the absorption of the skylight was studied, while a different DOAS implementation utilised the sun disc as the light source in slant-angle, long-path absorption measurements. Parallel measurements with the customary correlation spectroscopy method were also performed. Path length Monte Carlo simulations of the down-welling radiation through the volcanic plume at different sun altitude and azimuth angles have been performed taking into account also the effects of other geometric parameters as the plume height and extension. The results are discussed with special emphasis on systematic effects due to scattering. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: DOAS; Lidar; Correlation spectroscopy
E-mail address: [email protected] (P. Weibring). 0143-8166/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 0 8 4 - 7
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1. Introduction Volcanic sulphur dioxide (SO2) emissions constitute an important source for the atmospheric sulphur budget [1] and monitoring programmes are frequently carried out at major active volcanoes, see, e.g. [2,3]. The measurements have normally been done analysing the spectral contents of the down-welling sky radiation by mask correlation spectroscopy performed with instruments denoted correlation spectroscopy (COSPEC) [4,5]. Since the atmospheric scattering above, inside and below high-altitude plumes constitutes a considerable uncertainty in the results obtained, a research programme for comparing different optical techniques, including lidar, for volcanic monitoring started in 1990. Results from three previous ship-borne campaigns at Mt. Etna, Stromboli and Vulcano (Eolian Islands) were presented in Refs. [6–8]. The fourth campaign is described in this paper. Measurements were performed on the plumes of Mt. Etna and Stromboli by an improved lidar system with a range larger than in the previous campaigns. Long-path absorption measurements using the sun disc as the light source were also done since these measurements are easier to interpret than those performed employing the sky radiation. For interpretation and verification purposes Monte Carlo simulations of the light scattering through the plume have also been performed. In the next section the field experiment is described, followed by a presentation of the different optical sensors used. Measurement geometry and other considerations for the construction of the Monte Carlo model are also presented. Finally, the results of the measurements and Monte Carlo simulations are presented and discussed.
2. Field experiment The measurements were performed on August 23 and 26, 2000 as part of the SALE-3 hydrospheric/atmospheric campaign performed on board the oceanic vessel Urania operated by Sopromar S.p.A. for the CNR. The 2-week cruise started and ended in the Italian harbour of Civitavecchia. Atmospheric measurements were performed at Mt. Etna and at the Eolian island of Stromboli while the oceanographic studies by fluorescence lidar and by in-situ sampling were performed at many locations in the Tyrrhenian Sea and in the waters around Sicily. The present paper is restricted to the volcanic investigations. Some measurement data from Mt. Etna, collected during the SALE-2 campaign on August 17, 1997 are also used for inter-comparison with the scattering model. A schematic diagram of the Etna plume measurement is shown in Fig. 1. The volcanic plume from Mt. Etna was encountered south-east of the Sicilian city of Catania given the prevailing north-westerly wind direction. The ship travelled along a straight line and at a constant speed, normally 10 knots. The ship bearing was arranged to encounter the plume at approximately right angles as indicated in the map shown in Fig. 1. During some of the traverses with a more northerly wind, it was not possible to scan the complete plume since part of it was over the land. The distance to the Mt. Etna summit craters was about 30 km, and the plume was centred
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Fig. 1. Measurement scenario for volcanic sulphur dioxide monitoring with passive and active optical techniques. The passive techniques include blue sky monitoring as well as the use of the sun disc as the source of background illumination.
at an altitude of about 3 km matching the height of Mt. Etna (B3300 m). At Stromboli, with a volcano height of about 900 m, the plume was traversed at a distance of 3–5 km from the source. Normally, the ship retraced its track after one traverse was finished. The ship was equipped with a GPS-based navigational system with on-line data logging permitting the retrieval of the ship position, bearing and speed at any time. During part of the afternoon of August 26 at Etna, the geometrical conditions pertaining to the optical measurements were particularly simple, since the sun disc was close to the bow or stern directions of the ship. When the sun was in the stern direction, the vertically looking lidar, COSPEC and differential optical absorption spectroscopy system (DOAS) instruments would simultaneously encounter the plume while the sun-tracking data come delayed in time because of the slant angle observation. On the return track, the plume was firstly engaged in the sun-tracking data. At the end of the day, when the sun was at a low angle, the SO2 signal remains for a long time even if the ship moves large distances. Thus, it was difficult to use the sun-tracking DOAS very late during the day in the inter-comparison between the different techniques.
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For the inter-comparison, either the measured SO2 column content (g/m2) at different positions, or the total plume integrated value (g/m) for the sun-tracking were employed. All data can be used to calculate the SO2 flux by multiplying the plume integrated value by the appropriate wind velocity and taking into account the angle between the ship track and the wind direction. Wind data for different heights were obtained from local meteorological stations.
3. Experimental arrangements All gas measuring equipment, except the COSPEC, was hosted in the Swedish mobile laser radar system, which was hoisted into position on the aft deck of the research vessel Urania. Measurement scenarios for the active and passive gas monitoring are schematically shown in Fig. 1. Basic descriptions of the mobile lidar system used in the active remote sensing measurements of volcanic gases are available in Refs. [9,10]. Before these measurements, the system had been equipped with new cooling machines handling the laser transmitter heat dissipation as well as the laboratory air conditioning. A new 45 kW motor generator, housed on a trailer was also used for the first time. A schematic view of the differential absorption lidar (DIAL) system is provided in Fig. 2. The Nd : YAG/Dye laser transmitter operates at 20 Hz with a wavelength switching between the 300.02 (on-) and 299.30 nm (off-resonance) transmission at every shot. The pulse energy was typically of 25 mJ. In comparison to previous campaigns, the rooftop transmission dome had been removed to increase the optical efficiency of the receiving telescope by eliminating two first-surface aluminised mirrors. This allows an increase in the useful measurement range, which is very valuable to achieve a good return signal from the atmosphere above the plume. Previously, the laser beam was always transmitted coaxially with the telescope. Now, as an alternative, an off-axis arrangement could be chosen by transmitting the beam through a new aperture placed 45 cm away from the optical axis of the telescope. The strong near-field atmospheric return is then substantially reduced and also the signalinduced background in the detector. In this way more reliable DIAL signals at long ranges could be obtained. Software routines for system steering and data evaluation had also been updated by the use of LabVIEW. A description of the DOAS technique for atmospheric gas assessment can be found, for instance in Refs. [11,12]. The spectroscopic system and the data handling of the system employed in the experiment were described in Ref. [12]. In the present experiments, the light was collected in two ways. The normal mode uses a vertically looking 30 cm diameter telescope, directing the zenith-scattered light onto a 1 mm diameter quartz fibre bundle in the focus. This unit collects the light from a 1 mrad cone of the sky radiation and leads it, via the round-to-slit fibre bundle, to the entrance slit of the spectrometer, equipped with a flying exit slit arrangement. Alternatively, the down-looking fibre was removed from its holder at the telescope optical axis and was instead fixed along a 30 cm thin metal rod, at the end of which a white screen had been symmetrically attached. By using a flexible mount, the rod
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Fig. 2. Schematic layout of the differential absorption lidar system used in volcanic SO2 monitoring.
(and the fibre) could be directed towards the sun disc by minimising the shadow of the rod on the screen. Since the numerical aperture of the fibre was 0.22, the fibre accepted light in a rather wide cone of incident rays. Since the traverses of the plume were made using quite linear tracks, the sea was low to moderate, and the angle to the sun above the horizon changes only slowly, it was sufficient to perform manual adjustments of the rod pointing every few minutes. This simple set-up was quite adequate and there was no need for the servo system usually employed in suntrackers. If the sun disc is within the numerical aperture of the fibre, the direct sunlight will always be much larger than the scattered light. This was verified by directing the fibre against the sky away from the sun, yielding a signal level of about 1/40 compared to the correct sun-tracking position. In the DOAS measurements of SO2, a spectrum was recorded in the region 300–315 nm inside the plume and divided by a reference spectrum outside the plume. Normally, a spectrum recorded around
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solar noon was used as reference spectrum for all measurements of the same day. The further processing consists of a high-pass filtering and correlation with the SO2 and O3 absorption spectra [12]. The O3 correlation is used to compensate for possible crosstalk in the SO2 evaluation due to changing absorption path lengths through the stratospheric O3 layer at different solar angles and to different tropospheric O3 concentrations. Even after this compensation, a slight offset was normally registered in the DOAS values for smaller solar angles. This can in principle be removed by taking a reference spectrum more frequently during the day. However, any offset is evident in the scatter plots used in the inter-comparisons. A vertically looking COSPEC V instrument from Barringer Research, Inc. was also used in the measurements. The instrument receives the sky radiation in an opening angle of 10 30 mrad and directs it to a spectrometer. In the exit plane of the spectrometer there is a correlation mask arrangement matching the SO2 absorption spectrum in the 290–320 nm region. The instrument has a 4-step correlation mask that minimises the effects of the background changes. Furthermore, the instrument has an automatic gain control circuit which compensates the background variations. The zero level of the instrument was recorded when the ship had passed outside the plume, and the integrated over-head concentration values were calibrated by inserting two quartz cells, with a nominal gas load of 406 and 128 ppm m, respectively. These cells were also used to internally calibrate all instruments used during the measurements.
4. Radiative transfer model To quantitatively assess the influence of the light scattering on the results obtained from the passive instruments, a numerical model based on Monte Carlo simulations was employed. In this approach, the radiative transfer can be modelled by tracking a set of virtual ‘‘photon packets’’ through a scattering and absorbing medium. Each scattering event is governed by a pre-defined probability distribution based on known physical properties, together with a random number generated by the computer. In this way, each photon path may be described as a random walk process, and the paths, taken together, generate a statistical estimate of the property of interest. The Monte Carlo model is very well suited for this problem at hand, since the lengths of the photon paths may be recorded directly. Assuming that the errors in the passive instrument readings are based on variations in the path length due to scattering, thus disqualifying the use of the Lambert–Beer law and the direct line-ofsight assumption, this property is exactly what is needed. The computer software was based on the programme MCML by Jacques and Wang [13], modified to handle the present problem. This programme is thoroughly described in Ref. [13]. The programme allows a geometry of a stack of infinite layers, where each layer may have its own combination of scattering and absorption properties. To better approximate the plume geometry, in one of the layers it was made possible to define edges of the plume in relation to the position of the detector on the sea surface, see Fig. 3. The input parameters to the programme are, in
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Fig. 3. The geometry of the simulation. The plume may be translated along the z- and the x-axes. The detector point is actually the source point of the photon packets in the simulation, since the photons are tracked backwards. When a photon packet leaves the uppermost layer, its path length through the plume is recorded in an array indexed by the azimuth and altitude angles.
addition to the geometry, the absorption coefficient ma (1/m), the scattering coefficient ms (1/m), and a factor describing the amount of forward scattering. For Rayleigh scattering, the scattering is isotropic, but in the Mie regime where the scattering particles are larger, a phase function describing the angular intensity distribution is needed. For this purpose the programme utilises the Henyey– Greenstein phase function. The total extinction coefficient is defined as ma þ ms : Modelling the problem directly by assuming a very distant source (the sun), and detecting only those photons that scatter into the field of view of the instrument, is unrealistic because of the very small probability of a photon actually reaching the detector. To improve the photon economy, it was assumed that the problem is reciprocal, so that the photon paths may be tracked backwards from the detector. In this case, all photons which escape the upper limit of the atmosphere contribute with the direction of the photon as it leaves the atmosphere corresponding to specific solar altitude/azimuth angles. Thus, for a certain position of the sun in the sky, the path length of the detected light through the plume is estimated as the average path length of all photon packets leaving inside the solid angle defined by the solar disc. The atmosphere was modelled as a stack of layers with different scattering properties up to an altitude of 33 km, where it was assumed that the air is so thin that the scattering is negligible, see Fig 4. The Rayleigh component was calculated based on a standard formula and the number density of molecules as a function of altitude for a standard atmosphere. The Mie component (arising from the aerosols) was calculated based on the formula given in Ref. [14] and an observed estimated meteorological range of 20 km. It was assumed that the aerosol layers did not reach above 3 km. The scattering due to the aerosols in the plume itself was estimated by
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Fig. 4. Scattering cross-sections as a function of height used in the simulation.
the extinction coefficient measured with the lidar system, lifting off the contribution from molecular absorption and scattering in the atmosphere. The scattering coefficient at 300 nm due to aerosols in the plume was about 50% smaller than the Rayleigh component at this altitude (3000 m). Thus, the atmosphere above, inside, and below the plume constitutes a far larger source of scattering in the problem than the plume itself. The background absorption, mostly due to different concentrations of ozone at different heights in the atmosphere, was included in the model. The exact ozone abundance in the atmosphere at the time of measurement was unknown, but this was not regarded as a serious problem since the model turned out to be fairly immune to variations in the background absorption. Because each simulation provides data for all solar altitude and azimuth angles, the only parameters to be changed between simulations were the geometry of the plume and its position in relation to the detector. For comparison with the COSPEC/DIAL and DOAS/DIAL data, the plume height, width and altitude were taken from the DIAL scans. In addition to the comparison with experimental data, a series of simulations for different plume geometry, scanning across the plume, were done. These computations were performed to provide insight to which factors are the most relevant for the errant readings from the passive instruments.
5. Measurements and results Although all the instruments had independent calibrations, the COSPEC calibration cells mentioned above were transferred to the DIAL and DOAS
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instruments for local calibration, to avoid differences in measured concentration that are not due to the fundamental differences in scattering. All measurements were recorded or merged with a time resolution of 1 min, equivalent to a horizontal resolution of about 300 m with a 10 knots speed of the ship. The DIAL system was used to monitor range-resolved vertical SO2 concentration profiles along the ship traverses. A large number of such measurements were used to produce a 2-D plot of the gas plume, as shown in Fig. 5 for two different occasions during August 26, 2000. Integrated horizontal and vertical profiles are also given. The gas distributions are obtained from raw data of the type shown in Fig. 6 for the on- and off-resonance wavelengths. Analysing the plume shape, it can be concluded that the horizontal extension of the plume changed by as much as a factor of two during the day, while the vertical extension tends to be the same. The 2-D plots produced by the DIAL system were used in the model to predict the overestimation made with passive monitoring systems. The DIAL system has the unique property of yielding the height resolved concentration profile. However, the vertical integrated concentration or column content was used in the comparison with the other systems, only measuring this quantity. An example of an inter-comparison on a traverse under the Mt. Etna plume in the morning (09:30–10:30 local daylight saving time, 36–461 sun altitude angle) is shown in Fig. 7. The results are compared point-by-point by making scatter plots of the COSPEC versus DIAL and DOAS versus DIAL data. In this way, any offset in the passive instruments can be eliminated and the differential response is revealed from the slope K of the trendline. As a matter of fact, we note a clear off-set problem for both passive instruments in the particular scan. It is not necessary that the complete plume be fully traversed for this evaluation. DOAS measurements were performed alternatively employing the sky radiation as a distributed light source (with the spectrometer input fibre fixed at the focus of the vertically looking DOAS telescope) or the sun disc as a well-defined physical ‘‘lamp’’ (with the fibre pointing directly towards the sun using the manual sun-tracking arrangement). The location of the probed air volume will of course be different in the sun-tracking mode with respect to the DIAL. In Fig. 8, the results from DIAL and sun-tracking DOAS are compared for a traverse during the afternoon. The DOAS SO2 column content values have been corrected for the sun altitude angle of 48–571 during the measurement. The sun was towards the stern of the ship causing the horizontal displacement of the plume centre of gravity. In these measurements a point-by-point inter-comparison is not possible but the total integrated plume content has to be used. Monte Carlo simulations to obtain the average path length were performed for various plume widths and heights above the ground. The ratio between the average path length and the thickness of the plume was interpreted as corresponding to the ratio of the measured burden to the true burden for the passive instruments. The simulations were done with the detection point at different positions in relation to the plume edges, emulating a scan across the plume. In all simulations, 107 photon packets were launched. Irregularities in the data are caused by the stochastic nature of the simulations. Typical simulated scans are shown in Fig. 9, for two plumes of widths 6 and 14 km, respectively. For each plume, scans for three solar altitude
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Fig. 5. Distribution of SO2 in the volcanic plume as recorded by the off-axis DIAL system during two different occasions. (August 26, 2000, starting at 11:45 and 15:27 local time, respectively). Integrated horizontal and vertical profiles are also shown.
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Fig. 6. On- and off-resonance lidar curves recorded with the off-axis geometry for the vertically sounding lidar system. The curves are shown after distance-square compensation and logaritmation. For each of the curves, data from 300 laser shots were averaged and the total time for the recording took 1 min.
angles are shown: 251, 451 and 651. The solar azimuth angle was perpendicular to the plume, resulting in the skewed symmetry of the curves. Similar curves for a solar angle parallel to the plume would be symmetric. To emulate the normal case of measurement where the total integrated burden from the scan is calculated, the integrated ratios of the scans were computed. To test the hypothesis that the plume width influences the ratio, simulations for various widths were performed. The two cases of perpendicular and parallel solar azimuth angles are shown in Fig. 10, for solar altitude angles from 251 to 751. Results from simulations for various plume heights are shown in Fig. 11. The plume thickness was 1 km, the width was 10 km, and solar altitude angles varied from 251 to 751. The influence on the measurement of the aerosol concentration outside the plume is quite low. If the aerosol concentration below the plume was increased to the double value or was reduced to half the value in the calculations, it was found that the theoretical ratio changed by about 0.1 units. In Fig. 12a, the different measurement results during the measurement day at Mt. Etna are compared with the model taking into account different scattering situations as a function of the geometry, sun altitude and azimuth angle. The ratios between the recorded values for the three different passive techniques and the DIAL data are shown. What is plotted for COSPEC/DIAL and DOAS(sky)/DIAL is actually the fitted K value of the scattering plot as defined above. Thus, we did not have to consider the true out-of-plume values, which were not always available. Each data entry corresponds the value obtained from a single plume scan. In particular, the two K-values corresponding to average time 10:00 were marked, i.e. the data entries from Fig. 7. The two datapoints for DOAS(sky)/DIAL correspond to the total integrated plume value ratios. The highest sun altitude angle is 62.71 at 13:00 local time. A similar evaluation was also made on older DOAS(sky)/DIAL data from a previous
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Fig. 7. Comparison of data obtained with the different measurement systems in traverses under the Mt. Etna plume (August 26, 2000, 09:30–10:30 local time). Correlation diagrams (DOAS versus DIAL and COSPEC versus DIAL) are shown, illustrating systematic effects related to atmospheric scattering.
campaign in 1997 (Fig. 12b). Finally, in Fig. 12c the results from the measurements at the much lower plume from Stromboli are displayed. The DIAL system was not in operation during the Stromboli measurements, so here the sun-tracking DOAS data are used as the indicator of the true plume content. As can be seen, even if the model does not match perfectly the individual measurement results, the overall trend towards higher ratios at lower solar angles is in good agreement (Fig. 12a and b). The lack of trend in the Stromboli data (Fig. 12c) is also in agreement with the model. Scattering in the experimental data could be explained from variations in cloud/haze conditions.
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Fig. 8. Simultaneous recording of the Etna plume with DIAL and sun-tracking DOAS system (August 26, 2000, 14:20–15:20 local time).
Fig. 9. The model results of the ratio of the average path length to the plume thickness, shown as a scan across the plume. Two plumes of different width are shown: 6 and 14 km, for three solar altitude angles: 251, 451 and 651. The thickness of the plumes was 1 km. The solar azimuth angle was perpendicular to the plume.
The purpose of these experiments was primarily to compare the results of active and passive monitoring techniques and to create a scattering model for a better understanding of the differences encountered. However, the data can also be used to estimate the SO2 flux from the volcanoes. Using the wind data that were available, and correcting for the overestimation in the passive instruments, the SO2 flux from Etna was 1800 tons/day, and from Stromboli 300 tons/day, during the measurement days in August 2000.
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Fig. 10. The model results of the integrated ratio across the plume as a function of the plume width. The plume thickness was 1 km, and the height to its lower boundary 3 km. In (a) the solar azimuth angle was perpendicular to the plume, while in (b) it was parallel. Each curve represents a solar altitude angle, ranging from 251 to 751 in steps of 101.
6. Discussion By analysing the data obtained, we notice that there is a general agreement between the DIAL and the sun-tracking DOAS data, while the COSPEC and the vertically looking DOAS system employing the diffuse sky radiation also show internal agreement. However, the two groups of data exhibit a discrepancy by a factor of about 0.9–1.6, with the instruments employing diffuse sky light usually overestimating the total overhead SO2 burden. This is in agreement with earlier observations [6,8]. The simulated scans in Fig. 9 show that the scattering induces edge effects that may be significant, especially for low solar altitude angles. The simulations suggest that when looking at pointwise measurements, solar altitude angle, solar azimuth
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Fig. 11. The model results of the integrated ratio across the plume as a function of the plume height above the surface. The solar azimuth angle was perpendicular to the plume. Each curve represents a solar altitude angle, ranging from 251 to 751 in steps of 101.
angle, and plume width all may have large influence on the measurement. The situation is not so dramatic when considering the total integrated scans, however. The simulations in Fig. 9 show that for the plume widths relevant for the Mt. Etna measurements, approximately 7–16 km, the most significant factor determining the ratio is the solar altitude angle. Nevertheless, edge effects may still play an important role when performing total integrated scans, since the effect may give rise to a signal even as far as 3–5 km from the plume edge. This means that one has to be careful to be outside the range of the plume influence when calibrating the instruments and when starting and stopping the scan. In reality, the plume is gradually getting thinner near the edges which will be likely to reduce the importance of edge effects, but some residual signal is likely to be found up to a few kilometres outside the physical boundaries of the plume. For the comparison between the DIAL, DOAS and COSPEC measurements, the total integrated measurements could not be used, and instead the mean value of ratios taken along certain fractions of the scans were calculated utilising the correlation diagrams discussed before. Even though edge effects are automatically included, the plume geometry may still be of concern. A cross-section of the real plume certainly does not resemble a rectangle, and it is possible that irregularities in the shape will induce errors. Both the measurements from Stromboli and the simulations show that the ratio is close to unity when the plume is located very close to or in direct contact with the surface (see Fig. 12). A conclusion that may be drawn from this result, and which may seem counter-intuitive, is that a layer of scattering atmosphere below the plume is necessary for the ratio to increase significantly. The solar azimuth angle was of no major importance for these results.
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Fig. 12. Comparisons between the scattering model and results of different measurements as a function of time: (a) Mt. Etna, August 26, 2000; (b) Mt. Etna, August 17, 1997; (c) Stromboli, August 23, 2000.
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7. Conclusion Comparisons of different measurement techniques for volcanic plumes were successfully performed during several campaigns. A scattering model has been developed using Monte Carlo simulations of traced photon histories. The model extends the work presented in Ref. [5] for industrial plumes at low altitudes. Solutions were found to the transport equation for the atmospheric scattering conditions pertaining to the measurement situation which provide the necessary correction factors to allow straightforward and convenient passive measurements to improve the volcanic SO2 assessment. The modelling can be further improved by simulations of the exact plume shape in 3-D yielding even better accordance, but the expected error reduction is in the order of the present measurements error; furthermore, the result cannot be used by the passive instrument users due to the fact that it is difficult to access the 2-D geometry surface without an active system. Clearly, the correction factors will depend on the scattering conditions, which gives a residual uncertainty. Of the passive techniques, DOAS measurements using the solar disc as the light source will give much more reliable results. The active DIAL is the most general technique of wide applicability, but also by far the most expensive one.
Acknowledgements The authors are very grateful to Captain Emanuele Gentile and his crew for efficient and flexible navigation of the Urania. This work was supported by the Swedish Natural Sciences Research Council (NFR), the Knut and Alice Wallenberg Foundation and the Italian National Research Council (CNR). The authors are very grateful to Thomas Johansson for assistance during the measurements.
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Optical monitoring of volcanic sulphur dioxide emissionsFcomparison between four different remote-sensing spectroscopic techniques P. Weibringa, J. Swartlinga, H. Ednera, S. Svanberga, T. Caltabianob, D. Condarellib, G. Cecchic, L. Pantanic b
a Department of Physics, Lund Institute of Technology, P.O. Box 118, SE-22100 Lund, Sweden Istituto Nationale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma 2, I-95123 Catania, Italy c Consiglio Nazionale delle Ricerche, Istituto Ricerca sulle Onde Elettromagnetiche-Nello Carrara, Via Panciatichi 64, I-50127 Firenze, Italy
Received 12 July 2001; accepted 28 July 2001
Abstract The emissions of sulphur dioxide from the Italian volcanoes Mt. Etna and Stromboli were studied in ship-borne underpasses of their plumes. Four different optical spectroscopy techniques were used and inter-compared. All techniques utilise the absorption signature of the gas in the wavelength region of around 300 nm. A differential absorption lidar was employed in active gas concentration assessment. In parallel, a differential optical absorption spectroscopy system (DOAS) provided spectrally resolved absorption spectra. In one configuration the DOAS used a vertically looking telescope and the absorption of the skylight was studied, while a different DOAS implementation utilised the sun disc as the light source in slant-angle, long-path absorption measurements. Parallel measurements with the customary correlation spectroscopy method were also performed. Path length Monte Carlo simulations of the down-welling radiation through the volcanic plume at different sun altitude and azimuth angles have been performed taking into account also the effects of other geometric parameters as the plume height and extension. The results are discussed with special emphasis on systematic effects due to scattering. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: DOAS; Lidar; Correlation spectroscopy
E-mail address: [email protected] (P. Weibring). 0143-8166/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 3 - 8 1 6 6 ( 0 1 ) 0 0 0 8 4 - 7
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1. Introduction Volcanic sulphur dioxide (SO2) emissions constitute an important source for the atmospheric sulphur budget [1] and monitoring programmes are frequently carried out at major active volcanoes, see, e.g. [2,3]. The measurements have normally been done analysing the spectral contents of the down-welling sky radiation by mask correlation spectroscopy performed with instruments denoted correlation spectroscopy (COSPEC) [4,5]. Since the atmospheric scattering above, inside and below high-altitude plumes constitutes a considerable uncertainty in the results obtained, a research programme for comparing different optical techniques, including lidar, for volcanic monitoring started in 1990. Results from three previous ship-borne campaigns at Mt. Etna, Stromboli and Vulcano (Eolian Islands) were presented in Refs. [6–8]. The fourth campaign is described in this paper. Measurements were performed on the plumes of Mt. Etna and Stromboli by an improved lidar system with a range larger than in the previous campaigns. Long-path absorption measurements using the sun disc as the light source were also done since these measurements are easier to interpret than those performed employing the sky radiation. For interpretation and verification purposes Monte Carlo simulations of the light scattering through the plume have also been performed. In the next section the field experiment is described, followed by a presentation of the different optical sensors used. Measurement geometry and other considerations for the construction of the Monte Carlo model are also presented. Finally, the results of the measurements and Monte Carlo simulations are presented and discussed.
2. Field experiment The measurements were performed on August 23 and 26, 2000 as part of the SALE-3 hydrospheric/atmospheric campaign performed on board the oceanic vessel Urania operated by Sopromar S.p.A. for the CNR. The 2-week cruise started and ended in the Italian harbour of Civitavecchia. Atmospheric measurements were performed at Mt. Etna and at the Eolian island of Stromboli while the oceanographic studies by fluorescence lidar and by in-situ sampling were performed at many locations in the Tyrrhenian Sea and in the waters around Sicily. The present paper is restricted to the volcanic investigations. Some measurement data from Mt. Etna, collected during the SALE-2 campaign on August 17, 1997 are also used for inter-comparison with the scattering model. A schematic diagram of the Etna plume measurement is shown in Fig. 1. The volcanic plume from Mt. Etna was encountered south-east of the Sicilian city of Catania given the prevailing north-westerly wind direction. The ship travelled along a straight line and at a constant speed, normally 10 knots. The ship bearing was arranged to encounter the plume at approximately right angles as indicated in the map shown in Fig. 1. During some of the traverses with a more northerly wind, it was not possible to scan the complete plume since part of it was over the land. The distance to the Mt. Etna summit craters was about 30 km, and the plume was centred
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Fig. 1. Measurement scenario for volcanic sulphur dioxide monitoring with passive and active optical techniques. The passive techniques include blue sky monitoring as well as the use of the sun disc as the source of background illumination.
at an altitude of about 3 km matching the height of Mt. Etna (B3300 m). At Stromboli, with a volcano height of about 900 m, the plume was traversed at a distance of 3–5 km from the source. Normally, the ship retraced its track after one traverse was finished. The ship was equipped with a GPS-based navigational system with on-line data logging permitting the retrieval of the ship position, bearing and speed at any time. During part of the afternoon of August 26 at Etna, the geometrical conditions pertaining to the optical measurements were particularly simple, since the sun disc was close to the bow or stern directions of the ship. When the sun was in the stern direction, the vertically looking lidar, COSPEC and differential optical absorption spectroscopy system (DOAS) instruments would simultaneously encounter the plume while the sun-tracking data come delayed in time because of the slant angle observation. On the return track, the plume was firstly engaged in the sun-tracking data. At the end of the day, when the sun was at a low angle, the SO2 signal remains for a long time even if the ship moves large distances. Thus, it was difficult to use the sun-tracking DOAS very late during the day in the inter-comparison between the different techniques.
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For the inter-comparison, either the measured SO2 column content (g/m2) at different positions, or the total plume integrated value (g/m) for the sun-tracking were employed. All data can be used to calculate the SO2 flux by multiplying the plume integrated value by the appropriate wind velocity and taking into account the angle between the ship track and the wind direction. Wind data for different heights were obtained from local meteorological stations.
3. Experimental arrangements All gas measuring equipment, except the COSPEC, was hosted in the Swedish mobile laser radar system, which was hoisted into position on the aft deck of the research vessel Urania. Measurement scenarios for the active and passive gas monitoring are schematically shown in Fig. 1. Basic descriptions of the mobile lidar system used in the active remote sensing measurements of volcanic gases are available in Refs. [9,10]. Before these measurements, the system had been equipped with new cooling machines handling the laser transmitter heat dissipation as well as the laboratory air conditioning. A new 45 kW motor generator, housed on a trailer was also used for the first time. A schematic view of the differential absorption lidar (DIAL) system is provided in Fig. 2. The Nd : YAG/Dye laser transmitter operates at 20 Hz with a wavelength switching between the 300.02 (on-) and 299.30 nm (off-resonance) transmission at every shot. The pulse energy was typically of 25 mJ. In comparison to previous campaigns, the rooftop transmission dome had been removed to increase the optical efficiency of the receiving telescope by eliminating two first-surface aluminised mirrors. This allows an increase in the useful measurement range, which is very valuable to achieve a good return signal from the atmosphere above the plume. Previously, the laser beam was always transmitted coaxially with the telescope. Now, as an alternative, an off-axis arrangement could be chosen by transmitting the beam through a new aperture placed 45 cm away from the optical axis of the telescope. The strong near-field atmospheric return is then substantially reduced and also the signalinduced background in the detector. In this way more reliable DIAL signals at long ranges could be obtained. Software routines for system steering and data evaluation had also been updated by the use of LabVIEW. A description of the DOAS technique for atmospheric gas assessment can be found, for instance in Refs. [11,12]. The spectroscopic system and the data handling of the system employed in the experiment were described in Ref. [12]. In the present experiments, the light was collected in two ways. The normal mode uses a vertically looking 30 cm diameter telescope, directing the zenith-scattered light onto a 1 mm diameter quartz fibre bundle in the focus. This unit collects the light from a 1 mrad cone of the sky radiation and leads it, via the round-to-slit fibre bundle, to the entrance slit of the spectrometer, equipped with a flying exit slit arrangement. Alternatively, the down-looking fibre was removed from its holder at the telescope optical axis and was instead fixed along a 30 cm thin metal rod, at the end of which a white screen had been symmetrically attached. By using a flexible mount, the rod
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Fig. 2. Schematic layout of the differential absorption lidar system used in volcanic SO2 monitoring.
(and the fibre) could be directed towards the sun disc by minimising the shadow of the rod on the screen. Since the numerical aperture of the fibre was 0.22, the fibre accepted light in a rather wide cone of incident rays. Since the traverses of the plume were made using quite linear tracks, the sea was low to moderate, and the angle to the sun above the horizon changes only slowly, it was sufficient to perform manual adjustments of the rod pointing every few minutes. This simple set-up was quite adequate and there was no need for the servo system usually employed in suntrackers. If the sun disc is within the numerical aperture of the fibre, the direct sunlight will always be much larger than the scattered light. This was verified by directing the fibre against the sky away from the sun, yielding a signal level of about 1/40 compared to the correct sun-tracking position. In the DOAS measurements of SO2, a spectrum was recorded in the region 300–315 nm inside the plume and divided by a reference spectrum outside the plume. Normally, a spectrum recorded around
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solar noon was used as reference spectrum for all measurements of the same day. The further processing consists of a high-pass filtering and correlation with the SO2 and O3 absorption spectra [12]. The O3 correlation is used to compensate for possible crosstalk in the SO2 evaluation due to changing absorption path lengths through the stratospheric O3 layer at different solar angles and to different tropospheric O3 concentrations. Even after this compensation, a slight offset was normally registered in the DOAS values for smaller solar angles. This can in principle be removed by taking a reference spectrum more frequently during the day. However, any offset is evident in the scatter plots used in the inter-comparisons. A vertically looking COSPEC V instrument from Barringer Research, Inc. was also used in the measurements. The instrument receives the sky radiation in an opening angle of 10 30 mrad and directs it to a spectrometer. In the exit plane of the spectrometer there is a correlation mask arrangement matching the SO2 absorption spectrum in the 290–320 nm region. The instrument has a 4-step correlation mask that minimises the effects of the background changes. Furthermore, the instrument has an automatic gain control circuit which compensates the background variations. The zero level of the instrument was recorded when the ship had passed outside the plume, and the integrated over-head concentration values were calibrated by inserting two quartz cells, with a nominal gas load of 406 and 128 ppm m, respectively. These cells were also used to internally calibrate all instruments used during the measurements.
4. Radiative transfer model To quantitatively assess the influence of the light scattering on the results obtained from the passive instruments, a numerical model based on Monte Carlo simulations was employed. In this approach, the radiative transfer can be modelled by tracking a set of virtual ‘‘photon packets’’ through a scattering and absorbing medium. Each scattering event is governed by a pre-defined probability distribution based on known physical properties, together with a random number generated by the computer. In this way, each photon path may be described as a random walk process, and the paths, taken together, generate a statistical estimate of the property of interest. The Monte Carlo model is very well suited for this problem at hand, since the lengths of the photon paths may be recorded directly. Assuming that the errors in the passive instrument readings are based on variations in the path length due to scattering, thus disqualifying the use of the Lambert–Beer law and the direct line-ofsight assumption, this property is exactly what is needed. The computer software was based on the programme MCML by Jacques and Wang [13], modified to handle the present problem. This programme is thoroughly described in Ref. [13]. The programme allows a geometry of a stack of infinite layers, where each layer may have its own combination of scattering and absorption properties. To better approximate the plume geometry, in one of the layers it was made possible to define edges of the plume in relation to the position of the detector on the sea surface, see Fig. 3. The input parameters to the programme are, in
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Fig. 3. The geometry of the simulation. The plume may be translated along the z- and the x-axes. The detector point is actually the source point of the photon packets in the simulation, since the photons are tracked backwards. When a photon packet leaves the uppermost layer, its path length through the plume is recorded in an array indexed by the azimuth and altitude angles.
addition to the geometry, the absorption coefficient ma (1/m), the scattering coefficient ms (1/m), and a factor describing the amount of forward scattering. For Rayleigh scattering, the scattering is isotropic, but in the Mie regime where the scattering particles are larger, a phase function describing the angular intensity distribution is needed. For this purpose the programme utilises the Henyey– Greenstein phase function. The total extinction coefficient is defined as ma þ ms : Modelling the problem directly by assuming a very distant source (the sun), and detecting only those photons that scatter into the field of view of the instrument, is unrealistic because of the very small probability of a photon actually reaching the detector. To improve the photon economy, it was assumed that the problem is reciprocal, so that the photon paths may be tracked backwards from the detector. In this case, all photons which escape the upper limit of the atmosphere contribute with the direction of the photon as it leaves the atmosphere corresponding to specific solar altitude/azimuth angles. Thus, for a certain position of the sun in the sky, the path length of the detected light through the plume is estimated as the average path length of all photon packets leaving inside the solid angle defined by the solar disc. The atmosphere was modelled as a stack of layers with different scattering properties up to an altitude of 33 km, where it was assumed that the air is so thin that the scattering is negligible, see Fig 4. The Rayleigh component was calculated based on a standard formula and the number density of molecules as a function of altitude for a standard atmosphere. The Mie component (arising from the aerosols) was calculated based on the formula given in Ref. [14] and an observed estimated meteorological range of 20 km. It was assumed that the aerosol layers did not reach above 3 km. The scattering due to the aerosols in the plume itself was estimated by
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Fig. 4. Scattering cross-sections as a function of height used in the simulation.
the extinction coefficient measured with the lidar system, lifting off the contribution from molecular absorption and scattering in the atmosphere. The scattering coefficient at 300 nm due to aerosols in the plume was about 50% smaller than the Rayleigh component at this altitude (3000 m). Thus, the atmosphere above, inside, and below the plume constitutes a far larger source of scattering in the problem than the plume itself. The background absorption, mostly due to different concentrations of ozone at different heights in the atmosphere, was included in the model. The exact ozone abundance in the atmosphere at the time of measurement was unknown, but this was not regarded as a serious problem since the model turned out to be fairly immune to variations in the background absorption. Because each simulation provides data for all solar altitude and azimuth angles, the only parameters to be changed between simulations were the geometry of the plume and its position in relation to the detector. For comparison with the COSPEC/DIAL and DOAS/DIAL data, the plume height, width and altitude were taken from the DIAL scans. In addition to the comparison with experimental data, a series of simulations for different plume geometry, scanning across the plume, were done. These computations were performed to provide insight to which factors are the most relevant for the errant readings from the passive instruments.
5. Measurements and results Although all the instruments had independent calibrations, the COSPEC calibration cells mentioned above were transferred to the DIAL and DOAS
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instruments for local calibration, to avoid differences in measured concentration that are not due to the fundamental differences in scattering. All measurements were recorded or merged with a time resolution of 1 min, equivalent to a horizontal resolution of about 300 m with a 10 knots speed of the ship. The DIAL system was used to monitor range-resolved vertical SO2 concentration profiles along the ship traverses. A large number of such measurements were used to produce a 2-D plot of the gas plume, as shown in Fig. 5 for two different occasions during August 26, 2000. Integrated horizontal and vertical profiles are also given. The gas distributions are obtained from raw data of the type shown in Fig. 6 for the on- and off-resonance wavelengths. Analysing the plume shape, it can be concluded that the horizontal extension of the plume changed by as much as a factor of two during the day, while the vertical extension tends to be the same. The 2-D plots produced by the DIAL system were used in the model to predict the overestimation made with passive monitoring systems. The DIAL system has the unique property of yielding the height resolved concentration profile. However, the vertical integrated concentration or column content was used in the comparison with the other systems, only measuring this quantity. An example of an inter-comparison on a traverse under the Mt. Etna plume in the morning (09:30–10:30 local daylight saving time, 36–461 sun altitude angle) is shown in Fig. 7. The results are compared point-by-point by making scatter plots of the COSPEC versus DIAL and DOAS versus DIAL data. In this way, any offset in the passive instruments can be eliminated and the differential response is revealed from the slope K of the trendline. As a matter of fact, we note a clear off-set problem for both passive instruments in the particular scan. It is not necessary that the complete plume be fully traversed for this evaluation. DOAS measurements were performed alternatively employing the sky radiation as a distributed light source (with the spectrometer input fibre fixed at the focus of the vertically looking DOAS telescope) or the sun disc as a well-defined physical ‘‘lamp’’ (with the fibre pointing directly towards the sun using the manual sun-tracking arrangement). The location of the probed air volume will of course be different in the sun-tracking mode with respect to the DIAL. In Fig. 8, the results from DIAL and sun-tracking DOAS are compared for a traverse during the afternoon. The DOAS SO2 column content values have been corrected for the sun altitude angle of 48–571 during the measurement. The sun was towards the stern of the ship causing the horizontal displacement of the plume centre of gravity. In these measurements a point-by-point inter-comparison is not possible but the total integrated plume content has to be used. Monte Carlo simulations to obtain the average path length were performed for various plume widths and heights above the ground. The ratio between the average path length and the thickness of the plume was interpreted as corresponding to the ratio of the measured burden to the true burden for the passive instruments. The simulations were done with the detection point at different positions in relation to the plume edges, emulating a scan across the plume. In all simulations, 107 photon packets were launched. Irregularities in the data are caused by the stochastic nature of the simulations. Typical simulated scans are shown in Fig. 9, for two plumes of widths 6 and 14 km, respectively. For each plume, scans for three solar altitude
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Fig. 5. Distribution of SO2 in the volcanic plume as recorded by the off-axis DIAL system during two different occasions. (August 26, 2000, starting at 11:45 and 15:27 local time, respectively). Integrated horizontal and vertical profiles are also shown.
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Fig. 6. On- and off-resonance lidar curves recorded with the off-axis geometry for the vertically sounding lidar system. The curves are shown after distance-square compensation and logaritmation. For each of the curves, data from 300 laser shots were averaged and the total time for the recording took 1 min.
angles are shown: 251, 451 and 651. The solar azimuth angle was perpendicular to the plume, resulting in the skewed symmetry of the curves. Similar curves for a solar angle parallel to the plume would be symmetric. To emulate the normal case of measurement where the total integrated burden from the scan is calculated, the integrated ratios of the scans were computed. To test the hypothesis that the plume width influences the ratio, simulations for various widths were performed. The two cases of perpendicular and parallel solar azimuth angles are shown in Fig. 10, for solar altitude angles from 251 to 751. Results from simulations for various plume heights are shown in Fig. 11. The plume thickness was 1 km, the width was 10 km, and solar altitude angles varied from 251 to 751. The influence on the measurement of the aerosol concentration outside the plume is quite low. If the aerosol concentration below the plume was increased to the double value or was reduced to half the value in the calculations, it was found that the theoretical ratio changed by about 0.1 units. In Fig. 12a, the different measurement results during the measurement day at Mt. Etna are compared with the model taking into account different scattering situations as a function of the geometry, sun altitude and azimuth angle. The ratios between the recorded values for the three different passive techniques and the DIAL data are shown. What is plotted for COSPEC/DIAL and DOAS(sky)/DIAL is actually the fitted K value of the scattering plot as defined above. Thus, we did not have to consider the true out-of-plume values, which were not always available. Each data entry corresponds the value obtained from a single plume scan. In particular, the two K-values corresponding to average time 10:00 were marked, i.e. the data entries from Fig. 7. The two datapoints for DOAS(sky)/DIAL correspond to the total integrated plume value ratios. The highest sun altitude angle is 62.71 at 13:00 local time. A similar evaluation was also made on older DOAS(sky)/DIAL data from a previous
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Fig. 7. Comparison of data obtained with the different measurement systems in traverses under the Mt. Etna plume (August 26, 2000, 09:30–10:30 local time). Correlation diagrams (DOAS versus DIAL and COSPEC versus DIAL) are shown, illustrating systematic effects related to atmospheric scattering.
campaign in 1997 (Fig. 12b). Finally, in Fig. 12c the results from the measurements at the much lower plume from Stromboli are displayed. The DIAL system was not in operation during the Stromboli measurements, so here the sun-tracking DOAS data are used as the indicator of the true plume content. As can be seen, even if the model does not match perfectly the individual measurement results, the overall trend towards higher ratios at lower solar angles is in good agreement (Fig. 12a and b). The lack of trend in the Stromboli data (Fig. 12c) is also in agreement with the model. Scattering in the experimental data could be explained from variations in cloud/haze conditions.
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Fig. 8. Simultaneous recording of the Etna plume with DIAL and sun-tracking DOAS system (August 26, 2000, 14:20–15:20 local time).
Fig. 9. The model results of the ratio of the average path length to the plume thickness, shown as a scan across the plume. Two plumes of different width are shown: 6 and 14 km, for three solar altitude angles: 251, 451 and 651. The thickness of the plumes was 1 km. The solar azimuth angle was perpendicular to the plume.
The purpose of these experiments was primarily to compare the results of active and passive monitoring techniques and to create a scattering model for a better understanding of the differences encountered. However, the data can also be used to estimate the SO2 flux from the volcanoes. Using the wind data that were available, and correcting for the overestimation in the passive instruments, the SO2 flux from Etna was 1800 tons/day, and from Stromboli 300 tons/day, during the measurement days in August 2000.
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Fig. 10. The model results of the integrated ratio across the plume as a function of the plume width. The plume thickness was 1 km, and the height to its lower boundary 3 km. In (a) the solar azimuth angle was perpendicular to the plume, while in (b) it was parallel. Each curve represents a solar altitude angle, ranging from 251 to 751 in steps of 101.
6. Discussion By analysing the data obtained, we notice that there is a general agreement between the DIAL and the sun-tracking DOAS data, while the COSPEC and the vertically looking DOAS system employing the diffuse sky radiation also show internal agreement. However, the two groups of data exhibit a discrepancy by a factor of about 0.9–1.6, with the instruments employing diffuse sky light usually overestimating the total overhead SO2 burden. This is in agreement with earlier observations [6,8]. The simulated scans in Fig. 9 show that the scattering induces edge effects that may be significant, especially for low solar altitude angles. The simulations suggest that when looking at pointwise measurements, solar altitude angle, solar azimuth
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Fig. 11. The model results of the integrated ratio across the plume as a function of the plume height above the surface. The solar azimuth angle was perpendicular to the plume. Each curve represents a solar altitude angle, ranging from 251 to 751 in steps of 101.
angle, and plume width all may have large influence on the measurement. The situation is not so dramatic when considering the total integrated scans, however. The simulations in Fig. 9 show that for the plume widths relevant for the Mt. Etna measurements, approximately 7–16 km, the most significant factor determining the ratio is the solar altitude angle. Nevertheless, edge effects may still play an important role when performing total integrated scans, since the effect may give rise to a signal even as far as 3–5 km from the plume edge. This means that one has to be careful to be outside the range of the plume influence when calibrating the instruments and when starting and stopping the scan. In reality, the plume is gradually getting thinner near the edges which will be likely to reduce the importance of edge effects, but some residual signal is likely to be found up to a few kilometres outside the physical boundaries of the plume. For the comparison between the DIAL, DOAS and COSPEC measurements, the total integrated measurements could not be used, and instead the mean value of ratios taken along certain fractions of the scans were calculated utilising the correlation diagrams discussed before. Even though edge effects are automatically included, the plume geometry may still be of concern. A cross-section of the real plume certainly does not resemble a rectangle, and it is possible that irregularities in the shape will induce errors. Both the measurements from Stromboli and the simulations show that the ratio is close to unity when the plume is located very close to or in direct contact with the surface (see Fig. 12). A conclusion that may be drawn from this result, and which may seem counter-intuitive, is that a layer of scattering atmosphere below the plume is necessary for the ratio to increase significantly. The solar azimuth angle was of no major importance for these results.
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Fig. 12. Comparisons between the scattering model and results of different measurements as a function of time: (a) Mt. Etna, August 26, 2000; (b) Mt. Etna, August 17, 1997; (c) Stromboli, August 23, 2000.
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7. Conclusion Comparisons of different measurement techniques for volcanic plumes were successfully performed during several campaigns. A scattering model has been developed using Monte Carlo simulations of traced photon histories. The model extends the work presented in Ref. [5] for industrial plumes at low altitudes. Solutions were found to the transport equation for the atmospheric scattering conditions pertaining to the measurement situation which provide the necessary correction factors to allow straightforward and convenient passive measurements to improve the volcanic SO2 assessment. The modelling can be further improved by simulations of the exact plume shape in 3-D yielding even better accordance, but the expected error reduction is in the order of the present measurements error; furthermore, the result cannot be used by the passive instrument users due to the fact that it is difficult to access the 2-D geometry surface without an active system. Clearly, the correction factors will depend on the scattering conditions, which gives a residual uncertainty. Of the passive techniques, DOAS measurements using the solar disc as the light source will give much more reliable results. The active DIAL is the most general technique of wide applicability, but also by far the most expensive one.
Acknowledgements The authors are very grateful to Captain Emanuele Gentile and his crew for efficient and flexible navigation of the Urania. This work was supported by the Swedish Natural Sciences Research Council (NFR), the Knut and Alice Wallenberg Foundation and the Italian National Research Council (CNR). The authors are very grateful to Thomas Johansson for assistance during the measurements.
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