A C O R R E L A T I O N S T U D Y OF V E H I C L E - G E N E R A T E D

A I R P O L L U T A N T S

R O G E R P E R R Y and K E I T H W H I T E L A W

Public Health Engineering, Imperial College, London SW7 2AZ, U.K.

and

ROY M. H A R R I S O N

Department o f Environmental Sciences, University o f Lancaster, Lancaster LA1 4YQ, U.K.

(Received 27 December, 1977; revised 6 April, 1978)

Abstract. The concentrations of Pb, CO, CH 4 and total hydrocarbons have been measured at a roadside site and at the exit of a multistorey car park. Average concentrations over short periods (10 to 25 min) have been calculated for each pollutant and possible correlations between these average levels of the different pollutants investigated. Significant correlations were found at only one site and the conclusion was drawn that it is only under exceptional conditions that correlations in levels exist over these extremely short averaging periods.

1. Introduction

Extensive surveys of the concentrations of Pb and CO in urban atmospheres have been conducted by Brief et al. (1960), Chovin (1967), Colucci et al. (1968), Georgii and Jost (1971), Bayley and Dockerty (1972), Martens et al. (1973) and others. In some instances these workers have investigated possible correlations between the atmospheric concentrations of Pb and CO and between these pollutants and the traffic volume. Highly significant relationships have been demonstrated in some cases.

In the above studies relatively long-term average concentrations of the pollutants were determined (from several hours to several days). In contrast, in the work described here, total sampling periods of 4 to 7 h were divided into far shorter periods (10 to 25 min) in oyder to investigate possible relationships between Pb and CO concentrations existing over this short time scale. In addition, measurements of CH 4 and total hydrocarbon (THC) concentrations have been made over the same sampling periods and the data analysed in a similar manner. Measurements were made at two very different sites to emphasize possible contrasts between sampling locations.

2. Experimental

2.1. DETAILS OF SAMPLING SITES

2.1.1. L o n d o n W.14

Samples were collected at the exit of a multistorey car park. The sampling point was within the enclosed car park, about 5 m from traffic leaving the park, but the sampler

Water, Air, and Soil Pollution 10 (1978) 115-127. All Rights Reserved Copyright © 1978 by D. Reidel Publishing Company, Dordrecht, Holland

116 ROGER PERRY ETAL.

was also exposed to polluted air from the road running alongside the car park. Sampling was performed between 13.00 and 19.05 h on 22 October, 1975.

2.1.2. Exhibition Road, London S. W.Z

Measurements were made at the curbside of a busy London street, adjacent to a junction controlled by traffic lights. On the first occasion, samples were collected at the apex of the junction on 3 June, 1975 between 13.55 and 17.15 h, and subsequently samples were collected at about 10 m from the junction on 19 June, 1975 between 12.00 and 18.55 h.

2.2. SAMPLING AND ANALYTICAL PROCEDURES

A mobile laboratory was equipped for this study. A rapid continuous stream of air was drawn through an 8 cm diameter PVC sampling duct from a height of 3 m; 0.5 m above the laboratory roof. Subsamples of the air stream were withdrawn for analysis.

2.2.1. Lead.

Particulate Pb was sampled isokinetically at a rate of 5 1 rain -1 through a 0.22/~m Millipore filter type GSWP 047 00. Sampling periods were varied according to the expected level of Pb - from 10 to 20 min (Exhibition Road) to 25 min (London W. 14). The Pb was determined by extraction into nitric acid/hydrogen peroxide (Harrison et al., 1974) and subsequent flameless atomic absorption spectrophotometry using a Perkin-Elmer flameless graphite atomiser and Model 305 spectrophotometer.

2.2.2. Carbon Monoxide, Methane, Total Hydrocarbons.

These were all measured using a Beckman Model 6800 Air Quality Chromatograph, calibrated with a standard gas mixture (British Oxygen Co., Ltd.). The instrument

was run concurrently with the lead sampling and the three pollutants were deter- mined on a 5 rain cycle; CO and CH 4 from one sample of air, and THC from a separate sample. Thus instantaneous readings of concentration are given and these have been averaged over the concurrent collection periods for Pb at each site. Since the detection system of the instrument is based upon flame ionization, the THC concentrations are expressed as ppm CH 4 equivalents.

2.2.3. Traffic Volume.

At the London W.14 site the traffic volume was counted during each Pb sampling period.

3. Results

The ranges of pollutant concentration at the two sites are shown in Table I. The mean concentrations of Pb over the full extent of each sampling period are shown,

A CORRELATION STUDY OF VEHICLE-GENERATED AIR POLLUTANTS I 17

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TABLE II

Tr a f f i c v o l u m e s at the air sampl ing sites

Tr a f f i c v o l u m e (vehic les h -1)

Petro l Diese l Tota l Petro l vehic les vehicles vehicles vehicles leaving car park

Site (PV) ( D V ) (TV) ( P V C P )

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A C O R R E L A T I O N S T U D Y O F V E H I C L E - G E N E R A T E D A I R P O L L U T A N T S l 19

together with the corresponding standard deviation. In the case of CO, C H 4 and THC, several readings were taken in each individual Pb sampling period. Hence mean concentrations have been calculated, as well as the corresponding standard deviation. Consequently, when a range of means is given, a standard deviation accompanies the maximum and minimum mean level and indicates the deviation of that variable during the averaging period (the lead sampling period).

The ranges in measured traffic volume are given in Table II. As the Exhibition Road site was at a busy road junction, no accurate measurements were taken, and only typical average figures are presented.

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120 ROGER PERRY ET AL.

The temporal variations in the measured pollutant levels are shown graphically in Figures 1 to 3 for the two sites. Both individual and time-averaged readings are included.

4. Analysis of the Data

The data were subjected to a first, second and third order regression analysis to determine the relationships, if any, between the measured variables. The significance of the indicated correlations was determined by application of the F-test, and the results are tabulated in Table III.

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In calculation of the correlations of CO, THC, CH4 and traffic volume with Pb, the mean value of each of these variables over the Pb sampling period has been utilized. The correlations between these time-averaged values for CO, THC, CH4 and traffic volume have also been determined, as well as the correlations between the individual 5 min readings for CO and CH4, which are analyzed in the same sample of air. A correlation coefficient is indicated, and the equation of the linear regression line given for correlations of more than 90°7o significance (probability point <0.1) as shown by the F-test.

5. Discussion

5.1. THE POLLUTANT CONCENTRATIONS

The major difference between this and other studies is the use of concentrations averaged over short time periods. This is especially important for Pb, for which continuous monitoring instruments are not available, and almost all previously reported levels of airborne Pb have been long-term averages. In some instances, notably at the London W. 14 site, quite significant temporal variations in atmospheric Pb concentrations were observed.

At the sites chosen there is no known major industrial source of Pb, and the average Pb concentrations over the full sampling periods approximately reflect those anticipated for roadside sites within an urban area. The mean levels of CO and THC measured on Exhibition Road are also typical of a busy urban street.

The London W.14 car park site shows a high CO mean over the total sampling period (11.0 ppm), as well as elevated mean levels of THC (5.1 ppm) and CH 4 (2.4 ppm). These very high pollutant concentrations at a site where traffic volume was not high are indicative of the poor ventilation typical of many multistorey car parks. The fact that the mean Pb concentration (6.0//g m -a) is not especially high and the mean Pb/CO and Pb/THC ratios are lower than for the other site indicates a substantial contribution from cold choked vehicles, such as those being driven from the car park, as the sampling site was situated at the exit. Vehicles outside the car park can also contribute to pollution at the sampling point and Figure 4 shows the temporal variation of petrol traffic volume in the vicinity, the number of vehicles leaving the car park and the pollutant levels.

5 .2 . CORRELATIONS

In the P b - C O correlation studies of other workers, summarized in Table IV, good correlations have often been found. Only linear correlations have been reported, and it is not clear whether these workers examined their data for other than first order regression.

In the work reported here, the data were subjected to first, second and third order regression analysis so that any more complex forms of regression should be

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Fig. 4.

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identified. However, no significant P b - C O correlation was found. There are two possible reasons for this: the :sampling technique or the shorter averaging times. Previous workers have used continuous response monitoring methods for CO, or methods giving an integrated concentration over the sample collection period. In this study, instantaneous concentrations were measured at 5 min intervals and averaged over the Pb sampling period. At the London W.14 car park site this technique showed good correlations of THC with CO, and of CO with the volume of petrol vehicles leaving the car park but not of Pb with CO. This was despite a degree of variability in the THC, CO and CH 4 levels about the calculated means over the Pb sampling periods which was comparable with the other site. The coefficients of variation for CO, THC and CH 4 levels within the 20 min averaging periods for Exhibition Road on 3 June, 1975 are not, in general, different in magnitude from those over the 25 rain averaging period at London W.14. Consequently, it is clear that where correlations exist this procedure will indicate them. The other factor, that of shorter averaging periods is of far greater importance.

In explaining this apparent discrepancy, it is instructive to consider the reasons why other wakers have observed good P b - C O correlations. Over a long averaging period (say 24 h), the existence of a correlation between Pb and CO levels at a given site implies that over that period the local sources of CO and Pb (presumed to be motor vehicles) are emitting the pollutants with a time-averaged constant ratio of source strengths. Daily differences in average levels are due to such factors as traffic volume and meteorological conditions. For the general case of a linear regression

Pb (jag m -3) = mCO (ppm) + c •

widely differing values of the gradient, m, and intercept, c, have been observed. This is only to be expected as each sampling site has its own characteristic meteorology and topography, and equally important its own typical driving mode. Colucci et al.

(1968) noted that at a freeway site the gradient of the regression line, equal to the

A CORRELATION STUDY OF VEHICLE-GENERATED AIR POLLUTANTS 125

ratio Pb:CO after subtraction of the intercept, was substantially greater than at a residential or commercial site, consistent with high speed driving causing high emis- sions of Pb and low CO. Conversely, over a very short sampling period (say the time taken for passage of one car), the ratio of source strengths (Pb/CO) is highly variable, dependent upon the driving mode of the car since low Pb emissions are frequently accompanied by relatively high CO and vice versa. Consequently, no correlation of the resultant ambient levels would be expected. Over far longer sampling periods, such factors are 'averaged out' due to the very large numbers of contributing sources, and correlations emerge. The differences between sites are related to the predominant driving mode, as shown by Colucci et al. (1968), as well as contributions from neighboring sites and the distance of the air sampler from the road, as the rates of the sink processes for CO and Pb are not identical. For inter- mediate sampling periods (10 to 25 min), as used in this study, it was not clear whether significant correlations would be seen. At the Exhibition Road site distinct differences between the 10 to 25 min sampling periods arise due to changes in driving mode caused by varying traffic congestion, the starting of cold engines at commut- ing times etc. and P b - C O correlations are unlikely. The lack of significant CH4-CO correlations serves to emphasize this point as the two are analyzed in a single sample of air and if short-term correlations existed they should be readily apparent. Temporal variations in meteorological conditions may also influence ambient concentration ratios due to the different dispersion properties of the various pollutants and changes in the contribution of different sink mechanisms, e.g., precipitation scavenging or gravitational settling.

At the London W. 14 site, vehicles were leaving a car park and were all in a similar driving mode, hence providing a relative uniformity in the ratio of pollutant source strengths. Examination of Figure 4 suggests a relationship between the Pb level and the number of petrol vehicles leaving the car park, but the regression analysis did not find this to be of high significance. The CO level did however correlate closely with the number of petrol vehicles leaving the car park, indicating these to be the major pollution source. The levels of the THC and CO also correlate closely, as might be expected for cold choked vehicles. These were the only significant correlations found in this work, and are believed to exist solely due to the relative uniformity of sources at the car park site.

There are many similarities in the patterns of dependence of vehicle exhaust emis- sions of CO and THC upon driving mode (Perry and Slater, 1975). Although the situation is complicated somewhat by contributions to atmospheric levels from evaporative losses of THC and from crankcase blow-by gases, significant correlation of CO and THC might be anticipated over shorter averaging periods than are necessary for the C O - P b correlation. The correlation coefficients found in this study (Table V) are generally higher than for the CO-Pb correlations, but only one is significant. It is interesting to note that the calculated regression lines indicate levels of THC from 1.3 to 2.5 ppm for zero CO concentration, and this presumably

126 R O G E R P E R R Y E T A L .

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A CORRELATION STUDY OF VEHICLE-GENERATED AIR POLLUTANTS 127

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fuel losses to a t m o s p h e r i c levels.

6. Conclusions

Using shor t averag ing t imes (10 to 25 min) it is no t poss ib le to d e m o n s t r a t e c lose

cor re la t ions be tween the levels o f veh ic le -genera ted po l lu t an t s in s treet air , except

under unusua l cond i t ions such as a re f o u n d at the exit o f a mu l t i s to rey car p a r k

where mos t vehicles are. d r iven in the same m o d e . Thus it is on ly for far longer

averaging pe r iods tha t the level o f one po l l u t an t such as C O can be co r re l a t ed with

and used to p red ic t the level o f ano the r (e .g. , Pb) .

Acknowledgment

The au tho r s a re g ra te fu l to M r E. V. I n g i m u n d a r s o n for his ass is tance in d a t a

co l lec t ion a n d process ing.

References

Bayley, E. and Dockerty, A.: 1972, J. RoyalSoc. Health I, 6. Bov6, J. L. and Siebenberg, S.: 1970, Science 167, 986. Brief, R. S., Jones, A. R., and Yoder, J. D.: 1960, J. AirPollut. Contr. Assoc. 10, 384. Chovin, P.: 1967, Environ. Res. 1, 198. Colucci, J. M., Begeman, C. R., and Kumler, K.: 1968, Paper at 61st Annual Meeting, Air PoUut. Contr.

Assoc., St. Paul, Minn., June 1968. Harrison, R. M., Perry, R., and Slater, D. H.: 1974, Atmos. Environ. 8, 1187. Georgii, H. W. and Jost, D.: 1971, Atmos. Environ. 5, 725. Martens, C. S., Wesolowski, J. J., Kaifer, R., and John, W.: 1973, Atmos. Environ. 7, 905. Perry, R. and Slater, D. H.: 1975, In Benn and McAuliffe (eds.) Chemistry of Pollution, Macmillan,

London.