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Variation in airborne particulate matter concentration over the first three metres from ground in a street canyon
PII:
Atmospheric Environment Vol. 32, No. 21, pp. 3795 — 3799, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2310(98)00076–4 1352—2310/98 $19.00#0.00
SHORT COMMUNICATION VARIATION IN AIRBORNE PARTICULATE MATTER CONCENTRATION OVER THE FIRST THREE METRES FROM GROUND IN A STREET CANYON: IMPLICATIONS FOR HUMAN EXPOSURE ALFRED MICALLEF* and JEREMY J. COLLS Division of Environmental Science, School of Biological Sciences, Sutton Bonington Campus, University of Nottingham, Loughborough LE12 5RD, U.K. (First received 29 September 1997 and in final form 20 January 1998. Published August 1998) Abstract—This note presents vertical concentration profiles of various size fractions of suspended particulate matter, measured over the first three metres from ground in a street canyon, using a novel sampling system. The daily average percentage difference of airborne particulate matter concentration between receptor heights 0.81 and 2.88 m, with the lower height taken as reference, was more than 35% for PM 10 and more than 12% for the inhalable fraction, for 5 out of the 7 days of measurement. These preliminary measurements, carried out during June-August 1997, consisting of 37 h of data, corroborate the idea put forward by Colls and Micallef (Atmospheric Environment 31, 4253—4254), that different height groups of the population are exposed to different concentrations. Air-quality standards, based on human exposure estimates, should take this variation into account. These measurements also have implications for the siting of urban air quality monitoring systems. ( 1998 Elsevier Science Ltd. All rights reserved Key word index: Vertical concentration profiles, suspended particulate monitoring, human exposure, air quality standards, street canyon.
INTRODUCTION
Airborne particulate matter has been shown to have adverse effects on human health and the environment (QUARG, 1996). Government agencies in different countries have set limits and guide values for the concentration of various size fractions of suspended particulate matter (SPM) with the intention of protecting public health and the environment. At the same time, monitoring of airborne particulate matter concentration has continued and in some countries even intensified especially in urban locations where concentrations tend to be highest. The main driving force behind this is the correlation that exists between concentration of the PM and PM fractions of 10 2.5 airborne particulate matter, and morbidity and mortality among human subjects (Vedal, 1997). Air-quality monitoring data is used in estimating human exposure to air pollution, and hence it is crucial for the measured concentrations to be representative of what humans breath in. Most of the time this is not the case (Ott, 1980; Vostal, 1994). Colls and
*Author to whom correspondence should be addressed.
Micallef (1997) generated vertical concentration profiles of SPM and PM on both the leeward and 10 windward sides of a street canyon over the first 3 m from ground level, using the SLAQ model (Micallef et al., 1997). Two scenarios were considered; the first was for heavy traffic with relatively low turbulence, and the second with low traffic and high turbulence. The first scenario exhibited higher vertical concentration gradient and concentration as compared to the second scenario. These modelled results indicated that different height groups of the population might be exposed to different concentrations and that air-quality monitoring systems, which are normally sited at about 2.5 m or higher above ground, might not be measuring concentrations that are representative of the concentrations to which human subjects are exposed. The measurements presented here corroborate these ideas and confirm that further investigation into the problem is warranted.
METHODOLOGY AND SAMPLING SITE
Vertical concentration profiles of the inhalable, thoracic, alveolic, PM and PM airborne particle 10 2.5 3795
3796
Short Communication
Measurements were carried out using KSS system on high street, Loughborough, Leicestershire, United Kingdom. The street is confined on both sides by buildings with an approximate height of 17 m, and has a width-to-height ratio of 1.0. The variation in the hourly average traffic count is typical of a trunk road cutting across an urban area; the average flow is 600 vehicles per hour on each of the two lanes carrying traffic in opposite directions. The street is bound by two signalised intersections, with one end close to a large pedestrianised area in the town centre. Measurements were carried out close to this end of the street (see Fig. 1). Data were collected at different times of the day, and on different weekdays, for seven days spread over the time period June—August 1997. This ensured that different meteorological and traffic conditions were considered. A total of 37 hourly average concentration data were collected for each of the six receptor levels. The analysis of these data is discussed below.
RESULTS AND DISCUSSION
Fig. 1. Photograph showing the general features of the kinetic sequential sampling (KSS) system and the location in high street, Loughborough, United Kingdom where measurements were carried out.
fractions were measured using the kinetic sequential sampling (KSS) system shown in Fig. 1, which has been described in detail elsewhere (Micallef et al., in press) and hence only a brief description is included here. In the KSS system, airborne particle concentration is measured by a portable optical particle monitor (Model 1.104/5, Grimm Labortechnik Ltd., Ainring, Germany) strapped to a ‘‘lift’’ which ascends and descends vertically at predetermined time intervals. The lift stops at six separate, but equally spaced, levels for monitoring to take place. On completion of monitoring at the uppermost level, the lift returns directly to the lower level before starting to ascend, level by level, once again. This sequence ensures that the time for monitoring at each level is constant and that the time interval between successive measurements at the same level is also identical. The whole operation is controlled by a purpose-built electronic circuit that was interfaced with the optical monitor in order to relate accumulated data to the appropriate receptor heights. In this work, the heights considered are 0.35, 0.81, 1.23, 1.77, 2.30 and 2.88 m, measured from ground.
Figure 2 shows a sample from the hourly averaged vertical concentration profiles for the inhalable and PM particle fractions. Nearly, all of the profiles 10 shown exhibit a decrease in concentration with height. This feature was common to the majority of the profiles measured in the street canyon environment. Variation in concentration at a given height depends on several factors, which include vehicle-generated (thermal and mechanical) turbulence, environmental (convective and mechanical) turbulence and variation in traffic flow. The extent to which these factors are capable of altering the pollutant concentration at a given height depends on the geometry of the street, e.g. street orientation and aspect ratio. In Fig. 2 this variation is represented by an ‘‘error bar’’ of length equal to twice the standard deviation of concentration measured at each of the receptor heights. In most cases variation in concentration is not negligible but this does not undermine the hourly average concentration gradient and concentration. Figures 3 and 4 show time series of hourly average concentration measured at 0.81 and 2.88 m heights, for the inhalable and PM fractions, respectively. It 10 is clear that higher concentrations of both fractions were measured at the lower receptor height. Out of a total of 37 h, 29 exhibited higher concentration for the inhalable fraction at 0.81 m height, and 28 for PM . At both receptor heights, the measured hourly 10 average PM concentrations are well below the limit 10 value of 50 kg m~3 as a 24 h running average, set by the United Kingdom’s Expert Panel on Air-Quality Standards (1995), but one should bear in mind that as with all optical particle monitors employing the light scattering principle, the response very much depends on the diameter of the aerosol particles; efficiency tends to be low for relatively large particles (which are
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3797
Fig. 2. Sample of hourly average vertical concentration profiles for the inhalable and PM particle size 10 fractions of airborne particulate matter measured on High Street, Loughborough, United Kingdom on 10 July and 7 August 1997. The length of each of the ‘‘error bars’’ is equal to twice the standard deviation of the measured concentrations at the particular receptor level for the given particle size fraction.
pre-collected by a cyclone sampling head with a cutoff at 15 km in the case of the monitor used in this work) and also for particles that are small compared to the wavelength of the monochromatic light source used (in this case, 780 nm from a solid state laser (H. Grimm, personal communication)). The smallest particle diameter recorded by the optical particle monitor is 0.35 km (Grimm Labortechnik Ltd, 1996). Hence, for environments where the mode of the airborne particle distribution lies well below this value, concentrations may be under measured. In the street canyon environment, there are five principal sources of airborne particulate matter: vehicle exhaust, resuspended road dust, particulate matter originating
from tyre wear and brakes, secondary aerosol particles formed by gas-to-particle conversion and the background aerosol. Particles from these sources have different aerosol distributions but certainly the majority of particles from vehicle exhaust tend to be smaller than 1 km diameter (Greenwood, 1996). Consequently, airborne particulate matter concentration measured in the street environment, by an optical particle monitor, tend to be under measured. The under measurement of the optical particle monitor has implications for the absolute concentrations but less so for the concentration gradient. Figure 5 shows the daily average (for those hours during which measurements were carried out) per-
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Short Communication
Fig. 3. Time series of the inhalable fraction of airborne particulate matter measured at heights 0.81 and 2.88 m from ground in high street, Loughborough, United Kingdom.
Fig. 4. Time series of the PM fraction of airborne particulate matter measured at heights 0.81 and 2.88 m 10 from ground in high street, Loughborough, United Kingdom.
centage difference of airborne particulate matter concentration between receptor heights 0.81 and 2.88 m, with the lower height taken as reference. For 5 out of 7 days, the difference was more than 35% for PM 10 and more than 12% for the inhalable fraction. This discrepancy in the percentage difference between the two fractions may be explained by the fact that the main source of PM in the canyon is vehicle exhaust 10 that is released near ground for most vehicles. Only on 5 August 1997, concentration at 2.88 m height
was on average slightly higher than that at the 0.81 m receptor height, and this exception occurred for the inhalable fraction, part of which is background aerosol transported to the confines of the street and hence not associated with any particular height. This exceptional case may also be explained by the variation in the calculated percentage difference, whose standard deviation (the length of the ‘‘error bar’’ in Fig. 5) is at least five times the size of the average value.
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3799
Fig. 5. Daily average percentage difference of concentration for the inhalable and PM fractions of 10 airborne particulate matter between heights 0.81 and 2.88 m, with the lower receptor height taken as reference. The length of each of the ‘‘error bars’’ is equal to one standard deviation of the parameter for the given particle size fraction.
The measured concentration differences and gradients, although small, should be seen in the context of epidemiological studies of urban air pollution, which have shown that adverse health effects increase by 1% for each 1 kg m~3 increase in black smoke (Sunyer et al., 1991). Figure 5 implies two things. First, samples taken by urban air-pollution monitoring systems sited in the confines of streets may not be representative of the concentration to which human subjects are exposed, as most systems are located at heights greater than 2 m for ‘‘practical’’ purposes. Secondly, different height groups of the population may be exposed to different airborne particle concentrations and distributions. The latter aspect has implications for the health of certain groups in society, e.g. children, and babies in pushchairs. It is a problem that presents science with practical difficulties in ensuring health protection to these individuals but surely merits consideration at the earliest. Future setting of air-quality standards should take into account consideration relating to siting of air-pollution monitoring systems for realistic estimates of human exposure associated with different groups of the population. Acknowledgements—The authors would like to thank Christopher Norton Deuchar for his help during the monitoring campaign and the anonymous referees for their comments. A.M. wishes to thank the Commonwealth Scholarship Commission in London for sponsoring his doctoral research.
REFERENCES
Colls, J. J. and Micallef, A. (1997) Towards better human exposure estimates for setting of air quality standards. Atmospheric Environment 31, 4253—4254.
Expert Panel on Air Quality Standards (1995) Particles, Department of the Environment. HMSO, London. vii #30 p. Greenwood, S. J. (1996) Automotive particulate size distributions including 2-strokes, diesels, gasoline and CNG fuelled engines, Proceedings of the ‘Vehicle Emissions Meeting’ organised by The Aerosol Society at the University of Wales, Swansea, United Kingdom, 16 April. Grimm Labortechnik Ltd (1996) Manual for the Dust Monitor 1.104, 1.105, 1.106, Third edition. Ainring, Germany. 43 p. Micallef, A., Deuchar, C. N. and Colls, J. J. (in press) Kinetic Sequential Sampling (KSS) system: an automated sampling system for measuring vertical concentration profiles of airborne particles. Journal of the Air and ¼aste Management Association. Micallef, A., Singh, R. B. and Colls, J. J. (1997) SLAQ: a street level air quality model for vehicle-generated suspended particulate matter, PM and PM , Proceedings of the 10 2.5 11th Annual Conference of The Aerosol Society entitled ‘Aerosols: Their Generation, Behaviour and Applications’, held at the University of Warwick, Coventry, United Kingdom, 18—20 March. pp. 49—54. Ott, W. R. (1980) Models of Human Exposure to Air Pollution, SIMS Technical Report Number 32. Department of Statistics, Stanford University, California. Quality of Urban Air Review Group (1996) Airborne Particulate Matter in the ºnited Kingdom, Third Report. Department of the Environment, United Kingdom. ii#176 pp. Sunyer, J., Anto, J. M., Murillo, C. and Saez, M. (1991) Effects of urban air pollution on emergency room admissions for chronic obstructive pulmonary disease. American Journal of Epidemiology 134, 277—286. Vedal, S. (1997) Critical Review—Ambient particles and health: lines that divide. Journal of the Air and ¼aste Management Association 47, 551—581. Vostal, J. J. (1994) Physiologically based assessment of human exposure to urban air pollutants and its significance for public health risk evaluation. Environmental Health Perspectives 102 (Suppl. 4), 101—106.
Atmospheric Environment Vol. 32, No. 21, pp. 3795 — 3799, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2310(98)00076–4 1352—2310/98 $19.00#0.00
SHORT COMMUNICATION VARIATION IN AIRBORNE PARTICULATE MATTER CONCENTRATION OVER THE FIRST THREE METRES FROM GROUND IN A STREET CANYON: IMPLICATIONS FOR HUMAN EXPOSURE ALFRED MICALLEF* and JEREMY J. COLLS Division of Environmental Science, School of Biological Sciences, Sutton Bonington Campus, University of Nottingham, Loughborough LE12 5RD, U.K. (First received 29 September 1997 and in final form 20 January 1998. Published August 1998) Abstract—This note presents vertical concentration profiles of various size fractions of suspended particulate matter, measured over the first three metres from ground in a street canyon, using a novel sampling system. The daily average percentage difference of airborne particulate matter concentration between receptor heights 0.81 and 2.88 m, with the lower height taken as reference, was more than 35% for PM 10 and more than 12% for the inhalable fraction, for 5 out of the 7 days of measurement. These preliminary measurements, carried out during June-August 1997, consisting of 37 h of data, corroborate the idea put forward by Colls and Micallef (Atmospheric Environment 31, 4253—4254), that different height groups of the population are exposed to different concentrations. Air-quality standards, based on human exposure estimates, should take this variation into account. These measurements also have implications for the siting of urban air quality monitoring systems. ( 1998 Elsevier Science Ltd. All rights reserved Key word index: Vertical concentration profiles, suspended particulate monitoring, human exposure, air quality standards, street canyon.
INTRODUCTION
Airborne particulate matter has been shown to have adverse effects on human health and the environment (QUARG, 1996). Government agencies in different countries have set limits and guide values for the concentration of various size fractions of suspended particulate matter (SPM) with the intention of protecting public health and the environment. At the same time, monitoring of airborne particulate matter concentration has continued and in some countries even intensified especially in urban locations where concentrations tend to be highest. The main driving force behind this is the correlation that exists between concentration of the PM and PM fractions of 10 2.5 airborne particulate matter, and morbidity and mortality among human subjects (Vedal, 1997). Air-quality monitoring data is used in estimating human exposure to air pollution, and hence it is crucial for the measured concentrations to be representative of what humans breath in. Most of the time this is not the case (Ott, 1980; Vostal, 1994). Colls and
*Author to whom correspondence should be addressed.
Micallef (1997) generated vertical concentration profiles of SPM and PM on both the leeward and 10 windward sides of a street canyon over the first 3 m from ground level, using the SLAQ model (Micallef et al., 1997). Two scenarios were considered; the first was for heavy traffic with relatively low turbulence, and the second with low traffic and high turbulence. The first scenario exhibited higher vertical concentration gradient and concentration as compared to the second scenario. These modelled results indicated that different height groups of the population might be exposed to different concentrations and that air-quality monitoring systems, which are normally sited at about 2.5 m or higher above ground, might not be measuring concentrations that are representative of the concentrations to which human subjects are exposed. The measurements presented here corroborate these ideas and confirm that further investigation into the problem is warranted.
METHODOLOGY AND SAMPLING SITE
Vertical concentration profiles of the inhalable, thoracic, alveolic, PM and PM airborne particle 10 2.5 3795
3796
Short Communication
Measurements were carried out using KSS system on high street, Loughborough, Leicestershire, United Kingdom. The street is confined on both sides by buildings with an approximate height of 17 m, and has a width-to-height ratio of 1.0. The variation in the hourly average traffic count is typical of a trunk road cutting across an urban area; the average flow is 600 vehicles per hour on each of the two lanes carrying traffic in opposite directions. The street is bound by two signalised intersections, with one end close to a large pedestrianised area in the town centre. Measurements were carried out close to this end of the street (see Fig. 1). Data were collected at different times of the day, and on different weekdays, for seven days spread over the time period June—August 1997. This ensured that different meteorological and traffic conditions were considered. A total of 37 hourly average concentration data were collected for each of the six receptor levels. The analysis of these data is discussed below.
RESULTS AND DISCUSSION
Fig. 1. Photograph showing the general features of the kinetic sequential sampling (KSS) system and the location in high street, Loughborough, United Kingdom where measurements were carried out.
fractions were measured using the kinetic sequential sampling (KSS) system shown in Fig. 1, which has been described in detail elsewhere (Micallef et al., in press) and hence only a brief description is included here. In the KSS system, airborne particle concentration is measured by a portable optical particle monitor (Model 1.104/5, Grimm Labortechnik Ltd., Ainring, Germany) strapped to a ‘‘lift’’ which ascends and descends vertically at predetermined time intervals. The lift stops at six separate, but equally spaced, levels for monitoring to take place. On completion of monitoring at the uppermost level, the lift returns directly to the lower level before starting to ascend, level by level, once again. This sequence ensures that the time for monitoring at each level is constant and that the time interval between successive measurements at the same level is also identical. The whole operation is controlled by a purpose-built electronic circuit that was interfaced with the optical monitor in order to relate accumulated data to the appropriate receptor heights. In this work, the heights considered are 0.35, 0.81, 1.23, 1.77, 2.30 and 2.88 m, measured from ground.
Figure 2 shows a sample from the hourly averaged vertical concentration profiles for the inhalable and PM particle fractions. Nearly, all of the profiles 10 shown exhibit a decrease in concentration with height. This feature was common to the majority of the profiles measured in the street canyon environment. Variation in concentration at a given height depends on several factors, which include vehicle-generated (thermal and mechanical) turbulence, environmental (convective and mechanical) turbulence and variation in traffic flow. The extent to which these factors are capable of altering the pollutant concentration at a given height depends on the geometry of the street, e.g. street orientation and aspect ratio. In Fig. 2 this variation is represented by an ‘‘error bar’’ of length equal to twice the standard deviation of concentration measured at each of the receptor heights. In most cases variation in concentration is not negligible but this does not undermine the hourly average concentration gradient and concentration. Figures 3 and 4 show time series of hourly average concentration measured at 0.81 and 2.88 m heights, for the inhalable and PM fractions, respectively. It 10 is clear that higher concentrations of both fractions were measured at the lower receptor height. Out of a total of 37 h, 29 exhibited higher concentration for the inhalable fraction at 0.81 m height, and 28 for PM . At both receptor heights, the measured hourly 10 average PM concentrations are well below the limit 10 value of 50 kg m~3 as a 24 h running average, set by the United Kingdom’s Expert Panel on Air-Quality Standards (1995), but one should bear in mind that as with all optical particle monitors employing the light scattering principle, the response very much depends on the diameter of the aerosol particles; efficiency tends to be low for relatively large particles (which are
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3797
Fig. 2. Sample of hourly average vertical concentration profiles for the inhalable and PM particle size 10 fractions of airborne particulate matter measured on High Street, Loughborough, United Kingdom on 10 July and 7 August 1997. The length of each of the ‘‘error bars’’ is equal to twice the standard deviation of the measured concentrations at the particular receptor level for the given particle size fraction.
pre-collected by a cyclone sampling head with a cutoff at 15 km in the case of the monitor used in this work) and also for particles that are small compared to the wavelength of the monochromatic light source used (in this case, 780 nm from a solid state laser (H. Grimm, personal communication)). The smallest particle diameter recorded by the optical particle monitor is 0.35 km (Grimm Labortechnik Ltd, 1996). Hence, for environments where the mode of the airborne particle distribution lies well below this value, concentrations may be under measured. In the street canyon environment, there are five principal sources of airborne particulate matter: vehicle exhaust, resuspended road dust, particulate matter originating
from tyre wear and brakes, secondary aerosol particles formed by gas-to-particle conversion and the background aerosol. Particles from these sources have different aerosol distributions but certainly the majority of particles from vehicle exhaust tend to be smaller than 1 km diameter (Greenwood, 1996). Consequently, airborne particulate matter concentration measured in the street environment, by an optical particle monitor, tend to be under measured. The under measurement of the optical particle monitor has implications for the absolute concentrations but less so for the concentration gradient. Figure 5 shows the daily average (for those hours during which measurements were carried out) per-
3798
Short Communication
Fig. 3. Time series of the inhalable fraction of airborne particulate matter measured at heights 0.81 and 2.88 m from ground in high street, Loughborough, United Kingdom.
Fig. 4. Time series of the PM fraction of airborne particulate matter measured at heights 0.81 and 2.88 m 10 from ground in high street, Loughborough, United Kingdom.
centage difference of airborne particulate matter concentration between receptor heights 0.81 and 2.88 m, with the lower height taken as reference. For 5 out of 7 days, the difference was more than 35% for PM 10 and more than 12% for the inhalable fraction. This discrepancy in the percentage difference between the two fractions may be explained by the fact that the main source of PM in the canyon is vehicle exhaust 10 that is released near ground for most vehicles. Only on 5 August 1997, concentration at 2.88 m height
was on average slightly higher than that at the 0.81 m receptor height, and this exception occurred for the inhalable fraction, part of which is background aerosol transported to the confines of the street and hence not associated with any particular height. This exceptional case may also be explained by the variation in the calculated percentage difference, whose standard deviation (the length of the ‘‘error bar’’ in Fig. 5) is at least five times the size of the average value.
Short Communication
3799
Fig. 5. Daily average percentage difference of concentration for the inhalable and PM fractions of 10 airborne particulate matter between heights 0.81 and 2.88 m, with the lower receptor height taken as reference. The length of each of the ‘‘error bars’’ is equal to one standard deviation of the parameter for the given particle size fraction.
The measured concentration differences and gradients, although small, should be seen in the context of epidemiological studies of urban air pollution, which have shown that adverse health effects increase by 1% for each 1 kg m~3 increase in black smoke (Sunyer et al., 1991). Figure 5 implies two things. First, samples taken by urban air-pollution monitoring systems sited in the confines of streets may not be representative of the concentration to which human subjects are exposed, as most systems are located at heights greater than 2 m for ‘‘practical’’ purposes. Secondly, different height groups of the population may be exposed to different airborne particle concentrations and distributions. The latter aspect has implications for the health of certain groups in society, e.g. children, and babies in pushchairs. It is a problem that presents science with practical difficulties in ensuring health protection to these individuals but surely merits consideration at the earliest. Future setting of air-quality standards should take into account consideration relating to siting of air-pollution monitoring systems for realistic estimates of human exposure associated with different groups of the population. Acknowledgements—The authors would like to thank Christopher Norton Deuchar for his help during the monitoring campaign and the anonymous referees for their comments. A.M. wishes to thank the Commonwealth Scholarship Commission in London for sponsoring his doctoral research.
REFERENCES
Colls, J. J. and Micallef, A. (1997) Towards better human exposure estimates for setting of air quality standards. Atmospheric Environment 31, 4253—4254.
Expert Panel on Air Quality Standards (1995) Particles, Department of the Environment. HMSO, London. vii #30 p. Greenwood, S. J. (1996) Automotive particulate size distributions including 2-strokes, diesels, gasoline and CNG fuelled engines, Proceedings of the ‘Vehicle Emissions Meeting’ organised by The Aerosol Society at the University of Wales, Swansea, United Kingdom, 16 April. Grimm Labortechnik Ltd (1996) Manual for the Dust Monitor 1.104, 1.105, 1.106, Third edition. Ainring, Germany. 43 p. Micallef, A., Deuchar, C. N. and Colls, J. J. (in press) Kinetic Sequential Sampling (KSS) system: an automated sampling system for measuring vertical concentration profiles of airborne particles. Journal of the Air and ¼aste Management Association. Micallef, A., Singh, R. B. and Colls, J. J. (1997) SLAQ: a street level air quality model for vehicle-generated suspended particulate matter, PM and PM , Proceedings of the 10 2.5 11th Annual Conference of The Aerosol Society entitled ‘Aerosols: Their Generation, Behaviour and Applications’, held at the University of Warwick, Coventry, United Kingdom, 18—20 March. pp. 49—54. Ott, W. R. (1980) Models of Human Exposure to Air Pollution, SIMS Technical Report Number 32. Department of Statistics, Stanford University, California. Quality of Urban Air Review Group (1996) Airborne Particulate Matter in the ºnited Kingdom, Third Report. Department of the Environment, United Kingdom. ii#176 pp. Sunyer, J., Anto, J. M., Murillo, C. and Saez, M. (1991) Effects of urban air pollution on emergency room admissions for chronic obstructive pulmonary disease. American Journal of Epidemiology 134, 277—286. Vedal, S. (1997) Critical Review—Ambient particles and health: lines that divide. Journal of the Air and ¼aste Management Association 47, 551—581. Vostal, J. J. (1994) Physiologically based assessment of human exposure to urban air pollutants and its significance for public health risk evaluation. Environmental Health Perspectives 102 (Suppl. 4), 101—106.