Air mass back trajectories and particulate matter concentration at a site on the south coast of England

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1 Air mass back trajectories and particulate matter concentration at a site on the south coast of England A. R. Deacon&K. L. Smallbone Division of Geography, School of the Environment, The Universi@ of Brighton, UK, Abstract Recent concern over the health effects of PMIO(Hajat et al [1], Greenbaum et al [2]) combined with the legislative drive towards long-term air quality objectives has led to the need for better understanding of the nature and characteristics of particulate matter (PM) less than 10~m. This study has highlighted a number of issues relating to monitoring PMz.s.andPMIo. Monitoring of PMIOand PM2,5particulate matter was carried out at an urban background site in Eastboume, East Sussex, UK for nine months between March and December These data have been analysed alongside on-site meteorological data and atmospheric back trajectories (calculated using the British Atmospheric Data Centre back trajectory system). Overall, PM2,srepresented 69.5% of the PMIOconcentration at the site, with marked differences in this proportion between summer and winter months but not between weekdays and weekends. Analysis of back trajectories during periods when PM2,5contributes more than 85?40of PMIOhas shown that there were differences between summer and winter events. Under summer anticyclonic conditions, airmasses have travelled shorter distances and are more mixed, whilst winter events show back trajectories arriving at the site from Continental and Eastern Europe, Pollution rose plots for the site also show an increased proportion of PM2,5when winds are from North- West to North-East and from the South-East to South, Comparison with previous work has highlighted the importance of the regional component of PM2,5as well as PMIO.

2 202 Air Pollution X 1 Introduction Recent legislation governing the ambient concentrations of PMIO particulate matter has increased interest at both the national and local government level in the UK to comprehend the processes governing the behaviour and transport of particulate matter. In order to better understand PMIO, it is also necessary to examine data on freer fractions such as PM2,5,and PM1. Harrison et al [3] used inter site and season comparisons of data to examine PM10:PM2,5ratio, They found that during winter, PM2,5comprised approximately 80% of PM1oand was strongly correlated with NOX, indicating the importance of road traffic as a source of fme particulate matter, Hursthouse et al [4] compared two Scottish urban sites against a nearby rural location for total suspended particulate (TSP), PM1o and PM2.5Vahxx. He found that in the urban area PM2,5contributed significantly to concentrations of PMIOand TSP, whilst in the rural area, there were higher proportions of coarser material, It was assumed that this was due to the influence of primary vehicle emissions in the urban centre and the contribution of erosion derived material in the rural location, Similar work in Central London (Rickard et az [5]) has also highlighted the predominance of fine particles in roadside environments, Measurements made at four sites using a cascade irnpactor showed that PM2,1 contributions to PMIO could be as high as 700/o during particulate pollution episodes. Furthermore Rickard et al [5] indicated that the coarse particle concentration varied more markedly between sites than fme particles (<2.1 pm). There is a growing consensus that a significant proportion of PMIOmay be derived from sources well beyond the immediate area and that PMIO and to a certain extent PM2,5pollution may be regional and intercontinental in nature. This phenomenon was studied in the UK by King and Dorling [6]. Their analysis focussed on two pollution episodes from 1996, when regulatory standards were breached for long periods of time. Both episodes were characterised by elevated concentrations in both rural and urban areas and significant increases in PMIOconcentration when winds were from the east, The use of meteorological back trajectories indicated that significant contributions to PMIOlevels across the UK were made by particulate matter arriving from the continent. Muir [7] also analysed daily mean national network data for sites within and between Birmingham, Bristol, Cardiff and Leicester, He concluded that a substantial proportion of the PMIObeing measured in the UK may be the consequence of long range transport, possibly from continental Europe. Due to its proximity to continental European sources, the South-east of England tends to experience higher background PMIOpollution areas than other parts of the UK [8]. In addition to this, coastal ribbon development combined with the proximity to the marine environment may concentrate the problem into particular areas. This paper will, therefore, examine the relationship between the size fraction of particulate matter and wind direction, Furthermore, the relationship between particulate matter and air mass trajectories will be undertaken.

3 2 Methodology Air pollution X 203 PMIO and PM2,5 data were collected at the same site using Rupprecht and Patashnick Tapered Element Oscillating Microbalance (TEOM) instruments. Data were stored as 30 minute averages and subsequent post-processing was undertaken to give daily and longer term means. On-site meteorological data were also collected using a meteorological mast erected to a height above ground level of approximately 10m. The site used was at the University of Brighton campus in Eastboume, East Sussex (grid reference ,097900), The site was approximately 50 m above sea level, 1 km inland and 20 m back from the nearest minor road. There were no local sources of particulate pollution in the surrounding vicinity. Air mass back trajectories were obtained from the British Atmospheric Data Centre trajectory service for specific particulate pollution episodes. 3 Results and Discussion Daily mean pm2s and PM,o concentrations wererecorded for the whole of the study period (March to December 2001) (see Figure 1), Missing data during May and early June were due to power and subsequent communications failure at the site, During this period, the raw TEOM data indicated that there was only one day with a daily mean PMIOconcentration of greater than 50 ~g/m3. This related to the annual Bonfme night celebrations in the UK. Analysis of the data showed that overall, pmz,5accounted for 69.5% of PM1o. This matches similar ratios found in other work (e.g. [3]). 70 Figure 1: Daily mean PM2,sand PM,o concentrations over the study period.

4 204 Air Pollution X Further investigation indicated that there was a seasonal trend in particulate matter with a higher proportion of coarse material in the summer, while fme material dominates in the winter months (see Table 1), Table 1: Monthly mean ratio of fme and coarse particulate matter. Month Ratio PMZ,5:PM10 March 61.1 April 66.7 May 64.2 June 69.7 July 79,7 August 56.1 September 73.4 October 57,9 November 90,7 December 81.1 Conversely there is little variation in the ratio between pm2,5and PM1oby day of week (between 65% and 73 % on Wednesday and Friday respectively), indicating that the impact of local traffic activity is minimal at this site (see Table 2). Table 2: Daily mean ratio of fine and coarse particulate matter. Day of Week Ratio PM2,5:PM10 Monday 70.4 Tuesday 68.7 Wednesday 64.5 Thursday 70.9 Friday 72.8 Saturday 69.2 Sunday 70.4 Analysis of the data by wind direction indicated that above average contributions of PM2,5to PMIOoccurred when winds were from the north-west to north-east and from the south-east to south (see Figure 2). This suggests that there are inland sources of fine particulate matter (north-west to north-east) and that a contribution is also made either from the marine environment or from

5 Air Pollution X 205 Continental Europe (south-east to south). South-westerly winds appear to be associated with higher than average contributions of coarse particulate material. wind Direction (Degrees) [m PM2,5 i3pm10 Figure 2: Average proportion of fme and coarse particulate matter by wind direction (mean ratio across all wind directions (73.7 Yo)is shown for comparison). Further analysis of the data was undertaken using back trajectory modelling for the periods when PMZ,Smakes an above average contribution to PMIO, Over the whole of the sampling period, there were 28 days on which the daily mean PM2,5concentration was greater than or equal to 85% of the PMIOconcentration. Of those there were two extended periods of three or more days on which this was the case. The fust a summer period from 27/07/01 to 30/07/01 (Figure 3) and the second a winter period from 08/12/01 to 10/12/01 (Figure 4). These two periods provided a contrasting view of seasonal meteorology. Back trajectories for Midday on each of the event days were plotted using the British Atmospheric Data Centre trajectory service. Trajectories were plotted for an atmospheric pressure surface of 950 mb using the 3D vertical advection option over one hour time steps for output.

6 206 Air Pollution X ~ E070t o070i - Countriee N o Khnnwlr w -++ s E Figure 3: Hourly mean PMZ,Sand PMIOand back trajectories for 25/07/01-30/07/01.

7 Air Pollution X 207 DatefTlnw PM1O PM2,5 0 $00 Iaoo 1500 lal I Countries N E -+$+ Figure 4: Hourly mean PMZ,5and PMIO and back trajectories for 08/12/01-10/12/01, The July period (see Figure 3) shows trajectories looping and originating from near continental areas, typical of atmospheric behaviour under anticylconic conditions. The December period (Figure 4) shows trajectories passing over eastern and central Europe and originating in Scandinavia,

8 208 Air Pollution X 4 Conclusions The measurement of PMJ,5 alongside PMIO yields valuable additional information about sources of particulate matter, especially when combined with meteorological analyses, In line with previous studies, it is apparent that on average PM2,5contributes about 70% of PMIOwhen measured by TEOM, but that there is variation in this proportion, notably between summer and winter months. This work has shown an enhanced fme particle contribution arising from continental Europe and inland areas of the UK, contrasting with enhanced coarse particulate matter concentrations with south-westerly winds. References [1] Hajat, S., Haines, A,, Atkinson, R.W,, Bremner, S,A,, Anderson, H.R., Emberlin, J., Association between air pollution and daily consultations with General Practitioners for allergic rhinitis in London, United Kingdom. American Journal of Epidemiology, 153, pp ,2001. [2] Greenbaum, D.S., Bachmann, J.D,, Krewski, D,, Samet, J,M., White, R., Wyzga, R.E., Particulate air pollution standards and morbidity and mortality: Case study. American Journal of Epidemiology, 154, pp ,2001. [3] Harrison, R.M., Deacon, A,R., Jones, M.R. and Appleby, R,S., Sources and processes affecting concentrations of PMIO and PM2,5 particulate matter in Birmingham (UK), Atmospheric Environment, 31, pp , 1997, [4] Hursthouse, A., Muir, R. and Smith, F., Benzene, Particulate matter and associated priority pollutants in Renfrewshire, Scotland experience from enhancing local air quality monitoring, Clean Air, 30, pp , [5] Rickard, A., Milliken, C. and Neville, S., The mass concentration and size distribution of particles in Central London using non-continuous gravimetric sampling techniques, Clean Air, 29, pp , [6] King, A.M. and Dorling, S., PMIOparticulate matter - the significance of ambient levels. Atmospheric Environment, 31, pp , [7] Muir, D., PM1o particulate in relation to other atmospheric pollutants. Environmental Monitoring and Assessment, 52, pp , [8] APEG, Source Apportionment of Airborne Particulate Matter in the United Kingdom, APEG, London, 1999.