The relationship between traffic throughput and the associated primary pollutants in Surrey

Transcription

1 The relationship between traffic throughput and the associated primary pollutants in Surrey I.M. Cowan, E.E. Hellawell & S.J. Hughes University of Surrey, Guildford, Surrey G U2 7XH, UK Abstract Thls paper investigates the link between traffic-related primary pollutants and traffic volun~e in Surrey. This County, in the South East of England, was selected for the study as it has one of the highest levels of population density within the United Kingdom hence elevated levels of car ownership and traffic flow. In addition, there were very few major industrial sources within the County, thus air pollutants were likely to be predominantly caused by traffic. This assumption was confxmed by comparing diurnal patterns of pollution at one location on a typical weekday, and Christmas Day, when the traffic was minimal. A further investigation compared the diurnal patterns of pollution with the levels of traffic provided by the local transportation model. This demonstrated a direct link behveen the primary pollutant, nitrogen oxide (NO) and traffic volume. The results showed hvo daily peaks in pollutant levels, which coincided with morning and afternoon 'rush hours'. Similar distributions were also obtained for the other primary pollutants, carbon monoxide (CO) and particulate matter (PMlo), measured at the same monitoring site. The study was then extended to two further locations within Surrey. Both of these were found to produce similar daily distributions of NO and PMlo although the actual magnitude of pollutants varied. Lower values were measured at the background site. due to its distance from the pollutant source.

2 432.dry ~oiiut~on L\ 1 Introduction This paper presents the results of an investigation into the daily levels and distribution of traffic-related air pollution. The work was divided into two stages, which are outlined below: The fvst stage was to identify a suitable area for the study. In particular, it was important to try to select an area in which the pollution levels could be mainly attributed to traffic as opposed to other industrial sources. Once the area had been selected, all the measured data in the region was collected for analysis. The second stage involved a detailed analysis of the data, in which the primary objectives were to: investigate the relationship between traffic count and measured nitrogen oxide (NO) levels compare the daily distribution of the primary pollutants, NO, carbon monoxide (CO) and particulate matter (PMlo) from one measuring site extend this study to include other measuring sites across the county. 2 Location and Data Monitoring Unit Figure 1 - The County of Surrey & the location of the real-time monitoring units [l].

3 Time l --e 27-Oct-99 -c+ Christmas Day 1999 ~ Figure 2 - NO for Christmas Day 1999, and a Gical weekday The fvst criterion was to select an area where the measured air pollutants were primarily from traffic, rather than industrial or heating sources. Surrey County in the South East of England, UK has few industrial sources of pollution either within the County or to the South West in the direction of the prevailing winds. It has, associated with it, one of the highest levels of population density in the UK and, as a consequence of this, there is a higher than average traffic throughput [2]. Surrey comprises of 11 local Boroughs, as indicated in Figure 1. There are approximately 150 nitrogen dioxide (NOz) diffusion tubes and 6 realtime monitoring units that measure a variety of pollutants including NO, NOz, PMlo, CO and Ozone within the County. Not all of the real-time units measure all of the pollutants. A number of air pollution studies are also currently undenvay in Surrey, which would allow for comparison of results [3, 4. 5, 61. In order to determine whether the pollutants being measured were predominantly generated by traffic, as opposed to other sources, a small sample of data was analysed. Figure 2 shows the distribution of NO on Christmas Day compared with the same distribution on a typical weekday in a busy urban area. Christmas Day was selected, as this was the only day in the year when traffic levels were known to be significantly reduced, whilst other sources, for example heating, were unaffected. The results clearly showed that the maximum level of NO on Christmas Day was approximately 10% of the typical daily peak. This implied that the majority of the NO pollution generated could be directly attributed to traffic. Once the suitability of the study area had been confirmed, real-time data was collected from 3 of the 6 monitoring units in Surrey (in Guildford, Waverley and Mole Valley, see Fig. 1). The real-time measuring units in Guildford and Waverley were located adjacent to main roads, whilst that in Mole Valley was a background site (i.e. >50m from the road [7]). All of the units had sequential

4 _- ---pp Transactions on Ecology and the Environment vol 47, 2001 WIT Press, ISSN Table 1 - Chemical analysis of the pollutants (Horiba [S]) Pollutant Measured Chemical Technique NO & NOz Chemiluminescence CO Cross Flow Modulation, Inflared-Absorption ( Technology PMio I P-Ray Absorption data from April 1999 to March 2000, which was sampled every 15 minutes and stored in a central database. Table 1 outlines the analytical techniques used to measure the air pollutants discussed in this paper. The data were calibrated automatically using non-traceable gas sources on a daily basis with manual fortnightly calibrations using traceable gas sources. Every six months an independent audit of the unit was undertaken by the National Physical Laboratory [9]. 3 Results and discussion For this study, primary and secondary pollutants, together with local meteorological parameters, such as wind direction, wind speed and temperature, were collected. Figure 3 shows the relationship between the primary pollutant NO and traffic volume. The NO data were taken from the Guildford monitoring unit and are shown as the weekday averages between April 1999-March The traffic volume data were obtained from the Surrey Traffic and Transport Model [l01 for The traffic data were stored as hourly values between 07:OO and 19:OO and as a single 12-hour average between 19:OO and 07:OO. The hourly data were forward projected to reflect 1999 levels, using a 2% annual l I _ - Time o Gu~ldford Traffic R-edlcted ---c Guldford (Traffic Count) Average Leekday NO for pp Figure 3 - Traffic volume compared with levels of NO - _---l

5 p-- - Transactions on Ecology and the Environment vol 47, 2001 WIT Press, ISSN increment, as estimated by Surrey County Council [2]. In addition, the single 12 hour average traffic count was replaced with an estimated hourly distribution. These estimated values were taken fiom measured hourly nighttime readings in an area with a similar daytime traffic volume [l l]. The expected trends for traffic volume, namely two daily 'rush hour' peaks are shown in Figure 3. The morning 'rush hour' was more distinct, with maximum traffic throughput at approximately 08:OO. The afternoon 'rush hour' was more widespread, and could be attributed to the earlier school 'run' followed by daily commuters. The NO profile followed the traffic volume distribution with two similar peaks, corresponding to the 'rush hour' periods. This gave support to the earlier premise that the measured levels of NO in this study zone were directly attributed to traffic. The daily distribution of two more traffic related primary pollutants, namely CO and PMlo are presented in Figure 4. These were taken from the same measuring unit in Guildford and similarly represented the average weekday values between April 1999 and March These results showed that the distributions of both CO and PMlo closely followed that of NO and the traffic count as shown in Figure 3. This provided hrther evidence that the primary pollutants, CO and PMlo, measured at this monitoring site could also be attributed primarily to traffic. In order to investigate the overall distribution of primary pollutants across the County, the results were then compared with those from two other measuring units in the adjacent local authority regions of Waverley and Mole Valley (see Figure 1 for locations). Figure 5 shows the average daily distributions of NO over the same time period (April March 2000). The overall trends for NO were repeated at all three sites. This was expected, as the traffic volumes and throughput were similar along the roads adjacent to all the sites. However, it -e- CO - PM,, -- -pp Figure 4 - Comparison of PMIo and CO for Guildford - weekday average

6 I I S Mole Valley (Background) -6- Waverley (Keibs~de)-A- Gullclford (Kerbside) 1 ~i-~ure 5 -NO levels at 3 monitoring sites in Surrey Time -----p- 1 I =Mole Valley (Background) 4 Waverley (Kerbside) -A- Guildford (Kerbi~de) - Figure 6 - PMlo levels at 3 monitoring sites in Surrey i was noted that the magnitude of the NO levels recorded did show some variability. The monitoring units differed in their proximity to the road and this was reflected in the lower magnitudes recorded at the background site in Mole Valley. All three sites measured similar levels of pollutants during the night, which suggested that once the volume of traffic had dropped, the NO quickly dispersed and the background levels were attained. The equivalent distribution for the PMlo, measured at the three sites, is shown in Figure 6. The daily distributions were again similar at all three sites, with a double peak observed in the morning and evening 'rush hours'. This trend was less distinct at the background site in Mole Valley, which was located the greatest distance from the road source. The highest PMlo values were recorded at the Waverley site (the real-time unit located closest to the road source) followed by those in Guildford and finally those at Mole Valley. More variation

7 was observed in the magnitudes of the PMlo measurements between the 3 sites, compared with the NO measurements. Particulate matter has a greater mass than the gaseous pollutant, NO, hence was likely to be transported shorter distances from the original source. Therefore, the results for the PMlo were far more sensitive to the proximity of the real-time measuring unit to the road source as demonstrated in Figure 6. The night-time levels of PMlo were found to be comparable at all the monitoring locations, which again suggested that once the traffic-throughput was reduced, the PMlo levels quickly attained those of the ambient background readings. 4 Conclusions This paper has investigated the relationship between traffic throughput and traffic-related primary pollutants. Data were collected from three air qualitymonitoring units within Surrey County, and analysed with respect to traffic information extracted from the Surrey County Transport Model. Within Guildford, where one of the real-time measuring units was located, traffic was clearly the major source of air pollution. When traffic flow was minimal, on Christmas Day, it was found that the NO pollution levels were also minimal. In addition, it was found that the daily distribution of NO closely followed that of the traffic volume. There were two daily peaks, which corresponded to the morning and evening 'rush hours'. Investigations into other primary pollutants, PMlo and CO, produced similar results to those for NO, which also indicated that these pollutants were primarily attributed to traffic. The other monitoring units studied within the County showed similar trends to those for Guildford. During the weekdays, two peaks were prominent, occurring at the peak times of travel. The levels of the pollutants measured at each unit varied depending on whether the site was kerbside or background, i.e. adjacent to or over 50m from a main road. The current study is on-going and further data from real-time units across the county are in the process of being analysed. It is intended that once all the data has been processed, it will be possible to project the distribution of these trafficrelated primary pollutants across the county. 5 Acknowledgements Financial support for this work was provided by The UK Engineering and Physical Sciences Research Council, Surrey County Council and the eleven Borough Councils within Surrey. The authors wish to thank Surrey County Council, Guildford and Waverley Borough Councils and Mole Valley Borough Council in association with the South East Institute for Public Health, for providing the data and for supporting the ongoing work.

8 6 References [l] Surrey County Council; District and Borough Council Boundaries; web page [2] Surrey County Council; Surrey Local Transport Plan 2001/02 to 2005/06; Surrey County Council; 2000 [3] Lim, L.L., Hughes, S.J. & Hellawell, E.E.; Investigation of trafjic related pollutants in an urban area; 31~ International conference on urban air quality; Institute of Physics; Greece. March 2001 [4] Mavroulidou, M,, Hellawell, E.E., Hughes, S.J. & Lim, L.L.; A novel technique for air pollution predictions on a regional scale; 3rd International conference on urban air quality; Institute of Physics; Greece. March 2001 [5] Lythe, M.S., Hughes, S.J. and Hellawell, E.E.; Long-term countywide NO2 variations in Surrey; Air Pollution IX; 2001 [6] Hughes, S.J., Hellawell, E.E. & Stronitharm, G.; Evaluation of trajk related nitrogen dioxide data in Surrey; Air Pollution VIII; 2000 [7] Department of the Environment; The United Kingdom Air Quality Strategy; HMSO; 1997 [g] Horiba; Air pollution monitor AP-360: User Manual; Horiba [g] Fuller, G; Personal Correspondence; SEIPH-ERG; [l01 Engineering Consultancy Division & Scott Wilson Kirkpatrick; Surrey County Transportation Model (CTM95); Surrey County Council; 1996 [l l] Cambridge Environmental Research Consultants; Using ADMS-Urban (Version 1 S3); 1999