484 ANALYTICAL PROCEEDINGS, DECEMBER 1993, VOL 30

Something in the Air- Industrial Atmospheric Monitoring

The following is a summary of one of the papers presented at a Meeting of the Analytical Division held on February 16, 1993, in the Scientific Societies' Lecture Theatre, London W1.

Range-resolved Measurements of Air Pollution Using Lasers

M. J. T. Milton and P. T. Woods Environmental Standards Section, Division of Quantum Metrolog y, National Physical Laboratory, Teddington, Middlesex

There is an increasing demand for measurements of the atmosphere in and around industrial areas. Such quantification is difficult t o achieve because it involves measuring highly variable quantities over large areas and differing timescales. The Environmental Standards Section at the National Physical Laboratory (NPL) has responded in two ways to the need for better methods of measuring industrial air pollution. The first is through the provision of accurate standards of gas concent- ration, which are used for calibrating measurements, and the second is by the development of spectroscopic measurement techniques.

Standard Gases for Calibrating Measurements Measurements of air pollution are of value only if they are accurate and if their accuracy can be verified scientifically. The most reliable and cost-effective method to validate measure- ments of gas concentration is by the use of standard gases of known concentrations. Such mixtures are prepared at the NPL with accuracies of better then 1% and are traceable to the primary gas standards of the laboratory. Table 1 summarizes some of the gases and concentrations available. These mix- tures, which are used to calibrate equipment measuring pollution and air quality, are recognized internationally. They have been developed as part of the valid analytical measure- ment initiative of the UK's National Measurement System* and are supplied to organizations throughout Europe seeking to ensure the accuracy of their measurements.

Table 1 Standard gas mixtures available from the NPL. All mixtures are secondary standards supplied in passivated containers

Gas mixture Concentration Carbon monoxide in nitrogen

Nitric oxide in nitrogen Propane in air or nitrogen Methane in air 2%-100 ppm n-Hexane in nitrogen 100,100 ppm

10%-1 pprn

10%-1 ppm 1%-1 ppm

Carbon dioxide in nitrogen 10%-0.2%

Multicomponent mixtures for exhaust emissions CO (6%). C 0 2 (15'%0), C3Hs (2400 ppm) in nitrogen CO (3.5%). C 0 2 (14%), C3H8 (2000 ppm) in nitrogen

Multicomponent hydrocarbon mixtures (in nitrogen) at ppb levels for air quality measurements of volatile organic components

* The UK National Measurement System is supported by the National Measurement System Policy Unit of the Department of Trade and Industry.

Novel Methods for Air Pollution Monitoring The NPL has developed several optical remote-sensing systems and demonstrated their performance in a series of different field measurements. Two of these systems are described here, together with some results obtained with them on field measurement exercises. The first instrument uses an incan- descent source to perform integrated-path measurements with a range of up to 500 m. The monitor is portable, and of relatively good sensitivity and low cost. The second is a differential absorption light detection and ranging (LIDAR) system (DIAL) which uses a pair of pulsed lasers to perform range-resolved measurements with a range of up to 3 km. This system is large and complex but capable of highly sensitive and specific measurements. These two instruments provide com- plementary measurement capabilities.'

The performance of each instrument can be calibrated by reference to standard gas mixtures prepared gravimetrically and is therefore traceable to national standards of gas concentration.

Portable Long-path Monitor The design objectives for the NPL's long-path monitor were to develop a portable instrument capable of making accurate semi-automated measurements of gas concentration at rela- tively low cost. The monitor works in the near- and mid- infrared spectral region (1.5-5 pm) and uses an incandescent source to generate broad bandwidth radiation across the spectral region of interest. Light from the source is collimated and transmitted into the atmosphere by a telescope. The beam is reflected back by a corner-cube retro-reflector. The maxi- mum distance between the transmitter and the retro-reflector is about 500 m.

The monitor can be tuned to detect different gases by altering dispersive components within the instrument. The range of detectable gases includes methane, propane, carbon monoxide, nitrous oxide, hydrogen chloride and others. This monitor is now manufactured by Siemens-Plessey Controls Limited under the trade name 'Hawk'.

Differential Absorption LIDAR The differential absorption LIDAR technique enables range- resolved measurement of atmospheric gas concentrations using the radar principle at optical wavelengths.2 This is achieved by firing a pulse of high-powered laser radiation into the atmosphere and measuring the amount scattered back by particulates and aerosols. The distance to the point at which the light is scattered can then be determined by measuring the time taken for it to return to the receiver. The identity and

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ANALYTICAL PROCEEDINGS, DECEMBER 1993, VOL 30 485

quantity of any gas present along the measurement path can be determined by employing two separate wavelengths, one of which is absorbed by the target gas while the other is not. This principle enables quantitative measurements of atmospheric gas concentration to be made at ranges of up to 3 km with a range resolution of 10 m.

A mobile DIAL system developed at the NPL is shown in Fig. 1 . In the ultraviolet and visible spectral regions, it is capablc of detecting nitrogen monoxide, nitrogen dioxide, sulfur dioxide, ozone, toluene3 and benzene. The infrared channel is significantly more complex than the ultraviolet and visible channels. This is justified by the larger number of gases, including methane, ethane, higher alkanes, higher alkenes, hydrogen chloride, hydrogen sulfide, dinitrogen oxide and others, that can be detected with the system.

Applications A varied range of field measurements4 have been carried out to demonstrate the flexibility and advantages of these two instruments. In particular:

Three-dimensional concentration profiles of nitrogen oxides and sulfur dioxide have been measured in the atmosphere around industrial plants and in urban areas as part of a programme to demonstrate UK compliance with Euro- pean Community Directives.

Fluxes of methane emitted by landfill sites have been measured.

The concentrations of a range of gases have been monitored along the boundaries of industrial plants and within the working environment.

The total fluxes of fugitive hydrocarbons emitted by oil refineries have been measured directly. These measure- ments have demonstrated that vapour losses could cost refineries up to S0.5 million each year.

Measurement of Hydrocarbon Fluxes An example of the value of DIAL measurements to the chemical industry is the estimation of fugitive loss rates from oil

Fig. 1 by the NPL

The mobile differential absorption LIDAR system developed

refineries and petrochemical plants. The atmosphere at such sites usually contains a wide range of gaseous hydrocarbons, with similar spectral features in the 2.9-3.5 pm region. The DIAL system of the NPL is capable of analysing the gases present along a particular atmospheric path by scanning the wavelength of its laser source across this spectral region. This gives a preliminary estimate of the species present, as well as indicating wavelengths suitable for performing a more thorough area scan. This information is combined with more detailed spectroscopic data to calculate the exact concent- rations of the species.

Fig. 2 is an example of a measurement of a cloud of gaseous hydrocarbons performed downwind of a crude oil storage area. This was achieved by tuning the wavelength of the infrared DIAL system to a point in the spectral region at which butane, propane and some other alkanes have very similar absorption strengths. Information from this and other DIAL measure- ments has been combined with meteorological data to calculate the hourly mean hydrocarbon fluxes given in Table 2. These values were then annualized using standard calculations specified by the American Petroleum Institute, which considers the variation of ambient temperature and wind speed throughout the year.

Flux measurements by the DIAL technique have also been carried out for the Department of the Environment with the objective of improving the accuracy of the National Emission Inventory for the oil and petrochemical industries.

Validation of Optical Remote-sensing Systems Work has been performed at the NPL on the validation of these, and other, optical remote-sensing systems. This work has been based on the use of gravimetrically prepared gas mixtures to calibrate the systems and to determine their susceptibility to cross-interference. These experiments test the performance of the system directly by the use of large gas cells that can be placed in the transmitted beam of the system. The linearity of the system can be checked using different concentrations and the cross-sensitivity employing samples of

100

25

0 50 100 150 200 250 Rangelm

Fig. 2 Cloud of hydrocarbon vapour above a crude-produce storage area at an oil refinery measured using DIAL. The data show the measured column concentrations on a relative gravimetric scale

Table 2 Mean emission rates from different sources in an oil refinery. The data are corrected for the annual variation of temperature and wind speed

Mean measured flux

Crude tankage 238 Product tankage 402 Water treatment 36 Process area 250 Total 926

Source (kg h - 9

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386 ANALYTICAL PROCEEDINGS, DECEMBER 1993, VOL 30

possible interferents. The ease with which such calibration procedures can be carried out is one of the major advantages of optical remote-sensing systems.

Conclusion Gas standards have been developed for the calibration of in- situ air pollution monitors. In addition, optical remote-sensing systems have been demonstrated in several field measurement exercises. In particular, they have been used for the measure- ment of loss rates from industrial storage and production activities.

This work was sponsored by the U K Department of Trade and Industry (through its National Measurement System Policy Unit) and the Department of the Environment, together with industrial sponsors including BP International.

References 1 Partridge, R. H., Meas. Confrol, 1990, 23, 293. 2 Measures, R. M.. Laser Remoie SenAing, Wiley, New York,

1984. 3 Milton, M. J. T., Woods, P. T.. Jolliffc, B. W., Swann.

N. R . W., and Mcllveen, T. . I . . Appl . Phys., 1992, B55, 41. 4 Milton. M. J. T.. Woods. P. T., Jolliffe, B. W., Swann.

N. R. W., and Mcllvecn, T. J.. Proc SPIE. 1902, 1715.

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