AU2005205610A1 - Sulphur dioxide detection method - Google Patents

Description

WO 2005/068977 PCT/AU2005/000035 SULPHUR DIOXIDE DETECTION METHOD Field of the Invention 5 The invention relates to a sulphur dioxide detection method and apparatus. Background to the Invention 10 Volcanic ash and sulphur dioxide clouds constitute a serious hazard to aircraft even after the clouds have moved from the site of a volcanic eruption. Apart from containing ash particles, the clouds include gases such as SO 2 which after a few days oxidises and 15 hydrolises to form sulphuric acid droplets, either as an ash-acid mixture or as a coating over ash particles. Both the ash particles and the sulphuric acid droplets of volcanic ash clouds are capable of causing significant damage to and possible loss of an aircraft which 20 encounters an ash cloud. A number of aircraft encounters with volcanic ash clouds or sulphur dioxide clouds have been recorded in the past where significant damage has occurred. It will be 25 appreciated that the sulphur dioxide may be found in areas separate from the volcanic ash. In the year 2000, a National Aeronautics and Space Administration (NASA) DC-8 Airborne Sciences research airplane flew through what was described as a diffuse volcanic ash cloud from the mount 30 HEKLA Volcano when flying from Edwards, California to Kiruna, Sweden. The ash cloud was not visible to flight crew, however, the research airplane carried sensitive research equipment which was capable of detecting the sulphur dioxide. In-flight checks and post-flight visual 35 inspections revealed no damage to the airplane. However, detailed examination of the engines revealed damage to some of the turbine cooling passages. Furthermore, high WO 2005/068977 PCT/AU2005/000035 -2 levels of sulphur were found in the oil. It seems likely that this ash cloud actually was predominantly a sulphur dioxide cloud. Even if it was 5 not, it raises the possibility that an aircraft can fly through sulphur dioxide without passing through ash. The post encounter treatment of the engine in the case of sulphur dioxide encounter would be different to and considerably cheaper than the equivalent treatment 10 required of an engine during an ash encounter. Accordingly, it would be desirable to provide a sulphur dioxide cloud detection technique. 15 Summary of the Invention The present invention relates to a method of detecting sulphur dioxide clouds comprising: measuring infrared radiation at a viewing elevation 20 at or above the horizon and at a key wavelength at which there is a sulphur dioxide feature and in the vicinity of which there is a region where the amount of infrared radiation from water vapour in the atmosphere varies in accordance with a predetermined relationship; 25 measuring radiation at two or more subsidiary wavelengths in said region; determining the amount of radiation from water vapour at the key wavelength from the measured radiation at the subsidiary wavelengths using the predetermined 30 relationship; and determining whether a sulphur dioxide cloud is present from the measured infrared radiation at the key wavelength and the determined amount of radiation from water vapour. 35 Preferably, said subsidiary wavelengths are located either side of said key wavelength.

WO 2005/068977 PCT/AU2005/000035 -3 The inventor has determined that the key wavelength should be one of 7.3pm and 8.6gm and that 7.3pm is the preferred key wavelength. 5 Where the key wavelength is 7.3pm, it is preferred that subsidiary wavelengths at ±0.5pm are used. The inventor has established that for the region of these wavelengths the predetermined relationship is that 10 radiation from water vapour varies in a substantially linear manner. Accordingly, the radiation from water vapour at the key wavelength can be interpolated from the radiation at the subsidiary wavelengths on the basis of this predetermined relationship. The inventor has also 15 established that there is substantially less SO 2 absorption at this wavelength. The method may also involve compensating for background SO2 in the atmosphere. 20 The invention also provides a detection apparatus for detecting a sulphur dioxide cloud comprising: measurement means that measures infrared radiation at a viewing elevation at just below, or above 25 the horizon and at a key wavelength at which there is a sulphur dioxide feature and in the vicinity of which there is a region where the amount of infrared radiation from water vapour in the atmosphere varies in accordance with a predetermined relationship, said measurement means also 30 measuring infrared radiation at two or more subsidiary wavelengths in said region and processing means for determining the amount of radiation from water vapour at the key wavelength from the measured radiation at the subsidiary wavelengths using the 35 predetermined relationship and determining whether a sulphur dioxide cloud is present from the measured infrared radiation at the key wavelength and the WO 2005/068977 PCT/AU2005/000035 -4 determined amount of radiation from water vapour; and output means for generating an output signal indicative of the presence of a sulphur dioxide cloud when a sulphur dioxide cloud is present. 5 The inventor has also determined that the method and apparatus of the present invention can be used to detect sulphur dioxide clouds from the ground or from an aircraft. 10 Brief Description of the Drawings Figure 1 illustrates the SO 2 absorption feature in the region 1200cm~ 1 to 1500cm~ 1 and the preferred 15 measurement wavelengths of the invention; Figure 2 is a schematic diagram of a S02 detection apparatus of the preferred embodiment; Figure 3 illustrates two modes of operation of the apparatus; 20 Figure 4 is a schematic diagram of apparatus to be used from an aircraft; Figures 5a - Sc represent normal climatic conditions; Figures 6a and 6b represent variations on normal 25 conditions to allow testing of the invention; Figure 7 represents variations in S02 for testing; Figure 8 shows variation in temperature with S02 concentration; and 30 Figure 9 shows temperature plotted as a function of absorber amount. Description of the Preferred Embodiment 35 Herein, the term "key wavelength" is used to refer to a wavelength at which there is an appropriate SO 2 feature.

WO 2005/068977 PCT/AU2005/000035 Persons skilled in the art will appreciate that a "wavelength" in the context of this specification does not imply a single wavelength but rather encompasses a band of 5 radiation. Typically the width of the band will depend on the filter used to observe/measure light at the wavelength of interest. The numerical figures given in this specification are used to denote, in general terms, the centre of such bands, however, it will be appreciated by 10 persons skilled in the art that some variation of the centre wavelength is possible. The term "subsidiary wavelength" is used to refer to a wavelength in a region in the vicinity of the key 15 wavelength where a relationship can be established between radiation from water vapour at two or more subsidiary wavelengths and radiation from water vapour at the key wavelength. 20 The preferred embodiment provides a method and apparatus that allows identification of sulphur dioxide clouds in the free atmosphere. The apparatus of the preferred embodiment uses an infrared detector, interference filters and focussing optics. The filters 25 divide radiation within the band between 6.8 and 8.1 pm into three narrow bands. The central band corresponds to a strong SO 2 absorption feature caused by the anti symmetric stretch of the S02 molecule at 7.3 pm. The other bands are above and below this feature. The central band 30 B 0 , is sensitive to S02 concentrations. The lower band, B 1 and higher band B, are used to account for the effects of water vapour on the absorption in band B, Accordingly, B, is the key wavelength and B 1 and 35 Bh are the subsidiary wavelengths in the preferred embodiment.

WO 2005/068977 PCT/AU2005/000035 -6 Figure 1 illustrates the absorption feature due to S02 for the infrared region extending from 1200 cm~ 1 (8.33 pm) to 1500 cm" (6.67 pm). The ordinate in this plot is line strength and the abscissa is wavenumber (cm^ 1 ; 5 wavelength in pm = 10,000/wavenumber in cm- 1 ). Also, shown are three idealised filter response functions which isolate radiation within the three narrow regions corresponding to: Bh (7.633-8.065 pm) B, (7.143-7.57 pn) and B 1 (6.897-7.042 pm). 10 The response functions are normalised to unity and scaled appropriately for plotting. The central wavenumber for the S02 absorption is 1363 cm- 1 and the band extends from about 1320 cm- 1 to about 1390 cm-1. A filter 15 covering this region responds to all the radiation from this band; whether the S02 feature be due to absorption or emission. In the case of a detection apparatus viewing a cold background, i.e. viewing from the ground to space or from an aircraft towards the horizon, there would be more 20 radiation in this band in the presence of the S02 cloud than if it were absent. In practice, water vapour and clouds also absorb and emit radiation in the region 7-8 pim. The inventor has 25 realised that the two bands positioned either side of the central band can be used to eliminate the effects of water vapour. Water vapour absorbs and emits radiation 30 throughout the region 7-8 pm. The amount of radiation absorbed or emitted depends on the amount of water vapour and on its location in the atmospheric column. Water vapour near the boundary of the earth's surface is generally warm and abundant. Water vapour near the 35 tropopause (i.e. at jet aircraft cruising altitudes) is cold and sparse. The central band Be of the SO 2 detector of the preferred embodiment responds to radiation due to WO 2005/068977 PCT/AU2005/000035 -7 both SO 2 and water vapour. The lower and higher bands B 0 , Bh of the detector however, are only sensitive to water vapour. The inventor has determined that the radiation from water vapour in the region surrounding B, behaves in a 5 sufficiently linear manner to enable it to eliminate the effects of water vapour on the central band sufficiently for the purpose of detecting a sulphur dioxide cloud. The Planck blackbody radiation from B 1 and Bh are linearly interpolated to estimate the radiation detected in B, due 10 to water vapour only. This radiation amount is subtracted from the radiation actually measured by B,. The residual amount is due to SO 2 . Accordingly the preferred embodiment utilises a predetermined relationship that water vapour behaves in a linear manner. Persons skilled in the art 15 will appreciate that other predetermined relationships could be used, for example, relationships that are approximately linear. A schematic of the detection apparatus is shown 20 for illustrative purposes in Figure 2. The detection apparatus 6 consists of four major components: * Fore-optics 1 that focus a beam of incoming infrared radiation onto a detector. * A filter wheel 2 consisting of at least three narrow 25 band interference filters that isolate radiation into the bands: B 1 , B 0 and Bh. " An infrared detector array 3 sensitive to radiation in the 7-8 pm region. " Processing means 4 for processing the detector signal 30 to determine whether S02 and hence a sulphur dioxide cloud is present. Figure 3 is a schematic diagram illustrating two modes of operation of a detection apparatus 6 that senses 35 infrared radiation in order to detect S02 clouds. A first mode assumes that the detection apparatus 6 is on board an aircraft 7 and views the S02 cloud ahead at a small angle WO 2005/068977 PCT/AU2005/000035 -8 to the horizontal. The second mode assumes that the detection apparatus 6 is based on the ground and views the cloud at a large angle to the horizontal (e.g. zenith viewing). 5 The detection apparatus of the preferred embodiment may be operated from the ground viewing the sky above or from an aircraft viewing forwards at just below or above the horizon. The principal mode of operation is 10 anticipated to be from an aircraft with the instrument having an unobstructed view of the atmosphere ahead of the aircraft as the inventor has established that the method works best when water vapour path amount is less than 1g cm-2 . For example, at heights over 3000 m or in dry 15 atmosphere water vapour path is defined as the integral of the water vapour concentration with distance along the line of sight between instrument and target. Ideally the view should be horizontal or a few degrees (3-5*) above the horizon, so that the background radiation is cold. 20 Typically, aircraft fly with their nose at an angle of about 3 degrees to horizontal. However, the processor 4 can be configured to account for changes in viewing zenith angle, making the technique insensitive to the viewing direction. For the case of a detection apparatus 6 25 viewing ahead of an aircraft at a zenith angle of Z degrees, the detection apparatus 6 provides three signals to the processor 4. A synthetic signal corresponding to the amount of radiation from water vapour is determined through linear interpolation of the signals from B, and Bh. 30 This signal labeled B is compared to the signal from Be,. The processor 4 then computes the SO 2 amount at the key wavelength B: using B and the original signal B 0 . The processor 4 uses pre-defined look-up tables that 35 account for standard atmospheric conditions (tropical, mid-latitude, and polar) and the viewing geometry to compensate for background S02. The detector array 3 WO 2005/068977 PCT/AU2005/000035 -9 provides an image of the SO 2 amount with a spatial resolution that depends on the exact number of detector elements in the array (320x240 is recommended) and the distance to the S02 cloud. Distance information is 5 supplied by the detection apparatus 6, however, the S02 anomaly will be detected at distances of up to several 100 kms depending on the cruising altitude and clarity of the atmosphere ahead. The detection apparatus 6 produces an output 5, for example in the form of an amount of S02 or an 10 alarm signal indicating the presence of sufficient S02 to pose a problem. The alarm signal may cause an audible or visual alarm in an aircraft. Figure 4 illustrates how the apparatus works in 15 the case of being mounted in an aircraft. In addition to signals from the detector 3 the processor 4 also receives aircraft altitude information 8 from the aircraft and standard atmosphere information 9 20 from a memory associated with the processor. Examples A sophisticated radiative transfer model-MODTRAN 25 (Berk, et al., 1989) is used to model the response expected from a single-element detector viewing arealistic atmosphere. The viewing geometry is varied in the simulations to account for viewing from below the S02 cloud, viewing from above, and viewing at a small angle 30 along a nearly horizontal path. The amount of S02 is varied, as is the main other gaseous absorber in the region-water vapour. We refer to the amount of S02 as the cloud thickness. 35 1. Model Atmosphere Vertical profiles of the model atmosphere used in WO 2005/068977 PCT/AU2005/000035 - 10 the simulations are shown in Figure 5 and variations used to test the present invention are shown in Figure 6 and Figure 7. 5 (a) Temperature The temperature profile is shown in Figure 6a. Varying the profile has little effect on the retrieval and detection algorithm because the algorithm uses differences 10 in temperatures. No further simulations were performed on this parameter because of its insensitivity. (b) Water vapour 15 Water vapour was varied by increasing the amounts in the lowest layers from less than 0.1 cm of precipitable water to more than 3 cm. No effect was found on the detection or retrieval because the water vapour lies below the SO 2 cloud. Water vapour was also increased in the 20 layer that contained the SO 2 and this has a major effect. The perturbed water vapour profile is shown in Figure 6b. (c) Sulphur dioxide 25 The vertical profile of the background S02 is taken from the US standard atmosphere. The profile corresponds to a well-mixed gas with a constant vertical concentration of 10 5 ppmV (parts per million by volume). Perturbed profiles, with increasing SO 2 concentration, are 30 shown in Figure 7. Eight profiles are shown. The integrated amount of SO 2 in a vertical column for the profiles varies from 10 milli atm-cm to 100 milli atm-cm. Depending on the pathlength travelled the total absorber amount can be much larger. Results for SO 2 absorber 35 amounts of more than 1000 milli atm-cm are given. 2. Viewing the SO 2 cloud along horizontal paths WO 2005/068977 PCT/AU2005/000035 - 11 For the purpose of example, model simulations have been performed for the case of horizontal viewing from a platform (e.g. an aircraft) directly ahead and towards an 5 SO 2 cloud. The viewing direction is assumed to be horizontal at the altitude of the platform (8 km, or =26,000 feet is assumed). The cloud thickness (as measured in the viewing direction) is varied from 10 km to 500 km and the concentration within the cloud is varied 10 from background levels to =0.1 ppmV. This range of concentration covers the smallest eruptions (that are likely to reach these heights, e.g. Hekla-style eruptions) to the largest observed this century (e.g. Pinatubo-style eruptions). The results of these model simulations are 15 summarised in two figures. Figure 8 shows the variation of the temperature anomaly (the temperature difference between the synthetic signal and the measured signal as a function of cloud thickness). 20 The family of curves 20-27 generated from the modelling are lines of constant concentration for S02 concentration varying from 0.0136 ppmV 20 to 0.1083 ppmV 27. The points that lie on vertical lines correspond to lines of constant cloud thickness. As the cloud thickens 25 the curves follow the same trend with increasing anomalous signal until the cloud starts to become opaque. At this point, which varies with S02 concentration, the temperature anomaly increases towards a limiting value (AT = -2 K). Note that the opaque limit is reached either by increasing 30 concentration or increasing cloud thickness, since both quantities increase optical depth and hence absorption. Beyond a thickness of 500 km, the cloud is essentially opaque and the radiative process changes from absorption to emission. 35 Figure 9 provides an alternate way of understanding the physical processes involved in S02 WO 2005/068977 PCT/AU2005/000035 - 12 detection. Here the temperature anomaly is plotted as a function of absorber amount. The plot indicates that for a given anomaly, several values of absorber amount are possible, depending on the cloud thickness and 5 concentration. Thus, it is not possible to uniquely quantify the absorber amount from the temperature anomaly without knowing either the concentration or the cloud thickness. In practice it is not necessary to know these quantities, as the purpose of the invention is to detect 10 the presence of SO 2 gas in the free atmosphere, rather than quantify the amount. The modelling does give an indication of the limits within which detection of S02 is possible. At the lower end, for cloud thicknesses of 10 km or less, the S02 concentration must be larger than =0.06 15 ppmV. This corresponds to an absorber amount of =25 milli atm-cm. SO 2 clouds that intercept air-routes (i.e. heights >20,000 feet) will have horizontal dimensions of 10's of kilometres and absorber amounts well in excess of 25 milli atm-cm would be expected. 20 Persons skilled in the art will appreciate that various modifications may be made to the preferred embodiment without departing from the scope of the appended claims.

Claims (20)

1. A method of detecting sulphur dioxide clouds comprising: 5 measuring infrared radiation at a viewing elevation at or above the horizon and at a key wavelength at which there is a sulphur dioxide feature and in the vicinity of which there is a region where the amount of infrared radiation from water vapour in the atmosphere varies in 10 accordance with a predetermined relationship; measuring radiation at two or more subsidiary wavelengths in said region; determining the amount of radiation from water vapour at the key wavelength from the measured radiation at the 15 subsidiary wavelengths using the predetermined relationship; and determining whether a sulphur dioxide cloud is present from the measured infrared radiation at the key wavelength and the determined amount of radiation from 20 water vapour.

2. A method as claimed in claim 1, wherein said method is performed from a position or position where the water vapour path amount is less than lg cm- 2 . 25

3. A method as claimed in claim 1, wherein said subsidiary wavelengths are located either side of said key wavelength. 30

4. A method as claimed in claim 1, wherein said key wavelength is one of 7.3pm and 8.6pm.

5. A method as claimed in claim 1, wherein the key wavelength is 7.3pm. 35

6. A method as claimed in claim 4, wherein subsidiary wavelengths at ±0.5pm are used. WO 2005/068977 PCT/AU2005/000035 - 14

7. A method as claimed in claim 5, wherein subsidiary wavelengths at ±0.5pm are used. 5

8. A method as claimed in claim 5, wherein determining the amount of the radiation from water vapour at the key wavelength is performed by a linear interpolation based on the radiation measured at the subsidiary wavelengths. 10

9. A method as claimed in claim 1, further comprising compensating for background sulphur dioxide in the atmosphere.

10. A method as claimed in claim 1, wherein said method 15 is performed from an aircraft.

11. A method as claimed in claim 2, wherein said method is performed from the ground. 20

12. A detection apparatus for detecting a sulphur dioxide cloud comprising: measurement means that measures infrared radiation at a viewing elevation at just below, or above the horizon and at a key wavelength at which there is a sulphur 25 dioxide feature and in the vicinity of which there is a region where the amount of infrared radiation from water vapour in the atmosphere varies in accordance with a predetermined relationship, said measurement means also measuring infrared radiation at two or more subsidiary 30 wavelengths in said region; and processing means for determining the amount of radiation from water vapour at the key wavelength from the measured radiation at the subsidiary wavelengths using the predetermined relationship and determining whether a 35 sulphur dioxide cloud is present from the measured infrared radiation at the key wavelength and the determined amount of radiation from water vapour; and WO 2005/068977 PCT/AU2005/000035 - 15 output means for generating an output signal indicative of the presence of a sulphur dioxide cloud when a sulphur dioxide cloud is present. 5

13. Apparatus as claimed in claim 12, wherein said subsidiary wavelengths are located either side of said key wavelengths.

14. Apparatus as claimed in claim 12, wherein said key 10 wavelength is one of 7.3pm and 8.6gm.

15. Apparatus as claimed in claim 12, wherein the key wavelength is 7.3pim. 15

16. Apparatus as claimed in claim 14, wherein said subsidiary wavelengths are at ±0.5pm.

17. Apparatus as claimed in claim 15, wherein said subsidiary wavelengths are at ±0.5pm. 20

18. Apparatus as claimed in claim 15, wherein determining the amount of the radiation from water vapour at the key wavelength is performed by a linear interpolation of the radiation measured at the subsidiary wavelengths. 25

19. Apparatus as claimed in claim 12, wherein said processing means compensates for background sulphur dioxide in the atmosphere. 30

20. An aircraft having a detection apparatus as claimed in claim 12.