Automatic analytical methods for environmental monitoring and control

In order to obtain information on the concentration of a constituent, two wavelengths must be used. This gives rise to the differential absorption LIDAR, or DIAL, technique often utilized for atmospheric probing. In standard DIAL applications, one wavelength is tuned to a strong absorption line of the species of concern, whilst the other is tuned slightly off the absorption line, allowing the density or concentration of absorbing constituents to be calculated. As the wavelength of the absorption line is specific to each absorbing species, one can isolate the absorption of each constituent.

One of the main applications of the LIDAR techniques to environment monitoring has been in observations of ozone. Ozone absorbs strongly in the ultraviolet part of the spectrum, and furthermore the absorption coefficient varies rapidly with wavelength. These two characteristics make the DIAL method ideal for ozone measurement. The exact choice of wavelengths used in an ozone LIDAR depends on the atmospheric region to be probed: troposphere (< 300 nm) or stratosphere (> 300 nm).

An airborne ozone LIDAR has a lower vertical resolution (a few hundred meters) but is able to measure a curtain of ozone values ~10 km deep.
Pollutants measurement in the atmospheric regions

One of the most important applications of LIDAR lies in its ability to make remote measurements of pollutant emissions from industrial resources. In the modern regulatory environment such a capability is of great commercial value, and many LIDAR systems were developed around the world to provide it.

Typically, such a LIDAR is mounted in a truck or van and driven to the perimeter of the site under investigation. The LIDAR performs a two-dimensional scan of the region upwind and downwind of the emission source, mapping the pollutant concentration as a function of height and elevation angle.

Early applications of LIDAR to pollutant monitoring used the Raman technique. The Raman technique has the advantage that it does not require a specific laser wavelength, but has two main disadvantages: Raman backscatter is very weak and the rotational-vibrational bands of O₂ and N₂ can mask Raman lines from minor constituents. A typical application of the Raman technique has been to search for leaks from gas pipelines – concentrations of ~ 1 % methane may readily be detected 2 km away.

Recent work in this field, however has almost exclusively exploited the DIAL technique. As new and better tuneable laser transmitters are developed – especially in the infrared – new applications for DIAL are opened up. The two wavelength regions most commonly exploited are the ultraviolet between 230 and 300 nm, and the infrared between 3 and 5 m. In the former, gases such as nitric acid (226 nm), sulfur dioxide (287 nm), toluene (267 nm) and benzene (253 nm) have sharp absorption lines making them suitable for DIAL detection. This spectral region has also the advantage that is solar-blind and permits the operation for daytime.

Another pollutant that can be detected readily by UV LIDAR is mercury as vapor. Mercury is released from its ore, cinnabar, by roasting it, and in regions where this extraction is practiced (particularly the huge cinnabar deposits at Almadén in Spain). Since mercury has a very strong electronic transition at 253.6 nm, it is readily detected (in high enough concentration) by DIAL LIDAR.

The development of high-quality infrared non-linear optical materials has led to a new class of bright, stable infrared laser beams. These infrared sources are tuneable and have very narrow line widths, opening up the possibility of DIAL measurements in the infrared. Many molecules of atmospheric and environmental interest have vibrational – rotational bands in the near infrared (2 - 5 m), and provided that interference with water vapor and CO₂ bands can be avoided, can be measured with infrared DIALs. Examples are methane (CH₄), acetylene (C₂H₂), ethylene (C₂H₄) and ethane (C₂H₆). The most important application for such LIDARs is to measure emissions from petrochemical plants and storage facilities, although they are also useful for tracking plumes for several kilometers downwind of industrial sources.

An example of a mobile LIDAR designed for pollution measurements is that operated by the UK National Physical Laboratory, London. This uses two lasers, one for UV measurements and one for IR measurements. In Table III.3.1 are presented the gases measurable with this instrument (together with their typical detection limits for a 100 m diameter plume centered at 200 m range). The system is calibrated by reference to standard cells with known concentrations of the measured gases. It has been used to measure volatile organic compound emissions from more than twenty different petrochemical facilities, including oil refineries, retail petrol stations and ethylene processing plants.

1. 
   Encyclopedia of Atmospheric Sciences, Ed. J.R. Holton, J. Pyle, J.A. Currie, Academic Press, 2002.
   
2. 
   Optical and Laser Remote Sensing, D. K. Killinger, A. Mooradian, eds., Springer Verlag, New York, 1982.
   
3. 
   Sunesson, J. A., A. Apituley, D. P. J. Swart, Appl. Opt., 1994, 33, 7045-7058.
   
4. 
   Kempfer, U., W. Carnuth, R. Lotz, T. Trickl, Rev. Sci.Instrum., 1994, 65, 3145-3164.
   
5. 
   Ferrara, R., B. E. Maserti, H. Edner et al., Atmos. Environ., 1992, 26A, 1253-1258.
   
6. 
   Edner, H., P. Ragnarson, S. Svanberg et al., Science Total Environ., 1993, 133, 1-15.
   
7. 
   Milton, M. J. T., P. T. Woods, B. W. Jolliffe et al., Appl. Phys. B, 1992, 55, 41-45.
   

In environmental control DOAS is used as a method of measurement of gases. The idea of measuring gases with light may sound peculiar to laymen, but based on this idea in 1985 two Ph.D. students from the University of Lund, Sweden founded the company OPSIS (www.opsis.se) which became the leading supplier in the area. For this, many times the DOAS concept is identified with OPSIS.

F
igure III.3.2.

In an OPSIS system, the light from an emitter is projected through ambient air to a receiver, which may be up to 2 km away; the light is then transferred to the OPSIS analyzed via a fiber optic cable where it is analyzed and the concentration of a wide range of compounds determined.

The Beer-Lambert law (See Chapter II.4.3) can not directly be applied to atmospheric measurements because of several reasons:

a) Besides the absorption of the trace gases light extinction occurs also due to scattering on molecules and aerosols. Especially for aerosols this extinction can not be corrected for with the desired accuracy.

b) In the atmosphere the absorptions of several species always add up to the total absorption. Thus in most of the cases it is not possible to measure one specific species.

c) In the case of satellite measurements the detected intensity can strongly depend on the ground.

These restrictions can be avoided by applying the method of differential optical absorption spectroscopy (DOAS). The DOAS technique relies on the measurement of absorption spectra instead of the intensity of monochromatic light. Thus it is possible to separate the absorption structures of several atmospheric species from each other as well as from the extinction due to scattering on molecules and aerosols.

This technology is used in instruments that can measure a number of different pollutants along a single light beam that may be up to 2000 meters long. 

If two analytes both absorb at the same wavelength, for example, the resulting absorption will be the sum of the two individual absorptions. Therefore it is possible to mathematically treat the signal produced, to eliminate interferences, and to produce the spectrum for each analyte being sought. The ability to differentiate between adjacent absorption features can be improved with increased system resolution.

DOAS procedure can only be applied to species the spectrum of which contains reasonably narrow absorption features, thus limiting the number of molecules detectable by this technique. A possible continuous absorption of the trace gas will be neglected by this procedure. Nevertheless, slow varying absorption attenuates the total available light intensity that has an influence on the detection limit.

At producing measurements a spectral range of about 60 nm is chosen from the overall region 200-460 nm and the spectrum is registered in this range. Generally, at any spectral range a number of gases simultaneously have absorption bands. The only exception is the NO₂ spectrum in 400-500 nm region. The least square method provides for simultaneous determination of concentration of all gases having absorption in the selected spectral range. Nevertheless, inevitable small errors in knowledge of gases cross-section gives rise to specific interference errors in determination of gas concentration. For minimization of this type and other measurement errors the selected spectral range have to satisfy to following conditions:

1. 
   maximum possible light source spectral intensity
   
2. 
   absence of sharp peaks or another fine structures in light source spectrum
   
3. 
   maximum absorption cross section for gas component to be detected
   
4. 
   minimum absorption cross section for other gases.
   

The OPSIS system has been designated an Equivalent Method by the U.S. EPA for the monitoring of O₃, NO₂ and SO₂ in ambient air. In addition to these gases a wide variety of inorganic and organic gases can also be measured.

The DOAS based instrumentation is characterized by:

• 
  Total monitoring solution
  
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  Cost-effective, open-path technology
  
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  Multi-gas and multi-path system
  
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  High-performance monitoring of criteria pollutants
  
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  No sample required
  
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  Real time measurements
  
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  Easily calibrated
  
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  Operates with a minimum of maintenance
  
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  Low detection limits
  
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  Fully meeting the EU requirements
  
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  Withstands aggressive environment with high levels of particulates and gases
  
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  Remote service functions and servicing by highly skilled service network.