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TRANSPORT AND ROAD RESEARCH LABORATORY
Department of the Environment
SUPPLEMENTARY REPORT 89UC
A MOBILE LABORATORY FOR MONITORING AIR POLLUTION FROM MOTOR VEHICLES
by
M.G. Bevan A.J. Hickman
Any views expressed in this Report are not necessarily those of the Department of the Environment
Environment Division Transport Systems Department
Transport and Road Research Laboratory Crowthorne, Berkshire
1974
Abstract
1.
2.
3.
°
5.
6.
CONTENTS
In t roduc t ion
The Caravan
Ins t rumenta t ion
3.1 Gaseous pol lutants
3.1.1 Oxides o f nitrogen 3.1.2 Carbon monoxide and hydrocarbons 3.1.3 Volatile lead compounds
3.2 Particulate pol lutants
3.2.1 Smoke 3.2.2 Lead
3.3 Meteorological measurements
3.3.1 Wind speed and wind direction 3.3.2 Dewpoin t 3.3.3 Tempera tu re
3.4 Data collection
3.4.1 Digital vol tmeter 3.4.2 Data transfer unit 3.4.3 Data processing
3.5 Back-up facilities
3.5.1 Total hydrocarbons 3.5.2 Carbon -monoxide
Future deve lopments
Acknowledgements
References
Page
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2 3 3
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© CROWN COPYRIGHT 1°74
Extracts f rom the text may be reproduced, except for commercial purposes, provided the source is acknowledged.
Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April 1996.
This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.
A MOBILE LABORATORY FOR MONITORING AIR POLLUTION FROM MOTOR VEHICLES
A B S T R A C T
The equipping and instrumentation of a mobile laboratory for monitoring atmospheric pollution from motor vehicles is described. The instruments for measuring the concentrations of the various pollutants and associated meteorological conditions are described in detail together with the way in which the data is processed.
1. INTRODUCTION
The Laboratory is actively involved in evaluating the effects of traffic on the environment and one of the areas being investigated is the contribution made by vehicles to atmospheric pollution. As part of this research programme a mobile monitoring laboratory has been set up so that data can be obtained on atmospheric pollutant levels.
A large caravan has been instrumented, as described in this report, so that pollutants can be monitored at various sites. In selecting the instruments for use in this project, automatic operating systems, which require the minimum of staff attendance have been chosen whenever possible.
The pollutants from motor vehicles are emitted in two forms, gaseous and particulate. The representative compositions of the gaseous pollutants from petrol and diesel engines in various operating modes are shown in
Table 1.
TABLE 1
Representative composition of exhaust gases (from Pegg and Ramsden 1 ) (concentrations in parts per million)
Pollutant Idling Accelerating Cruising Decelerating
Petrol Engines:-
Diesel Engines:-
Carbon monoxide Hydrocarbons Nitrogen oxides
Carbon monoxide Hydrocarbons Nitrogen oxides
69000 5300
30
Trace 400
60
29000 1600 1020
1000 200 350
27000 1000
650
Trace 100 240
39000 10000
20
Trace 300
30
Carbon monoxide and hydrocarbons are produced as the result of the incomplete combustion of the fuel in the engine. Additional hydrocarbons are emitted from the fuel system on the vehicle. The mobile laboratory has instru- mentation to monitor carbon monoxide, total hydrocarbons, methane, ethylene and acetylene. Methane occurs naturally in the atmosphere at approximately two parts per million (p.p.m.) and not in significant quantities in vehicle exhaust emissions. This component of the total hydrocarbon concentration can be taken into account when
attributing hydrocarbon pollution to motor vehicles.
The oxides of nitrogen in vehicle exhausts are produced by the combination of atmospheric nitrogen and oxygen at high temperatures and pressures in the combustion chambers of the engine. Both nitric oxide and nitrogen dioxide are formed with nitric oxide being the dominant component. On dispersion in the atmosphere the nitric oxide slowly oxidises to nitrogen dioxide. Nitrogen dioxide is also present in the atmosphere from other forms of combustion.
The particulate pollutants are lead and smoke. Lead compounds are added to petrol to increase the octane number, (present maximum level is 0.55 g/l). The lead is emitted from vehicles mainly in particulate form, as lead halides, lead sulphate and lead oxide; there are also some volatile lead compounds emitted. Smoke includes all particulates, black smoke emitted by diesel engines consists mainly of fine carbon particles.
2. T H E C A R A V A N
External and internal views of the caravan are shown in Plates 1 and 2. The external dimensions are 5.4 metres long, 2.2 metres wide and 2.7 metres high. It is divided into two compartments, of 4.1 metres and 1.2 metres length, the smaller compartment being nearer to the tow bar. The large room is entered by double doors at the back of the caravan, the other by a single door in the side. The front compartment acts as a cloakroom and as store room for gas cylinders and back-up instrumentation. It has two small windows, lighting, heating, chemical toilet and sink with water supply. The main room has fitted benches, drawers and cupboards along three walls. Set into one of the benches is a sink with water supplied from a small tank at ceiling level. The small tank is fed by an electric pump from a main storage tank below this bench. There is also an electric water heater mounted above the sink. There are two large windows which can be protected by metal shutters. As good lighting is essential five fluorescent tubes are mounted above the bench space. Two electric heaters and an extraction fan are mounted on the walls. Specially designed clamps and bases are fitted either sid'e of the doors to hold the large hydrogen and air cylinders needed when operating the instruments with flame ionisation detectors. The mains power sockets for the instruments are situated in pairs on the benches. There are in all twenty sockets and when the laboratory is in full operation all the sockets are in use. Power for the laboratory when operating in the field is obtained from any accessible street lighting circuit. This is connected by armoured cable to the fuse box mounted on the wall in the main compartment. (Plate 3)
3. INSTRUMENTATION
The instruments used in this work can be classified into three main groups, those used to monitor the individual pollutants, those used to record meteorological parameters and those used to collect and process the data.
3.1 Gaseous pollutants
3.1.1 Oxides of nitrogen
A chemiluminescent method is employed to measure the concentration of nitric oxide (NO) in the atmosphere. Higher oxides of nitrogen are measured by firstly reducing them to NO by heating in the presence of a stainless steel catalyst and then using the chemiluminescence method to find the resulting NO concentration. The NO is blended with ozone (03) in a flow reactor to produce excited nitrogen dioxide (NO2) molecules, which revert to the ground state by emitting photons of a wavelength unique to this reaction. A schematic flow diagram of these operations is shown in Fig. 1. The chemiluminescent reaction is represented by the folloffing equation.
NO + 0 3 - - - - ~ N 0 2 + 02 + hv
Thus the concentration of NO and hence NOx, where NO x represents total oxides of nitrogen, can be found by measuring the amount of radiation emitted in this waveband using a system of filters and a photo-multiplier.
This method of measurement suffers little interference from other constituents of the air and the ouput from the photomultiplier is linearly proportional to the NO concentration. The zero calibration is obtained electronically by backing off the photomultiplier dark current when the ozonizer is switched off, that is, when there is no chemi- luminescent reaction. Span gases of known concentractions of NO and NO2 are used to calibrate the output f rom the photomultiplier. The meter reading can then be set to give direct readout in parts per million over eight ranges from 1000 p.p.m, to 0.01 p.p.m, full scale. The instrument used is a Chemiluminescent NO - NO x Gas Analyser
model 12A manufactured by Thermo Electron. (Plate 4)
To enable both the NO and NO x modes to be sampled automatically at frequent intervals a signal conditioning module was incorporated into the system. This switches the sample gas through and past the NO2 converter alternatively at intervals of seventy seconds. The NO and NO x values are retained in electronic memories until the next cycle. There is also an arithmetic unit to produce the function N O x - N O so that a reading for the NO 2 concentration can be obtained directly from the instrument. The three 10 volt outputs, NO, NO2, and NO x, are supplied to a multi-pen recorder and to the data logger.
3.1.2 C a r b o n m o n o x i d e a n d h y d r o c a r b o n s
• A chromatographic method of analysis is used to measure the concentrations of carbon monoxide and hydrocarbons present in the air. The ifistrument used for this purpose is a Beckman Model 6800 Air Quality Chromatograph
(Plate 3).
The operating principle of this instrument is the separation of the pollutants in the sample of air into components by column chromatography and to measure the magnitude of the components using a flame ionization detector (See 3.1.5). Carbon monoxide is catalytically converted to methane before it is passed through the flame ionization detector. In this way, a determination of the concentrations of carbon monoxide, methane, ethylene, acetylene and total hydrocarbons is made. The instrument performs six analyses of the air each hour.
Two outputs are available from the instrument, a continuous, 0-10mV output which gives a series of peaks, the heights of which indicate the concentrations of the components.and five, O-5V outputs, one for each pollutant mon- itored. The value of each component is stored electronically for the duration of the analysis cycle and is thus available for use with a multi-pen chart recorder and data logging equipment.
The chromatograph is calibrated periodically, using a standard mixture of the pollutants concerned in air. Tests of the instrument's stability over such periods of time have been made and an example of the results obtained is shown in Table 2. This shows that over a period of 14 days, the average error in the analysis of the standard gas
was 3.3 per cent.
The minimum concentrations of the pollutants required to produce full scale readings are 10 p.p.m, for carbon monoxide, methane and total hydrocarbons and 1.0 p.p.m, for ethylene and acetylene. These scales can be expanded to 10,000 p.p.m. (carbon monoxide, methane, total hydrocarbons) and 1000 p.p.m. (ethylene,acetylene) full scale readings, so that the instrument is capable of measuring all concentrations likely to be encountered under
normal conditions.
3.1.3 Volatile lead compounds
The method of sampling used is based on that described by Moss and Browett 2 .
A measured volume of air is drawn through the sampling system (see Fig 2) at a rate of approximately 120 1/h. The particulate matter is removed by a Millipore AAWP 04700 filter (0.8/am pore size) and the non particulate lead is absorbed by solutions of iodine monochloride. Corrosive vapours are removed by a sodium thiosulphate solution and a calcium chloride drying tower before the air is passed through a gas meter and pump. A total volume of approximately 20m 3 is sampled for each measurement.
6.2.73*
7.2.73
8 .2 .73
9.2.73
12.2.73
13.2.73
14.2.73
15.2.73
16.2.73
19.2.73
TABLE 2
Results o f stability check for Model 6800 Air Quality Chromatograph
0 - 1 0 m V output concentration (ppm)
0 - 5 V output concentration (ppm)
CH4 CO C2H4 C2H2 THC
47.0 52.0 45.5 10.5 16.0
49.0 54.0 46.0 10.5 16.0
45.5 51.0 44.0 10.0 15.5
45.0 51.0 45.0 10.0 15.5
43.0 49.0 42.5 9.5 15.0
43.0 49.5 42.0 I 0.0 15.5
43.0 49.5 42.5 11.0 16.5
43.0 49.5 42.0 10.0 15.0
43.0 51.0 43.0 10.0 15.5
45.5 52.5 44.0 10.0 15.5
CH4 CO C2H4 C2H2 THC
46.0 52.5 46.5 10.5 16.5
47.5 54.5 48.0 10.5 16.5
44.5 52.0 46.0 10.5 16.5
44.5 52.0 47.0 10.5 16.5
43.0 50.0 44.0 10.5 16.5,
42.0 50.0 43.0 10.0 16.5
41.5 49.0 43.0 10.5 16.5
42.5 50.5 44.0 10.5 16.5
43.5 51.0 45.0 10.5 16.5
45.0 53.5 46.0 10.5 16.5
* Calibrated values
Key CH4 Methane CO Carbon monoxide C2H4 Ethylene C2H2 Acetylene THC Total hydrocarbons
The lead absorbed into the solutions of iodine monochloride is determined by the method of Wallin et al 3 The lead is extracted into a solution of dithizone in 4 methyl pentan-2-one and its concentration measured by atomic absorption spectrophotometry.
3.2 Particulate pollutants
3.2.1 Smoke
A sequential smoke sampler manufactured by Glass Developments Ltd (Plate 5) is used to produce 1 hr, 3 hr, 8 hr, or 24 hr stains on Whatman No. 1 filter paper. The amount of particulate matter accumulated on the paper is then estimated using an Evans Electroselenium smoke stain meter. This gives a measure of the amount of light reflected from the surface of the stained paper. The meter is calibrated against a standard reflecting disc. The smoke concentrations are calculated from the diesel smoke calibration curve described by Wallin 4.
The smoke sampler consists of a small air pump and a gas meter mounted inside the instrument to draw a known volume of sample air through the falter paper, which is in the form of a roll and clamped at the sampling point. The paper is moved automatically at the desired intervals of time. The gas meter and reflectometer
readings are used to calculate the concentration of smoke in the air.
3.2.2 Particulate lead
A continuously operating instrument, similar in principle to that used for smoke, is used to collect the particulate lead in the atmosphere. A metered volume of air is drawn through glass fibre f'dter paper (Whatman GFA) at a rate of 1.5m3/h and after a predetermined period of time the falter paper is moved to a new position so that another sample can be taken. The delicate glass fibre filter is protected by a backing reel of Whatman No.1. The high air flow leads to a pressure drop being produced across the filter, this pressure drop has an effect on the response of the metering device which is taken into account by a correction according to Boyle's Law. It also produces a drag on the fdter paper at the changeover time which is overcome by automatically switching off the pump a few seconds before winding on so that the pressure can equilibrate on both sides of the falter paper. Sampling periods of 1,3,8 and 24 hours are available on the instrument.
The lead deposited on the filter is extracted into a solution of 5 per,cent nitric acid and 0.2 per cent of 30 per cent (W/V) hydrogen peroxide and the concentration of lead in the solution is measured by atomic absorption spectrophotometry using an air/acetylene flame and the 2170 fit line of the lead spectrum. The absorption is compared with that obtained with freshly prepared lead solutions of known concentration and the concentration of the lead found on the falter calculated. The volume of air drawn through the fdter is used to calculate the concentration of lead in the atmosphere in terms of micrograms per cubic metre.
3.3 Meteorological measurements
Meteorological conditions have a significant effect on the concentration of pollutants in the atmosphere. Of the many climatic factors,wind speed, wind direction, temperature and dewpoint have been selected for monitoring
alongside the pollutants.
3.3.1 Wind speed and wind direction
A mast is mounted on the caravan adjacent to the back door (see Plate 1). A Porton Wind vaneand a three cup Porton anemometer capable of measuring wind speeds up to 100 knots, manufactured by Spembly Ltd., are mounted at a height of four metres from the ground. Both instruments require a 12 volt d.c. supply, the output from the anemometer is recorded on a 1 volt d.c. Rustrack chart recorder, the output from the wind vane is recorded on a centre zero-6 volt to +6 volt chart recorder. Both recorders are incorporated in one unit (see Hate 6). This unit is connected to the data logger so that spot readings can be taken every ten minutes. The anemometer is calibrated against known frequencies from a signal generator. The wind vane axis is orientated at each site, so that a south wind is indicated by a central reading on the chart.
3.3.2 Dewpoint
The percentage relative humidity is a good indicator of the moisture content of the atmosphere. Although there are instruments on the market that will measure relative humidity in constant temperature environments there is none available which will operate in ambient air. However, dewpoint can be measured independently of temperature and together with temperature readings can be used to determine percentage relative humidity from
tables.
A hygrometer operating on a capacitance principle manufactured by Moisture Control and Measurement Ltd. (Plate 7) is used to give continuous readings of dewpoint. The sensor consists o f an aluminium rod, anodised to produce a porous aluminium oxide surface, around which a fine insulated copper wire is helically wound. The aluminium rod and copper wire act as the electrodes of a capacitor and the capacitance is varied by the number of
water molecules held in the pores of the oxide coating. The sensor is maintained at a constant 40°C so that the operating temperature is always above the maximum dewpoint temperature to be measured. The hygrometer is calibrated to measure dewpoints in the range-10°C to +20°C. A small pump is used to bring air from the. sampling point to the sensor. The hygrometer has a floating ouput of 0 to 10mV which is used to supply a chart recorder and a data logger.
3.3.3 Temperature
Four resistance thermometers are used to monitor temperature and the values are recorded on a Foster Cambridge four pen chart recorder giving a continuous record of ambient air temperatures. The electrical balancing cir-
. \ O cuits are contained inside the recorder and are attenuated to give a direct readout over the range-10°C to +40 C.
Unfortunately it is not possible to obtain an output signal from the therometers directly or from the recorder for use with the data logger. A small unit containing three Wheatstone Bridge networks and a suitable transformer to enable mains supply to be used has been constructed for this purpose. Up to three resistance thermometers can be used with this unit to supply the data logger with input voltages over the required temper- ature range.
3.4 Data collection
It is essential, with this continuous monitoring regime, to be able to collect the data in a form that a computer can handle. The output signals from each instrument are fed in turn into a digital voltmeter which is used as an analogue to digital converter. A data transfer unit then codes the information which together wi/.h the channel numbers and the time of day, is punched onto paper tape. The equipment used is shown in Plate 8. The papertape is processed on the Laboratory's computer.
3.4.1 Digital voltmeter
A Solartron digital voltmeter accepts the input voltages between the ranges of 25 mV and 1000 V. In this case all the inputs are attenuated to the 100 mV range which has a sensitivity of 10/aV. Before use, the digital voltmeter is zeroed and calibrated using an internal Western Standard cell.
3.4.2 Data transfer unit
The Solartron data transfer unit can accept up to twenty inputs. Each input is sampled in turn at preset time intervals. The time interval between sampling is determined by the 'scan command' switch on the clock unit. Twelve settings are available ranging from 1 second to 2 hours. The cycle time for the gas chromatograph is ten minutes and so the scan command is set to sample every ten minutes as this is the minimum time interval that can be usefully used without repeating measurements. The twenty four hour clock is set by means of the 'set clock' and 'load' switches so that real time is recorded. When sampling is initiated the output driver unit is switched on to transfer the coded data to the paper tape punch unit. The format of the data presented to the tape punch is:
1. Time at the initiation of sampling sequence, 24 hour representation showing hours, minutes and seconds.
2. Channel number, corresponding to the input terminals. The channels are numbered 00 to 19.
3. A positive or negative sign followed by a four digit voltage reading corresponding to the component sampled.
This data is then transferred consecutively to eight hole paper tape on a Facit 4070 tape punch unit.
3.4.3 Data processing
The monitoring system described above produces, in normal operation, enormous quantities of data in the form of paper tapes. In order for the data to be assimilated, they must be summarized and presented in an easily readable form. For this purpose, a computer program has been written in Fortran IV language for use on the Laboratory 's ICL System 4-70 Computer. The paper tape records collected during monitoring are analysed and their contents tabulated in the way shown in Fig.3. This presentation of the data gives one hour, twelve hour and twenty-four hour average concentrations, the length of time each day during which the concentration of each pollutant exceeded a'given limit and the four highest individual values measured during each day, together with the times at which they
occurred.
For many purposes, this gives a sufficiently detailed account of the pollution trends. The paper tapes, however, provide a permanent record of each individual reading and are available for more detailed analysis appropriate to the
needs of particular monitoring exercises.
3.5 Back-up facilities
In the event of one or more of the instruments becoming temporarily out of action, or if comparative studies necessitate monitoring simultaneously at more than one site, back-up facilities are available.
3.5.1 Total hydrocarbons
A Beckman Model 400 flame ionisation detector is used for the continuous monitoring of the total hydrocarbon
content of the air.
A potential is applied across a hydrogen flame and when the sample to be analysed is introduced into the flame, the hydrocarbons are ionised, causing a change in the current. The magnitude of the current gives a measure of the concentration of hydrocarbons in the air sample. By calibration with a standard gas mixture, a direct reading in p.p.m. (expressed as methane equivalents) can be obtained.
The minimum full scale sensitivity of the instrument is 1.0 p.p.m. This can be increased by factors of ten to
10,000 p.p.m.
3.5.2 Infra-red gas analyser
A Maihak Unor 2 single beam infra-red gas analyser is used for the determination of the carbon monoxide content
of the air.
Infra red radiation is passed through the sample gas. Absorption of the radiation by carbon monoxide mole- cules present in the gas causes a change in pressure in the detector which is measured by its action on the diaphragm of a capacitor. The changing capacitance of the measuring capacitor is converted by an amplifier into a D'.C. signal
proportional to the carbon monoxide concentration in the sample gas.
The instrument has a range of 0 - 100 p.p.m, of carbon monoxide and is calibrated by use of a standard gas
mixture.
4. FUTURE DEVELOPMENTS
Sulphur dioxide and ozone are two pollutants which will be monitored to make the facility already available more
comprehensive in its monitoring of atmospheric contaminants.
Sulphur dioxide is principally produced by processes other than the internal combustion of petroleum fuels but in densely trafficked areas the concentrations emitted by vehicles may be sufficiently large to have some environmental impact, particularly on vegetation. Ozone is produced by the interaction of other pollutants in the formation of photochemical smog; although smog from motor vehicles is not a problem at present in the United Kingdom, ozone monitoring will provide an indication of any worsening situation.
The data handling system will be reviewed at a later stage to assess the feasibility of installing on-line data processing in the mobile laboratory. This could lead to a significant saving in staff time with the elimination of the tape editing process.
In order to increase the ease of mobility of the monitoring laboratory the caravan will shortly be mounted on a lorry chassis so that it can be moved independently. This will greatly facilitate the location of the laboratory at a site and the movement between sites.
5. ACKNOWLEDGEMENTS
The work described in this report was carried out in the Environment Division (Division Head - Mr L H Watkins) o f the Transport Systems Department of the Transport and Road Research Laboratory. The research team was
D.M. Colwill, L.E. Hogbin, M.G. Bevan, A.J. Hickman and J.B. Cornel.
6. REFERENCES
1. PEGG, R.E. and A.W. RAMSDEN Towards cleaner diesel engines. Proc. Int. Clean Air Congress 1966, Part 1,154-156.
2. MOSS R. and E.V. BROWETT. Determination of tetra-alkyl lead vapour and inorganic lead dust in air. Analyst 1966, 9:1, 428-438.
3. WALLIN S.C. et al. The evaluation of a filter for the removal of lead from the exhausts of petrol engines. L.R. 170 (A.P.) (Warren Spring Laboratory, Department of Trade and Industry)
4. WALLIN S.C. Calibration of DSIR Standard Smoke Filter for diesel smoke. Int. J. Air Water Poll. 1965, 9, 351.
8
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Neg No B 817/73
Wind speed and direction equipment
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ABSTRACT
A mobile laboratory for monitoring air pollution from motor vehicles: M.G. BEVAN, A. J. HICKMAN: Department of the Environment , T R R L Supplementary Repor t 89 UC: Crowthorne, 1974 (Transport and Road Research Laboratory). The equipping and instru- mentation of a mobile laboratory for monitor ing atmospheric pol lut ion f rom m o t o r vehicles is described. The instruments for measuring the concentrat ions o f the various pol lutants and associated meteorological conditions are described in detail together with the way in which the data is processed.
ABSTRACT
A mobile laboratory for monitoring air pollution from motor vehicles: M.G. BEVAN, A. J. HICKMAN: Department of the Environment , T R R L Supplementa ry R e p o r t 89 UC: Crowthorne, 1974 (Transport and Road Research Laboratory) . The equipping and instru- mentat ion of a mobile laboratory for monitoring a tmospher ic po l lu t ion f rom m o t o r vehicles is described. The instruments for measuring the concentra t ions o f the various pol lu tants and associated meteorological conditions are described in detail together wi th the way in which the data is processed.
