Abstract:
Methods for measuring emissions of gaseous substances to the atmosphere using scattered sunlight spectroscopy and an optical measuring device are disclosed in which the device includes a telescopic member defining a field-of-view of the optical measuring device and a scanner for controlling variation of the direction of the field of view to scan a predetermined layer of the atmosphere, the method comprising scanning the field-of-view to scan the predetermined layer of the atmosphere in the form of at least a part of a cone having its apex positioned at the optical measuring device and having a cone angle β. Optical measuring devices themselves are disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for measuring emissions of gaseous substances to the atmosphere using scattered sunlight spectroscopy and an optical measuring device. The present invention also relates to an optical measuring device for use in such a method. 
       BACKGROUND OF THE INVENTION 
       [0002]    Gaseous substances are constantly emitted to the atmosphere from both anthropogenic sources, such as vehicles and industries, and natural sources, such as volcanoes. Many of these substances have an effect on human health and/or on the global climate, the protective ozone layer in the stratosphere or other environmental effects. In order to study such effects and to quantify different sources etc., it is important that there are suitable equipment and methods available for measuring emissions and flux of such gaseous substances. The principles of some different methods are described in e.g. WO 02/04902. 
         [0003]    Measurement of gases from volcanoes are important not only for studying their environmental impact and their effect on atmospheric chemistry, but also with regard to geo-science, as the gases emitted from a volcano carry important information on the geophysical and geochemical status of the interior of the volcano, as well as to risk mitigation/assessment, since the gas emission and composition adds to our understanding of the volcanoes status. 
         [0004]    In measurements of gases emitted from relatively well demarcated sources, such as discharges from industries and volcanoes, the emissions can be measured at, or at least relatively close to, the source. In such measurements methods based on scattered sunlight spectroscopy are useful. In principle, scattered sunlight spectroscopy involves the use of a spectrometer to record the light of the zenith sky, i.e. the sky straight above the spectrometer, wherein an integration is made over the whole of the cross section of the emitted air mass. This results in a spectrum of the zenith sky including the vertically integrated concentration of the gases which are present in the atmosphere. By moving the spectrometer in such a way that the vertical layer that is measured cuts the emitted gas mass, e.g. an emission plume from a source, it is possible, after geometrical correction for deviations in direction of movement from a direction perpendicular to the propagation of the gas mass and after correcting for background concentration, to determine the integrated concentration over a cross section of the emitted gas mass. The emission is obtained after multiplying this integrated concentration with the concentration-weighed wind speed 
         [0005]    A scattered sunlight spectroscopy instrument that has been used extensively in studies of volcano emissions is the so-called COSPEC-instrument, which is an optical remote sensing instrument developed in the 70&#39;s. It consists of a spectrometer that receives scattered UV-light from a narrow solid angle of the blue sky, typically at zenith. SO 2  has a characteristic absorption spectrum around 300 nm. By means of a mask correlation technique the instrument is able to derive the total column of SO 2  molecules along the vertical path or column defined by the instruments field-of-view (FOV). Thus the vertically integrated number of SO 2  molecules in a segment of a volcanic plume may be derived. By installing the instrument on a moving platform, and move the platform in a direction such that the instruments FOV cuts the volcanic gas plume, the total number of SO 2  molecules in a cross-section perpendicular to the direction of propagation of the plume may be derived. Multiplying this number with the molecular weight and the wind-speed gives the mass-flux in kg/s. The COSPEC-instrument has the advantage that the measurements can be made remotely at distances of several km from the source and that mass flux can be derived. For these reasons it has become the most important technique for volcanic gas emission monitoring. 
         [0006]    Technological development during the recent decades has resulted in sensitive and fast multi-channel array detectors, powerful computers, and algorithms for modelling of radiative transfer and accurate analysis of differential absorption spectra. This has led to an alternative to the COSPEC-instrument: a miniature fiber optic ultraviolet differential optical absorption spectrometer: the mini-DOAS, which is described in Galle B., Oppenheimer C., Geyer A., McGonigle A., Edmonds M. and Horrocks L., “A miniaturised ultraviolet spectrometer for remote sensing of SO 2  fluxes: a new tool for volcano surveillance”.  Journal of Volcanology and Geothermal Research , Volume 119, Issues 1-4, 1 Jan. 2003, Pages 241-254. The basic mini-DOAS system consists of a pointing telescope fiber-coupled to a miniature spectrograph. Ultraviolet light from the sun, scattered from aerosols and molecules in the atmosphere, is collected by means of the telescope (length 10 cm, diameter 3 cm) with a quartz lens defining a field-of-view of 8 mrad. Light is transferred from the telescope to the spectrometer by means of a 2 m long optical quartz fiber with 800 μm diameter. The spectrometer uses a 2400 lines/mm grating combined with a 50 μm slit, providing an optical resolution of ˜0.6 nm over a wavelength range of 245-380 nm. The software used for controlling the spectrometer as well as for evaluating the spectra is a custom-built program. 
         [0007]    In a typical zenith sky measurement the mini-DOAS is carried by car or foot, and spectra are recorded whilst the instrument is moved under the volcanic gas plume in a direction approximately perpendicular to the plume transport direction. The platform position is tracked using a GPS receiver making it possible to geometrically correct for deviations in movement from the ideal direction perpendicular to the plume transport direction. In this manner the total number of molecules in a thin layer of the plume can be determined, and after multiplication with wind speed at plume height, the mass flow of the emission can be obtained, e.g. in kg/s. Besides being a cost-effective alternative to the COSPEC, the introduction of the mini-DOAS represented a major step forward in terms of field-operability, mobility and flexibility. The small size and power consumption of the device made possible measurements in hitherto inaccessible areas and situations. 
         [0008]    The mini-DOAS has been further developed in that the instrument was coupled to a scanning device consisting of a quartz-prism attached to a computer-controlled stepper-motor, providing a means to scan the field-of view of the instrument over 180°, see Edmonds M., R. A. Herd, B. Galle and C. Oppenheimer, “Automated, high time-resolution measurements of SO 2  flux at Soufrière Hills Volcano, Montserrat”,  Bulletin of Volcanology , Vol 65, 578-586, 2003. In a typical measurement the instrument is located under the plume, and scans are made, from horizon to horizon, in a plane perpendicular to the wind-direction. Thus, instead of moving the measurement equipment under the gas plume, this version of the mini-DOAS is capable of scanning a plane in the sky from a stationary position. Of course, this feature simplifies the measurement considerably. A stepwise scan of the plane typically use a 10 seconds integration time with 7.5° angular resolution, providing a full emission measurement every 4 minutes. The scanning mini-DOAS provides time resolved measurement of the gas emission, making it possible to correlate gas emission data with other geophysical data, e.g. seismic signals. A similar approach but using a mirror instead of a prism in the scanning device has also been used for volcanic gas measurements, McGonigle A. J. S., C. Oppenheimer, A. R. Hayes, B. Galle, M. Edmonds, T. Caltabiano, G. Salerno and M. Burton, “Volcanic sulphur dioxide flux measurements at Etna, Vulcano and Stromboli obtained using an automated scanning ultraviolet spectrometer”,  Journal of Geophysical Research , vol 108, No B9, 2455, 2003. 
         [0009]    As mentioned above one of the main advantages of the scanning mini-DOAS is that it is a stationary instrument that does not have to be moved during the measurement. However, this also means that the emission can only be measured when the wind direction is such that the gas plume from the volcano is passing over the instrument within a certain angular interval around zenith. If the wind direction changes such that it falls outside this interval the gas plume will, at least partly, be intersected by the instruments field-of-view at too high zenith angles to facilitate a good measurement. Typically it will not be able to determine unambiguously the clean air background below the plume. An effect of this is that it becomes difficult to evaluate the integrated concentration in the gas plume which leads to an unreliable determination of the amounts of emissions in the plume. In order to solve this problem efforts have been made to improve the mathematical algorithms for calculating the area of such incomplete plume scans by assuming certain plume dispersion characteristics and applying meteorological dispersion models. Another, more practical way, to solve this problem is to move the instrument or to install a number of instruments around the emission source such that the measurement ranges of two adjacent instruments overlap. 
         [0010]    Another problem related to this measurement strategy is caused by scattering of light in the lower part of the atmosphere. In the described measurement strategy it is assumed that all the light that reach the instrument originates from a volume above the gas plume to be studied. If the Sun is located at an angle off zenith, direct light from the Sun may be scattered by aerosols and molecules located between the instrument and the gas plume, into the field-of-view of the instrument. This part of the recorded light has never passed the gas plume and thus causes a “diluted” spectrum, giving rise to an underestimation of the gas content in the plume. This problem gets worse the more aerosols (haze, particles, ash, rain, etc.) there is in the lower atmosphere. It also gets worse when the plume is intersected at high zenith angles (close to the horizon) and when the distance between the instrument and the gas plume is long. 
         [0011]    To calculate the emission using the above mentioned methods, a knowledge of the transport speed of the gas plume is necessary. This can be obtained either by meteorological modeling, by extrapolating meteorological measurements made elsewhere or by measurement. A method for measuring the transport speed of the gas plume directly has been developed using DOAS techniques. In this method two DOAS instruments, similar to the systems described above, are located under the gas plume and made to point in two different directions, one beam upwind and the other downwind the plume, along the plume propagation direction. A time series of total column variations are registered in both directions, and from the temporal delay in variations in the total column, the wind-speed can be calculated if the plume height is known. Instead of two separate DOAS instruments one single system may be used where a flipping mirror instead alternates the pointing direction of the system between two directions. Ideally the parameter to be measured is the wind speed at plume height, weighted by plume concentration. As this approach uses variations in the total column of SO 2  to determine the plume speed, a weighting with concentration is automatically made. Examples of wind measurements using this approach at Etna Volcano on Sicily and Popocatepetl Volcano in Mexico are given in Galle B., M. Johansson, C. Rivera and Y. Zhang, “Dual-Beam mini-DOAS spectroscopy, a novel approach for volcanic gas emission monitoring”,  EGU General Assembly , Vienna, 24-29 Apr. 2005. 
         [0012]    One object of the present invention is to provide a method and a device for measurements of the type described above, which method and device exhibit improved properties compared to conventional instruments and methods, in particular with regard to changing wind directions. 
       SUMMARY OF THE INVENTION 
       [0013]    In accordance with the present invention, this and other objects have now been realized by the discovery of a method for measuring emissions of gaseous substances to the atmosphere using scattered sunlight spectroscopy and an optical measuring device comprising a telescopic member defining a field-of-view of the optical measuring device, and a scanner for controlling the variation of the direction of the field-of-view to scan a predetermined layer of the atmosphere, the method comprising scanning the field-of-view to scan the predetermined layer of the atmosphere in the form of at least a part of a cone having its apex positioned at the optical measuring device and having a cone angle β. Preferably, the cone angle β is up to 80°, more preferably at least 20°. In a preferred embodiment, the angle β is from 40 to 70°. 
         [0014]    In accordance with another embodiment of the method of the present invention, the cone has its axis of symmetry in a horizontal direction. In another embodiment, the cone has an axis of symmetry exhibiting an inclination angle (δ) relative to a horizontal plane. Preferably, the inclination angle (δ) is 90°−β. 
         [0015]    In accordance with the present invention, this and other objects have now been overcome by the discovery of an optical measuring device for measuring emissions of gaseous substances to the atmosphere using scattered sunlight spectroscopy, the device comprising a telescopic member defining a field-of-view of the measuring device, and a scanner for controlling the variation of the direction of the field-of-view to scan a predetermined layer of the atmosphere, the scanner being adapted to scan the predetermined layer of the atmosphere in the form of at least a part of a cone having its apex positioned at the optical measuring device and having a cone angle β. Preferably, the cone angle β is up to 80°, and in another preferred embodiment, the cone angle β is at least 20°. In a preferred embodiment, the cone angle β is from 40 to 70°. In accordance with another embodiment of the device of the present invention, the scanner comprises a light refracting member arranged to deflect the field-of-view by directing a portion of incoming sunlight towards the telescopic member. Preferably, the light refracting member is arranged in such a way that the non-deflected field-of-view and the deflected field-of-view form the angle β. In another embodiment, the light refracting member is rotatably arranged in relation to the telescopic member whereby rotation of the light refracting member allows the field-of-view to scan the predetermined layer of the atmosphere. In a preferred embodiment, the device includes a motor having a motor axis, and the light refracting member is arranged on the motor axis connected to the motor. In another embodiment, the light refracting member is a mirror. In yet another embodiment, the light refracting member is a prism. 
         [0016]    In accordance with another embodiment of the device of the present invention, the device includes a light analysis unit optically connected to the telescopic member. Preferably, the light analysis unit comprises a spectrometer. 
         [0017]    In accordance with another embodiment of the device of the present invention, the telescopic member comprises a light focusing member adapted to focus light that enters the telescopic member and an opening adapted to direct the focused light out from the telescopic member. Preferably, the light focusing member is a lens. In another embodiment, the light focusing member is a mirror. 
         [0018]    The present invention concerns a method for measuring emissions of gaseous substances to the atmosphere using scattered sunlight spectroscopy and an optical measuring device, said device comprising a telescopic member defining a field-of-view (FOV) of the device, and a scanning arrangement allowing a controlled variation of the direction of the field-of-view (FOV) in such a way that the field-of-view (FOV) is capable of scanning a certain layer ( 12 ,  22 ) of the atmosphere during operation of the device. The inventive method is characterized in that it comprises the step of scanning a scanning layer that has the form of, at least a part of, a cone having its apex positioned at the device and having a cone angle β. 
         [0019]    Such a scanning layer is capable of being directed towards an emission source and partly surrounding the source resulting in several advantageous effects. One advantage is that it increases the wind direction interval within which a reliable measurement can be performed. A consequence of this is that a wider range of wind directions can be covered by a single instrument, or alternatively that a smaller number of stationary measuring devices are required to cover all possible wind directions around an emission source. Another advantage is that the distance between the measuring device and the location where an emission plume is scanned is decreased which improves the reliability of the measurement as the previously described scattering effects are reduced. This is of particular importance when the wind direction deviates considerably from a direction right above the place where the measuring device is positioned. 
         [0020]    In a preferred embodiment of the inventive method, the cone angle β is ≦80° and ≧20°. Preferably, the cone angle β is in the interval 40-70°. 
         [0021]    The invention also concerns an optical measuring device for measuring emissions of gaseous substances to the atmosphere using scattered sunlight spectroscopy, said device comprising a telescopic member defining a field-of-view (FOV) of the device, and a scanning arrangement allowing a controlled variation of the direction of the field-of-view (FOV) in such a way that the field-of-view (FOV) is capable of scanning a certain layer of the atmosphere during operation of the device. In the inventive device, the scanning arrangement is adapted to form a scanning layer that has the form of, at least a part of, a cone having its apex positioned at the device and having a cone angle β. 
         [0022]    In a preferred embodiment of the inventive device, the scanning arrangement comprises a light refracting member arranged to deflect the field-of-view (FOV) by directing a portion of incoming sunlight towards the telescopic member. Such a design makes it possible to control the light refracting member, instead of the telescopic member, as to vary the direction of the FOV, which is less complicated. 
         [0023]    Preferably, the light refracting member is arranged in such a way that the non-deflected FOV and the deflected FOV forms the angle β. No further parts are thus needed for achieving this angle. 
         [0024]    In a further preferred embodiment of the inventive device, the light refracting member is rotatably arranged in relation to the telescopic member in such a way that rotation of the light refracting member allows the field-of-view (FOV) to scan the scanning layer. Preferably, the light refracting member is arranged on a motor axis connected to a motor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    In the description of the present invention set forth in the following detailed description, reference is made to the following figures, in which: 
           [0026]      FIG. 1  is a front, perspective, schematic view of a principal illustration of the use of a prior art optical measuring device, 
           [0027]      FIG. 2  is a front, perspective, schematic view of a principal illustration of the use of the method and device of the present invention, 
           [0028]      FIG. 3  is a graphical representation of an example of the measurement results obtained with the prior art device according to  FIG. 1 , 
           [0029]      FIG. 4  is a graphical representation of an example of the measurement results obtained with the inventive method and device according to  FIG. 2 , 
           [0030]      FIG. 5  is a schematic, representational view of an example of how the present invention can be used to surround an emission source, 
           [0031]      FIG. 6  is a top, schematic, representational view of the example of  FIG. 5  as seen from above, together with a similar view of a prior art device, 
           [0032]      FIG. 7  shows another example of the use of the invention, 
           [0033]      FIG. 8  is a schematic, representational view of the use of four prior art devices arranged around an emission source, 
           [0034]      FIG. 9  is a schematic, representational view of the use of four devices of the present invention arranged around an emission source, 
           [0035]      FIG. 10  is a top, elevational, schematic view of a preferred embodiment of the optical measuring device of the present invention, and 
           [0036]      FIG. 11  is a graphical representation of the present invention actual measurement results obtained with a prior art device and with the device of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIGS. 1 and 2  show a 3-d representation of the measurement strategies for the prior art and the present invention, respectively.  FIG. 1  gives a principal illustration of the use of a prior art optical measuring device  10  in the form of a scanning mini-DOAS in measurements of a gas emission from a source in the form of a volcano  5 . In particular,  FIG. 1  illustrates the effect of different plume directions relative to the location of the measuring device  10 . Three different plumes P 1 , P 2  and P 3  represent three different wind directions W 1 , W 2  and W 3 . The first wind direction W 1  is directed from the gas emission source  5  towards the measuring device  10  resulting in that the corresponding plume P 1  passes right above the measuring device  10 . The second wind direction W 2 , and thus the direction of the second plume P 2 , exhibit an angle Φ 1  relative to the direction of W 1  and P 1  such that the second plume P 2  passes above the measuring device  10  somewhat at the side. The third wind direction W 3 , and thus the direction of the third plume P 3 , exhibit an angle Φ 1 +Φ 2  relative to the direction of W 1  and P 1  such that the third plume P 3  passes above the measuring device  10  further at the side.  FIG. 1  further shows a scanning layer  12  which in this case has the form of a vertical plane, and which will be further described below. 
         [0038]    As described further above the prior art scanning mini-DOAS scans the field-of view (FOV) of the instrument step by step from horizon to horizon. In each scanning position, i.e. in each step, the FOV defines a more or less slanting column extending through the atmosphere which columns contain the air to be measured. A number of such columns are indicated by lines  14  in  FIG. 1 . Each of these lines  14  represents a solid angle defined by the FOV of the instrument. For clarity reasons these lines  14  have been given a certain length although the columns or FOV&#39;s strictly speaking have an infinite length. All the columns  14  taken together define the scanning layer  12  which in  FIG. 1  has the form of a vertical plane. Indicated in  FIG. 1  is also a scan step angle α. When the FOV is in a zenithal position, i.e. when it is directed vertically α is 0° and when it is directed horizontally α is either 90° or −90°.  FIG. 1  further shows sectional views of the plumes P 1 , P 2  and P 3  taken along the scanning layer  12 . 
         [0039]      FIG. 2  shows the principles of the present invention in a situation similar to what is shown for the conventional equipment in  FIG. 1 . In the example shown in  FIG. 2  the inventive measuring device  20  is arranged such that in each scanning position the FOV defines a column (indicated by lines  24 ) that exhibits an angle β of 45° relative to a direction defined by an imaginary, horizontal line  21  drawn between the measuring device  20  and the emission source  5 . This results in a curved scanning layer  22  that has the shape of an upper half of a horizontal cone having its axis of symmetry  21  corresponding to the imaginary line  21 , having its apex positioned at the measuring device  20  and having a cone angle β=45°. This means that the cone, and thus the scanning layer  22 , has an opening angle in the horizontal plane, i.e. the angle between the lowest, substantially horizontal, columns  24 ′, that is 2·β=90° and a vertical opening angle, i.e. the angle between the most vertical column  24 ″ and ground, that is β=45°. The scan step angle a has the same meaning in  FIG. 2  as in  FIG. 1 ; when the FOV is in its most vertical position, i.e. when it is directed as indicated by line  24 ″, a is 0° and when it is directed horizontally, i.e. when it is directed as indicated by lines  24 ′, α is either 90° or −90°.  FIG. 2  further shows sectional views of the plumes P 1 , P 2  and P 3  taken along the scanning layer  22 . 
         [0040]    In contrast to the planar scanning layer  12  shown in  FIG. 1 , the scanning layer  22  of the present invention exemplified in  FIG. 2  has an opening angle in the horizontal plane that is less than 180° (which it will have as long as the cone angle β is less then 90°). For this reason the scanning layer  22  becomes funnel-shaped which make it possible to direct the scanning layer  22  towards the emission source  5  and thereby letting the scanning layer  22  at least partly surround the source  5 . This has some advantages which will be described further below. 
         [0041]      FIG. 3  shows a principal absorption column representation of a derived content of a gaseous substance, in this example SO 2 , in a particular direction of the FOV, i.e. in a particular air column, as a function of scan step angle α obtained with a prior art measuring device  10  in a measurement corresponding to the conditions of  FIG. 1 . As plume P 1  passes straight above the measuring device  10  at α=0° the maximum slant column for P 1  is positioned in the middle of the scan with background values on each side of the peak resulting in that a good measurement of the integrated mass in plume P 1  can be performed. Regarding plume P 2  a reasonably good measurement can still be performed as a background measurement is obtained below the plume at about −80°. Regarding plume P 3  no reliable measurement can be performed as only a part of the plume peak is registered, i.e. the FOV is still directed towards the plume at the lowest scan angle which in this example is −80°. A change of the wind direction from W 1  to W 3 , i.e. a change of Φ 1 +Φ 2  degrees, thus makes the measurement unreliable. Considering the peak of P 2  in  FIG. 3  to be sufficiently defined for an acceptable reliability of the measurement the acceptable change in wind direction for a reliable measurement with the prior art mini-DOAS is thus +/−Φ 1  if the measuring device  10  initially is placed right below the plume. 
         [0042]    As a comparison to  FIG. 3 ,  FIG. 4  shows a similar principal absorption column representation as shown in  FIG. 3  but obtained with an inventive measuring device  20  in a measurement corresponding to the conditions of  FIG. 2 . Plume P 1  passes straight towards and above the inventive measuring device  20  such that at α=0° the maximum slant column for P 1  is, in similarity with  FIG. 3 , positioned in the middle of the scan with background values on each side of the peak resulting in that a good measurement of the integrated mass in plume P 1  can be performed. Regarding plume P 2  a good measurement can be performed as a background measurement is obtained below the plume at about −60°. In contrast to  FIG. 3  where only a part of the plume peak of P 3  is registered, in this case also plume P 3  can be reasonably well measured as a background measurement is obtained below the plume at about −80°. Thus, a change of the wind direction from W 1  to W 3 , i.e. a change of Φ 1 +Φ 2  degrees, does not make the measurement unreliable. Considering the peak of P 3  in  FIG. 4  to be sufficiently defined for an acceptable reliability of the measurement the acceptable change in wind direction for a reliable measurement with the inventive measuring device  20  is thus +/−(Φ 1 +Φ 2 ) if the measuring device  20  initially is placed right below the plume. 
         [0043]    Due to topographical and atmospheric radiation limitations, there is normally a lower limit in scan angle (α) below which no reliable slant column density can be derived. Under good conditions this angle is typically 80°-85°. 
         [0044]    As mentioned above, the inventive scanning layer  22  is funnel-shaped which makes it capable of partly surrounding the source  5 . This leads to at least two major advantages. Firstly, it increases the wind direction interval within which a reliable measurement can be performed. This is illustrated in  FIGS. 2 and 4 , as compared to  FIGS. 1 and 3 , showing that also the third plume P 3  now can be measured reliably since background data can be obtained also for scan step angles α below the plume P 3 . This is a major improvement compared to the measurement with the conventional equipment. A consequence of this is that a wider range of wind directions can be covered by a single instrument, or alternatively that a smaller number of stationary measuring devices  20  are required to cover all possible wind directions around an emission source  5 . Secondly, the distance between the measuring device  20  and the location where the plume is scanned is decreased which improves the reliability of the measurement as the previously described scattering effects are reduced. This is illustrated in table 1 where the distance between instrument and gas plume is shown as a function of wind direction deviation Φ for the two different measurement strategies, using as an example a typical volcano with a height of 1 km, located 3 km away and the instrument having a cone angle β=45°. 
         [0045]    Table 1 shows that the distance from the measuring device to the gas plume, for wind direction deviations Φ of more than around 30°, is considerably shortened with the scanning layer  22  according to the invention compared to the scanning layer  12  according to conventional technique. It should be noted that for Φ&gt;80°, the conventional technique starts to get useless, while the inventive technique still give useful data even beyond Φ&gt;100°, thus making possible the total surrounding of a source using only two measuring systems (see  FIG. 5 ). For wind direction deviations Φ of less than around 30°, corresponding to the plume center passing close to zenith, the present invention gives a slightly longer distance between instrument and plume. However, this distance is here still relatively short so this minor disadvantage is well compensated by the strong advantage at larger, more critical, wind direction deviations. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Distance from instrument to the point where 
               
               
                 the measurement intersects the gas plume, for different plume 
               
               
                 direction deviations. Plume height 1 km, distance to volcano 3 km, 
               
               
                 β = 45°. 
               
             
          
           
               
                   
                 Conventional 
                 Invention 
               
               
                 Wind direction 
                 geometry 
                 geometry 
               
               
                 deviation Φ 
                 scan layer 12 
                 scan layer 22 
               
               
                 (degrees) 
                 (km) 
                 (km) 
               
               
                   
               
             
          
           
               
                 0 
                 1.00 
                 1.41 
               
               
                 10 
                 1.13 
                 1.50 
               
               
                 20 
                 1.48 
                 1.69 
               
               
                 30 
                 2.00 
                 1.94 
               
               
                 40 
                 2.72 
                 2.21 
               
               
                 50 
                 3.72 
                 2.51 
               
               
                 60 
                 5.34 
                 2.83 
               
               
                 70 
                 8.32 
                 3.20 
               
               
                 80 
                 17.37 
                 3.65 
               
               
                 90 
                 ∞ 
                 4.26 
               
               
                 100 
                 N/A 
                 5.13 
               
               
                 110 
                 N/A 
                 6.59 
               
               
                   
               
             
          
         
       
     
         [0046]    Another reason for the improvement of the measurement reliability with the new scanning geometry is that, ideally for the flux measurement, and even more important for the plume-speed measurement, the scanning should be made in a plane relatively close to perpendicular to the plume propagation. In table 2 is shown the angle between the horizontal projection of the direction of the instruments FOV and the plume propagation direction, for different plume propagation directions Φ. As the wind direction is deviating more and more from 0, the angle under which the plume is scanned is deviating more and more from perpendicular to the plume propagation. With the inventive scanning layer  22  this effect is strongly suppressed. For wind direction deviations  0  of less than around 60°, the deviation from the ideal perpendicular traverse is still less than 20°, and even for Φ=90° the deviation is not more than 47° with the inventive device. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Angle between the horizontal projection of 
               
               
                 the direction of the instruments FOV and the plume propagation 
               
               
                 direction, for different plume direction deviations. Plume 
               
               
                 height 1 km, distance to volcano 3 km, β = 45°. 
               
             
          
           
               
                   
                 Conventional 
                 Invention 
               
               
                 Wind direction 
                 geometry 
                 geometry 
               
               
                 deviation Φ 
                 scan layer 12 
                 scan layer 22 
               
               
                 (degrees) 
                 (degrees) 
                 (degrees) 
               
               
                   
               
             
          
           
               
                 0 
                 90 
                 90 
               
               
                 10 
                 100 
                 82 
               
               
                 20 
                 110 
                 81 
               
               
                 30 
                 120 
                 86 
               
               
                 40 
                 130 
                 93 
               
               
                 50 
                 140 
                 100 
               
               
                 60 
                 150 
                 109 
               
               
                 70 
                 160 
                 118 
               
               
                 80 
                 170 
                 127 
               
               
                 90 
                 N/A 
                 137 
               
               
                 100 
                 N/A 
                 146 
               
               
                 110 
                 N/A 
                 155 
               
               
                   
               
             
          
         
       
     
         [0047]    In principal, these improvements gradually increase as the cone angle β is reduced from 90°. However, the effect is not likely to be significant when β&gt;80°. For geometrical reasons it is likely that the error in the flux calculations increases when the cone angle β gets too small. Nevertheless, as a low value of β makes it possible to completely surround the source with fewer instruments, low values of β, less than 45°, may still be attractive in some applications where a detection of a dramatic change in emission is interesting, e.g. the opening of a conduit system of a volcano. An example of a surrounding arrangement is given in  FIG. 5 . Here two systems are set up around a volcano at a distance of 2 km. The volcano is 1 km high. The cone angle β was 45° and the scan angle α was ranging between ±70° with 5° steps. As can be seen the two scanning systems puts up an effective “grid” facilitating the detection of gas emission in all possible directions, providing a cost effective way to detect dramatic changes in the gas emission. 
         [0048]    The lower part of  FIG. 6  shows a top view of  FIG. 5  where the intersection between the FOV columns  24 , i.e. the conical scanning layer  22 , and the plume plane at 1 km height are indicated with crosses. As a comparison, the upper part of  FIG. 6  also shows a top view of two conventional measuring devices  10  located in the same positions as the inventive devices in  FIG. 5  and lower part of  FIG. 6 . It is clear that the two parallel, planar, vertical scanning layers  12  formed by the conventional devices do not cover varying wind directions from the volcano  5  as well as the inventive devices. 
         [0049]    In a variant of the inventive cone-shaped scanning layer  22  described above the axis of symmetry of the cone is no longer horizontal but instead have an inclination δ. A special case of this embodiment is when δ=90°−β. Then the scanning direction will point towards zenith when α=0. This may be advantageous in some applications, e.g. if the earlier described dual-beam method is used for plume speed measurements. A further special case of this embodiment is if the cone inclination δ equals the inclination angle from the instrument to the source. In this special case the cone axis point towards the source. This has the additional advantage that the scan makes an almost vertical intersection of the plume when α is around ±90°. It also makes the scan perpendicular to the plume for low scan angles α. This reduces the error in the flux calculation caused by geometrical inaccuracies as well as improves the possibility to make successful measurements using the dual-beam plume speed measurement approach. An example of such a special embodiment set up for measurements of emission from a volcano using two measurement systems is shown in  FIG. 7 . Here ∂=90°−β=30°=inclination angle between instrument and volcano. 
         [0050]    Further advantages of the inventive scanning layer are that it improves the possibility to make 2-d representations of the concentration field of the gas plume, using two or more measuring devices. For calculations of the flux from the data obtained with the instrument a knowledge of the height of the gas plume, or ideally the concentration distribution in the plane of the scanning, is necessary. This is needed for three reasons; Firstly, because the wind speed usually shows a strong gradient with height. Thus knowledge of the plume height is important in order to be able to establish the correct plume speed to calculate the mass-transport. Secondly, when calculating the total number of molecules in a cross-section of the plume each scanning direction represents a certain scanned area along the cone surface. This area increases linearly with the distance from the instrument. Thus, information about the plume height, and ideally also the distribution of the gas concentrations along the scanned cone surface, is important to obtain a correct number of molecules integrated over the scanned area of the plume. Finally information of the plume height is also necessary if the earlier described dual-beam method to derive plume speed is used. Information about the geographical distribution of the gas over the scanned area can be obtained from modelling, e.g. calculation of the expected plume lift, and using an appropriate dispersion model. Alternatively the distribution of the gas concentration over the scanned cone surface may be measured using two or more instruments scanning the plume from different directions, and making a tomographic reconstruction of the concentration field. This technique is relatively straightforward when the two systems are scanning in the same plane. When several systems are installed around a source a major reason for doing so is, however, to be able to cover varying wind directions. To maximize the coverage of different wind directions a more optimal setup is to distribute the measuring devices around the source. A disadvantage with such a setup when using the conventional technique is that the two systems now no longer scan the plume in the same geographical plane, and a tomographic reconstruction of the concentration field gets more complicated and less accurate. With the cone scanning method, choosing appropriate scanning parameters, a higher degree of overlap may be obtained, while also improving the coverage of varying wind directions. It can be shown that at the altitude where the plume intersection is expected, the overlap between the scans from the two instruments is almost perfect. A more extreme demonstration of this advantage, surrounding a volcano with four systems, is shown in  FIGS. 8 and 9 . Here the four systems in the conventional configuration (see  FIG. 8 ) are scanning orthogonal to each other, with no overlap at all, while the inventive configuration (see  FIG. 9 ) still shows an almost perfect overlap. 
         [0051]      FIG. 10  shows a preferred embodiment of an optical measuring device  20  according to the invention. The device  20  comprises a telescopic member  30  provided with a lens  39  and a focal point  40  at which an optical fiber  33  is attached as to optically connect the telescopic member  30  to a spectrometer  31  via its entrance slit  41 . The telescopic member  30  defines an optical axis  38  and a field-of-view (FOV)  24  directed towards a scanning arrangement  42 . The scanning arrangement  42  comprises a computer-controlled stepper-motor  36  that, via a motor axis  35 , is connected to a mirror  32  mounted onto a supporting member  34 , wherein the mirror  32  faces the telescopic member  30  as to establish an optical connection. The mirror  32  is arranged so that the direction of the FOV  24  forms an angle β with the optical axis  38 . A certain portion of scattered sunlight  37  falling within the FOV  24  into the mirror  32  will thus be directed towards the telescopic member  30  and thus to the spectrometer  31 . When the motor axis  35 , and thus the mirror  32 , is rotated the FOV  24  of the telescopic member  30  defines the scanning layer  22  (see  FIG. 2 ) in the atmosphere which layer  22  forms a part of a cone with an axis of symmetry along the optical axis of the telescopic member  30 , a top at the mirror  32  and a cone angle β. 
         [0052]    If the measuring device  20  is used in a horizontal direction, i.e. if the optical axis  38  and the co-axially arranged motor axis  35  are horizontal, the symmetry axis of the scanning cone will also be horizontal. As mentioned above, the measuring device  20  can however be tilted vertically with an angle δ, resulting in that the symmetry axis of the scanning cone gets a vertical inclination δ. 
         [0053]    From geometrical laws it follows that the mirror  32  exhibits an angle γ with a plane  44  perpendicular to the motor axis  35  and the optical axis  38  where γ=β/2. It also follows that the mirror  32  exhibits an angle with the axes  35 ,  38  that is equal to (90−γ)°. In this example β=45° which means that γ=22.5°. 
         [0054]    An instrument based on the above mentioned ideas has been built and its performance was tested in a field test on the gas-plume from the active&#39;volcano Popocatepetl in Mexico. The measuring device  20  used in the field test consists of a small telescope  30  (quartz lens, focal length=50 mm, diameter=20 mm, field of view approximately 0.4°) coupled to a quartz fiber bundle  33 , which transmits the light  37  into a commercial miniature fiber optic spectrometer  31  (OceanOptics Inc., USB2000). The telescope  30  is optically connected to a scanning device  42  consisting of a mirror  32  attached to a computer-controlled stepper-motor  36 , providing a means to scan the field-of view  24  of the instrument over 180°, i.e. the motor axis is rotated 180°. The mirror  32  was positioned such that an angle β was formed with the optical axis  38  as shown in  FIG. 10  as to form a cone shaped scanning layer  22 . In this particular experiment the angle β was 45°. Typically a seconds integration time was used, with 7.5° angular resolution, providing a full emission measurement every 4 minutes. The 2 m long quartz fiber bundle  33  consisted of four individual 200 μm core diameter fibers, arranged in rectangular configuration in the focal point  40  of the telescope  30  and a linear arrangement at the spectrometer entrance slit  41 . A Hoya U330 filter blocked the visible light &gt;400 nm to reduce the stray light in the spectrometer. The USB2000 spectrometer  31  used was a crossed Czerny-Turner arrangement (1/f=2.2, UV grating 2400 grooves/mm) with a CCD detector (2048 elements at 12.5 μm centre−centre spacing) coupled to a 12-bit ADC, which connects to a PC via a serial RS232 interface. The quartz fiber  33  transmits the light to the entrance slit  41  (width 50 μm), which guarantees a spectral resolution of 0.7 nm. The wavelength region observed by the detector was set from 245 nm to 380 nm. The wavelength to pixel mapping as well as the instrumental line shape was determined by taking a spectrum of a low-pressure mercury emission lamp. The entire system (notebook PC, spectrometer and stepper motor) operates on 12 V for about 24 hours from a standard car battery. The known Differential Optical Absorption Spectroscopy (DOAS) technique was applied to identify and quantify the atmospheric trace gases by their specific narrow band absorption features in the UV and visible spectral regions. 
         [0055]    The fundamental equation of absorption spectroscopy is the Beer-Lambert&#39;s Law. It describes the decrease of light intensity while light passes matter. 
         [0000]        I (λ, L )= I   0 (λ)* e   −S*σ   
         [0000]    I 0  (λ)—light intensity outside the gas layer to be measured
 
I (λ, L)—light intensity after passing the gas layer to be measured
 
λ—wavelength of the radiation
 
c (l)—concentration of the trace gas as function of the position l along the light path
 
σ(λ, T)—trace gas absorption cross section
 
T—temperature
 
L—length of light path
 
         [0000]    
       
         
           
             S 
             = 
             
               
                 
                   ∫ 
                   0 
                   L 
                 
                  
                 
                   
                     c 
                      
                     
                       ( 
                       l 
                       ) 
                     
                   
                    
                   
                       
                   
                    
                   
                      
                     
                       l 
                        
                       
                         ( 
                         
                           
                             = 
                             
                               c 
                               * 
                               L 
                             
                           
                           , 
                           
                             
                                 
                             
                              
                             
                                 
                             
                           
                            
                           
                             
                               if 
                                
                               
                                   
                               
                                
                               c 
                             
                             = 
                             
                               const 
                               . 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
                
               
                   
               
               - 
               
                   
               
                
               
                 Definition 
                  
                 
                     
                 
                  
                 of 
                  
                 
                     
                 
                  
                 slant 
                  
                 
                     
                 
                  
                 column 
                  
                 
                     
                 
                  
                 density 
                  
                 
                     
                 
                  
                 
                   ( 
                   
                     S 
                      
                     
                         
                     
                      
                     C 
                      
                     
                         
                     
                      
                     D 
                   
                   ) 
                 
               
             
           
         
       
     
         [0056]    SCD were directly determined here as results of the measurements. Fluxes can however be calculated using further estimates. Automatic data acquisition, real time spectral evaluation and flux calculations was performed by a home-made C-program running on a notebook computer under the WINDOWS operating system. 
         [0057]      FIG. 11  shows some results from the testing of the inventive measuring device  20 . For comparison, tests were also made with a conventional instrument, using a flat vertical scanning plane  12  passing zenith, operated in parallel from the same location.  FIG. 11  shows the results from the different devices in the form of a representation of SO 2  in a particular air column/FOV-direction  14 ,  24  as a function of scan step angle α as given by the extent of rotation of the motor axis  35 . Note that in contrast to  FIGS. 3 and 4 , the x-axis of  FIG. 11  starts at 0°. Thus only “half” scans was performed, starting at zenith going down to one horizon. Over a time period of 76 minutes  22  simultaneous scans was performed and the averaged results are shown. The averaged results obtained with the inventive measuring device  20  are given by solid lines  120  and the results obtained with the conventional equipment are given by dashed lines  110 . The approximate centre position of each peak is indicated by arrows. At the time of the experiment the wind direction was such that the volcano plume passed the measuring device at some distance at the side, in similarity with plume P 3  in  FIGS. 1 and 2 .  FIG. 11  clearly shows that the peak recorded with the inventive measuring device is positioned sufficiently far away from the end of the scan allowing a reliable measurement. In contrast, the peak recorded with the conventional equipment is too close to the end of the scan to allow for a reliable measurement. 
         [0058]    The present invention is not limited by the embodiments described above but can be modified in various ways within the scope of the claims. For instance, a variation of the direction of the field-of-view (FOV) can be achieved by a computer controlled variation of the position of the telescopic member  30 . Alternatively, a computer controlled mirror could be used instead of mounting the mirror  32  to a rotatable motor axis  35 . 
         [0059]    Moreover, the spectrometer  31  may be replaced by another light analysis unit by which the slant column density of a gas or aerosol may be determined. In some applications it may for instance be sufficient to be able to detect a certain, narrow wavelength interval. 
         [0060]    Further, a mirror could be used instead of the lens  39  as light focusing member, and a prism could be used instead of the mirror  32  as light refracting member. The optical fiber is optional; the spectrometer  31  or other light analysis unit may be connected directly to the telescopic member  30 . 
         [0061]    It should also be noted that it is not necessary that the measurement is carried out stepwise; the scan angle α can be varied more or less continuously as to let the FOV  24  continuously scan the scanning layer  22 . 
         [0062]    In a preferred variant of the inventive device the mirror  32  is adjustable such that the angle β becomes adjustable. 
         [0063]    The angles can vary within their defined ranges; −180°≦α≦180°, 0°≦β&lt;90°, −90°≦δ≦90°. Although the inclination angle δ will be ≧0° in most applications, i.e. the cone is either horizontal or has its wider part pointing upwards as to point towards a source positioned at a higher level, it is possible to use a negative δ, i.e. where the cone has its wider part pointing downwards. It may also be noted that the horizontal pointing direction of the cone, i.e. the direction of the symmetry axis of the cone as projected onto the horizontal plane if the cone is inclined relative to this plane, does not necessarily have to be identical to the horizontal direction between instrument and source. For instance, if the view towards the source for topographical reasons is obstructed, one may “aim” with the cone at the side of the source. 
         [0064]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.