Marylebone Road Research Articles

Correlations in polycyclic aromatic hydrocarbon (PAH) concentrations in UK ambient air and implications for source apportionment

The extent of correlation in polycyclic aromatic hydrocarbon (PAH) concentration data obtained by the UK PAH Monitoring and Analysis Air Quality Network from March 2008 to November 2010 has been assessed. Application of principal component analysis (PCA) to the dataset has revealed that the concentrations of the vast majority of PAHs are very highly correlated. The use of diagnostic PAH ratios (including a new benzo[b]naph[2,1-d]thiophene/benzo[a]pyrene diagnostic ratio), and a novel 'combined diagnostic ratio - PCA' approach has revealed information about the main sources of PAH at individual Network sites, allowing the sites to be grouped in terms of those influenced by solid fuel use, industry and traffic, and those of an urban or rural nature. Solid fuel use has also been delineated from other fuel burning. Conclusions are drawn about a number of UK PAH Network sites - four sites are shown to be influenced significantly by solid fuel use, and the sites in South Wales and at London Marylebone Road are found to be located in distinct and unusual PAH pollution climates - the identification of a unique PAH pollution climate in South Wales is a key and novel conclusion of the work.

Wind tunnel measurements of pollutant turbulent fluxes in urban intersections

Wind tunnel experiments have been carried out at the EnFlo laboratory to measure mean and turbulent tracer fluxes in geometries of real street canyon intersections. The work was part of the major DAPPLE project, focussing on the area surrounding the intersection between Marylebone Road and Gloucester Place in Central London, UK. Understanding flow and dispersion in urban streets is a very important issue for air quality management and planning, and turbulent mass exchange processes are important phenomena that are very often neglected in urban modelling studies. The adopted methodology involved the combined use of laser Doppler anemometry and tracer concentration measurements. This methodology was applied to quantify the mean and turbulent flow and dispersion fields within several street canyon intersections. Vertical profiles of turbulent tracer flux were also measured. The technique, despite a number of limitations, proved reliable and allowed tracer balance calculations to be undertaken in the selected street canyon intersections. The experience gained in this work will enable much more precise studies in the future as issues affecting the accuracy of the experimental technique have been identified and resolved.

Modelling Street-Scale Flow and Dispersion in Realistic Winds—Towards Coupling with Mesoscale Meteorological Models

Further to our previous work—simulations of flow and dispersion in an oblique wind over the DAPPLE site (Xie and Castro, Atmos Environ 43:2174–2185, 2009)—large-eddy simulations of flows and dispersion over the same site in a wind perpendicular to Marylebone Road and the windward surfaces of most of the buildings were performed. The DAPPLE site is located at the intersection of Marylebone Road and Gloucester Place in central London. In order to investigate the effects of wind direction on flows and dispersion, the velocity and scalar fields in the perpendicular wind were compared with those in the oblique wind. Furthermore, realistic wind conditions measured on the BT Tower at 190 m above street level were processed and used to drive the numerical simulations of flows and dispersion at the DAPPLE site. This leads to significant predictive improvements of the dispersion compared with field measurements, which provides validation and confidence for coupling mesoscale meteorological models, e.g. the UK Met Office’s Unified Model and the NCAR’s Weather Research & Forecasting Model, with the street-scale large-eddy simulation of urban environments.

PMF Analysis of Wide-Range Particle Size Spectra Collected on a Major Highway

Particle number concentration data have been collected on a very busy road in central London (Marylebone Road). Continuous size distributions from 15 nm to 10 μm diameter, collected over 21 days, were analyzed using positive matrix factorization which identified 10 factors, five of which were observed to make major contributions (greater than 8%) to either the total number or volume of particulate matter. The sources associated with each factor were identified from the size distribution, directional association, diurnal variation and their relationship to meteorological pollution and traffic volume variables. The factors related to the emissions on Marylebone Road accounted for 40.5% of particle volume and 71.9% of particle number. These comprised nucleation mode exhaust particles (3.6% of total volume and 27.4% of total number), solid mode exhaust particles (18.8% of total volume and 38.0% of total number), brake dust (13.7% of total volume and 1.7% of total number and resuspension (4.4% of total volume and 4.8% of total number). Another six factors were associated with the urban background accounting for 59.5% of total volume and 28.2% of total particle number count. The method is extremely successful at separating the components of on-road emissions including brake wear and resuspension.

Understanding peak pedestrian exposures due to traffic emissions within the urban environment

This paper investigates pedestrian exposure to traffic emissions at the Marylebone Road–Gloucester Place intersection in central London, taking a particular interest in the peak exposures occurring within minutes. The approach is based on the large eddy simulation modelling methodology, which enables high spatial resolution of air movements and pollutant concentrations to be achieved at relatively moderate computational cost. The results show the complexity of the air flows in the vicinity of the intersection at pedestrian height and the large variability of peak exposures due to the turbulent air flows.

Particulate Oxidative Burden Associated with Firework Activity

Firework events are capable of inducing particulate matter (PM) episodes that lead to exceedances of regulatory limit values. As short-term peaks in ambient PM concentration have been associated with negative impacts on respiratory and cardiovascular health, we performed a detailed study of the consequences of firework events in London on ambient air quality and PM composition. These changes were further related to the oxidative activity of daily PM samples by assessing their capacity to drive the oxidation of physiologically important lung antioxidants including ascorbate, glutathione and urate (oxidative potential, OP). Twenty-four hour ambient PM samples were collected at the Marylebone Road sampling site in Central London over a three week period, including two major festivals celebrated with pyrotechnic events: Guy Fawkes Night and Diwali. Pyrotechnic combustion events were characterized by increased gas phase pollutants levels (NO(x) and SO(2)), elevated PM mass concentrations, and trace metal concentrations (specifically Sr, Mg, K, Ba, and Pb). Relationships between NO(x), benzene, and PM(10) were used to apportion firework and traffic source fractions. A positive significant relationship was found between PM oxidative burden and individual trace metals associated with each of these apportioned source fractions. The level of exposure to each source fraction was significantly associated with the total OP. The firework contribution to PM total OP, on a unit mass basis, was greater than that associated with traffic sources: a 1 μg elevation in firework and traffic PM fraction concentration was associated with a 6.5 ± 1.5 OP(T) μg(-1) and 5.2 ± 1.4 OP(T) μg(-1) increase, respectively. In the case of glutathione depletion, firework particulate OP (3.5 ± 0.8 OP(GSH) μg(-1)) considerably exceeded that due to traffic particles (2.2 ± 0.8 OP(GSH) μg(-1)). Therefore, in light of the elevated PM concentrations caused by firework activity and the increased oxidative activity of this PM source, there is value in examining if firework derived PM is related to acute respiratory outcomes.

Global comparison of VOC and CO observations in urban areas

Speciated volatile organic compound (VOC) and carbon monoxide (CO) measurements from the Marylebone Road site in central London from 1998 through 2008 are presented. Long-term trends show statistically significant decreases for all the VOCs considered, ranging from −3% to −26% per year. Carbon monoxide decreased by −12% per year over the measurement period. The VOC trends observed at the kerbside site in London showed greater rates of decline relative to trends from monitoring sites in rural England (Harwell) and a remote high-altitude site (Hohenpeissenberg), which showed decreases for individual VOCs from −2% to −13% per year. Over the same 1998 through 2008 period VOC to CO ratios for London remained steady, an indication that emissions reduction measures affected the measured compounds equally. Relative trends comparing VOC to CO ratios between Marylebone Road and Hohenpeissenberg showed greater similarities than absolute trends, indicating that emissions reductions measures in urban areas are reflected by regional background locations. A comparison of VOC mixing ratios and VOC to CO ratios was undertaken for London and other global cities. Carbon monoxide and VOCs (alkanes greater than C 5 , alkenes, and aromatics) were found to be strongly correlated (>0.8) in the Annex I countries, whereas only ethene and ethyne were strongly correlated with CO in the non-Annex I countries. The correlation results indicate significant emissions from traffic-related sources in Annex I countries, and a much larger influence of other sources, such as industry and LPG-related sources in non-Annex I countries. Yearly benzene to ethyne ratios for London from 2000 to 2008 ranged from 0.17 to 0.29 and compared well with previous results from US cities and three global megacities.

In-Street Wind Direction Variability in the Vicinity of a Busy Intersection in Central London

We present results from fast-response wind measurements within and above a busy intersection between two street canyons (Marylebone Road and Gloucester Place) in Westminster, London taken as part of the DAPPLE (Dispersion of Air Pollution and Penetration into the Local Environment; www.dapple.org.uk ) 2007 field campaign. The data reported here were collected using ultrasonic anemometers on the roof-top of a building adjacent to the intersection and at two heights on a pair of lamp-posts on opposite sides of the intersection. Site characteristics, data analysis and the variation of intersection flow with the above-roof wind direction (θ ref ) are discussed. Evidence of both flow channelling and recirculation was identified within the canyon, only a few metres from the intersection for along-street and across-street roof-top winds respectively. Results also indicate that for oblique roof-top flows, the intersection flow is a complex combination of bifurcated channelled flows, recirculation and corner vortices. Asymmetries in local building geometry around the intersection and small changes in the background wind direction (changes in 15- min mean θ ref of 5°–10°) were also observed to have profound influences on the behaviour of intersection flow patterns. Consequently, short time-scale variability in the background flow direction can lead to highly scattered in-street mean flow angles masking the true multi-modal features of the flow and thus further complicating modelling challenges.

Analysis of the air pollution climate at a central urban background site

Measurements of air pollutants from a background site in central London are analysed. These comprise hourly data for CO, NO, NO 2, O 3, SO 2 and PM 10 from 1996 to 2008 and particle number count from 2001 to 2008. The data are analysed in terms of long-term trends, annual, weekly and diurnal cycles, and autocorrelation and cross-correlation functions. CO, NO and NO 2 show a typical traffic-associated pattern with two daily peaks and lesser concentrations at the weekend. Particle number count and PM 10 show a similar cycle, but with smaller amplitude. Ozone has an annual cycle with a maximum in May, influenced by the spring maximum in background ozone, but the diurnal and weekly cycles are dominated by losses through reaction with nitric oxide. Particle number count shows a minimum corresponding with maximum air temperatures in August, whereas the CO, NO NO 2 and SO 2 show a minimum in June/July. There is a lower particle count to NO x ratio at the background site compared to a central London kerbside site (Marylebone Road) and a seasonal pattern in particle count to NO x and PM 10 ratios consistent with loss of nanoparticles by evaporation during atmospheric transport. Sulphur dioxide peaks in the morning in summer, but at midday in winter consistent with emissions from elevated sources mixing down from aloft as the diurnal mixed layer deepens. Implications for epidemiological studies of air quality and health are discussed. Sulphur dioxide, carbon monoxide, nitric oxide and nitrogen dioxide show clear downward trends over the measurement period, PM 10 declines initially before levels stabilised, and ozone concentrations increased.

The wind speed dependence of the concentrations of airborne particulate matter and NO x

The wind speed dependence of concentrations of PM 10, chloride, sulphate, nitrate, organic carbon, elemental carbon, particle number and NO x has been determined at three separate sites, Marylebone Road (kerbside), North Kensington (urban background) and Harwell (rural). The data are best described by a general dilution term multiplied by up to three separate source-related terms which we interpret as representing long-range transport sources, discrete local (including area) sources and marine sources respectively. Using this approach, the various particulate metrics can be quantitatively disaggregated according to the contributions of the three source types. The behaviour of nitrate is anomalous, probably due to an influence of wind speed upon the dissociation of ammonium nitrate.

Cluster Analysis of Rural, Urban, and Curbside Atmospheric Particle Size Data

Particle size is a key determinant of the hazard posed by airborne particles. Continuous multivariate particle size data have been collected using aerosol particle size spectrometers sited at four locations within the UK: Harwell (Oxfordshire); Regents Park (London); British Telecom Tower (London); and Marylebone Road (London). These data have been analyzed using k-means cluster analysis, deduced to be the preferred cluster analysis technique, selected from an option of four partitional cluster packages, namelythe following: Fuzzy; k-means; k-median; and Model-Based clustering. Using cluster validation indices k-means clustering was shown to produce clusters with the smallest size, furthest separation, and importantly the highest degree of similarity between the elements within each partition. Using k-means clustering, the complexity of the data set is reduced allowing characterization of the data according to the temporal and spatial trends of the clusters. At Harwell, the rural background measurement site, the cluster analysis showed that the spectra may be differentiated by their modal-diameters and average temporal trends showing either high counts during the day-time or night-time hours. Likewise for the urban sites, the cluster analysis differentiated the spectra into a small number of size distributions according their modal-diameter, the location of the measurement site, and time of day. The responsible aerosol emission, formation, and dynamic processes can be inferred according to the cluster characteristics and correlation to concurrently measured meteorological, gas phase, and particle phase measurements.

Comparison of average particle number emission factors for heavy and light duty vehicles derived from rolling chassis dynamometer and field studies

Fleet aggregate emission factors for particle number for light duty vehicles and heavy duty vehicles are available from a number of field studies by calculation from airborne concentrations. Data are also available from laboratory chassis dynamometer studies in which particle number emissions from engines of a range of capacity and technologies have been measured. In this study the data from the PARTICULATES project database of laboratory test bed measurements have been aggregated to estimate fleet average particle number emission factors for light duty and heavy duty vehicles which have then been compared with the results of field measurement studies. Measurements carried out by [Jones, A.M., Harrison, R.M., 2006. Estimation of the emission factors of particle number and mass fractions from traffic at a site where mean vehicle speeds vary over short distances. Atmospheric Environment 40, 7125–7137] at the Marylebone Road London, represented an ‘urban’ fleet and were estimated by the aggregated laboratory measured emission factors within one or two standard deviations of their values. For light and heavy duty vehicles travelling with an average speed of 39.5 km h −1 the NO x emission factors were estimated to be 0.62 and 5.88 g veh −1 km −1, respectively, and the PM 2.5 emission factors were estimated to be, respectively, 0.017 and 0.17 g veh −1 km −1. The total particle number emission factors (including volatiles) were estimated for light and heavy duty vehicles travelling according ‘urban’ Common ARTEMIS Driving Cycle (CADC) to be 6.31 × 10 13 and 70.6 × 10 13 num veh −1 km −1 and these were both within one standard deviation of the values measured using the field data. Similarly, the fractionated solid particle number emission factors (excluding volatiles), measured for the aerodynamic diameter ranges, 30–50 nm, 50–100 nm and 100–1000 nm, were estimated to within one standard deviation of the equivalent measured emission factors for the light duty vehicles and to within two standard deviations of the equivalent measured emission factors for the heavy duty vehicles. Extending this work to estimate the total number emission factors measured by other research groups, conditions representing both the ‘rural’ and ‘highway’ Common ARTEMIS Driving Cycles were considered. For the light duty vehicles, a general increase in the calculated value of the total number emission factor was observed with increased fleet speed and the observed scatter was accounted for by the split in the fleet according to the vehicle's fuel, whether gasoline or diesel. The calculated values of the total number emission factors for the heavy duty vehicles also show an increase with speed after 60 km h −1. High values are observed at 20 km h −1 suggesting an underlying inverted binomial trend similar to that observed for gaseous emission factors and arguably supported by the field measured values. The scatter of the calculated points about this trend was accounted for by the distribution of the fleet across the vehicle weight and emission legislation categories. With the exception of three of the field research studies considered – where the measured emission factors were significantly underestimated by the aggregated laboratory emission factors – there was a general agreement in the order of magnitude between the calculated and field emission factors, but not to the high degree shown for the Marylebone Road data. Furthermore, dilution temperature, fuel sulphur content and tunnel deposition effects did not individually account for the observed discrepancies in this work and point to the need for correction factors derived from these parameters together with additional information such as dilution ratio in giving a more complete description.

Multi-objective signal control of urban junctions – Framework and a London case study

This paper presents an approach to multi-objective signal control using fuzzy logic. The signal control uses fuzzy logic where the membership functions are optimised according to the Bellman–Zadeh principle of fuzzy decision-making. This approach is both practical for the decision-maker and efficient, as it leads directly to a Pareto-optimal solution. Signal control priorities are ultimately a political decision. Therefore the tool developed in this research allows the traffic engineer to balance the objectives easily by setting acceptability and unacceptability thresholds for each objective. Particular attention is given in the example to pedestrian delays. The membership functions of the fuzzy logic are optimised by a genetic algorithm coupled to the VISSIM microscopic traffic simulator. The concept is illustrated with a case study of the Marylebone Road–Baker Street intersection in London at which pedestrians as well as vehicle flows are high. The results prove the feasibility of the framework and show the vehicle delays for a more pedestrian friendly signal control strategy.

Estimation of particle resuspension source strength on a major London Road

Non-exhaust particles from road traffic arise from both abrasion sources and the resuspension of particles from the road surface. This paper reports a new combination of existing methods for indirect estimation of resuspension emission factors for Marylebone Road, London, a busy multi-lane highway in a street canyon. The method involves firstly estimating the total source strength of coarse particles (PM 2.5–10) arising from the road by calculating the roadside incremental concentration of coarse particles above the urban background. This is converted to a source strength by its ratio to NO x whose source strength is estimated from the knowledge of the traffic mix and mean speed. This coarse particle source strength is assumed to represent the sum of resuspension emissions and the coarse particle component of abrasion emissions. Using information on the traffic mix and speed, the abrasion emissions have been calculated from the EMEP/CORINAIR emissions factor database, the result subtracted from the total coarse particle emissions in order to yield resuspension emissions, and combined with traffic count data to derive fleet-average emission factors. Using the fact that the traffic mix differs substantially between weekdays and weekends, separate average emission factors for light- and heavy-duty vehicles have been estimated. In addition to traffic mix, the influence of wind speed and the time elapsed since the last rainfall upon resuspension have been estimated. Wind speed was found to have by far the larger influence, although this was still secondary to the number of heavy-duty vehicles. Uncertainties arising from the choice of urban background site and poor data quality are discussed.

What are the sources and conditions responsible for exceedences of the 24 h PM 10 limit value (50 μg m −3) at a heavily trafficked London site?

The European Union has set limit values for PM 10 to be met in 2005. At Marylebone Road, London, where the traffic is heavy, the daily limit value of 50 μg m −3 is exceeded more than 35 times a year. A total of 185 days with daily PM 10 concentrations exceeding the limit value of 50 μg m −3 measured between January 2002 and December 2004 (data capture of 89.5%) are discussed in this paper. These exceedences were more frequent in early spring and in autumn. Concentrations have been disaggregated into regional, urban (background) and local (street) contributions. Most of the episodes of gravimetric PM 10 above the limit value were associated with a high regional background and very often the regional contribution dominated the PM 10 mass. The secondary aerosol (especially the particulate nitrate) made a major contribution to the PM 10 load. These situations were frequently observed when air masses came from the European mainland (showing that both emissions from the UK and other EU countries contributed to the exceedences), and less frequently with maritime air masses that have stagnated over the UK (showing that emissions from the UK alone less frequently contributed to the high regional background). However, the higher frequency of episodes breaching the limit value at the roadside site than at the rural site and the higher frequency of PM 10 concentrations above the limit value on weekdays show that the high regional contributions are additional to local and urban emissions. Local emissions mainly due to traffic were the second important contributor to the exceedences, while the contribution of the urban background of London was less important than the local emissions and the regional background. Applying the pragmatic mass closure model of Harrison et al. [2003. A pragmatic mass closure model for airborne particulate matter at urban background and roadside sites. Atmospheric Environment 37, 4927–4933], revealed that the regional aerosol is comprised very largely of ammonium nitrate and sulphate and secondary organic aerosol. Findings suggest that international abatement of secondary aerosol precursors may be the most effective measure to fulfil the requirements of the European Directive 1999/30/CE by lowering the regional background.

Investigation into the use of the CUSUM technique in identifying changes in mean air pollution levels following introduction of a traffic management scheme

There is an increasing need for statistical techniques to identify and quantify the effects of traffic management schemes on ambient pollution levels. Cumulative sum (CUSUM) charts have been used extensively in industrial process control to detect deviations in production parameters from pre-determined values. This study investigates the use of the CUSUM procedure to identify change in ambient air pollution levels following the introduction of a traffic management scheme at a specific location in Central London. The CUSUM methods of Lucas first compute the standardised deviations of time series observations from the desired process mean. These are accumulated over time to compute the CUSUM at each time point. Data for the analysis were taken from a kerbside monitoring site on Marylebone Road, a six lane trunk route in Central London. In August 2001 the lane adjacent to the monitoring site was designated as a permanent bus lane. The CUSUM analysis clearly identifies a sustained decrease in carbon monoxide concentrations beginning in 2002. However, seasonality and other factors precluded precise characterisation of the timing of the change. When the analysis was repeated using a reference mean that extrapolated the pre-intervention trend in carbon monoxide concentrations, the CUSUM chart no longer identified a sustained decrease. CUSUM appears to offer a simple and rapid method for identifying sustained changes in pollution levels, but the range of confounding influences on carbon monoxide concentrations, most notably underlying trends, seasonality and independent interventions, complicate its interpretation. Its application in assessing the presence or timing of a stepped change in pollution or similar environmental time series data is recommended in its basic form only where the predicted change is large by comparison with other independent influences. The authors believe that further development of the technique beyond this initial study is worthwhile in order to improve the technique's sensitivity.

Evidence for increasing concentrations of primary PM 10 in London

Although annual mean concentrations of PM 10 in and around London reduced during the 1990s, concentrations have been relatively stable since, with small increases during 2001–2003. A modelling method was used to apportion the measured annual mean concentration of PM 10 between primary, and secondary and natural sources at monitoring sites in and around London between 1997 and 2004. The annual mean concentration of PM 10 from primary sources reached a minimum during 1998. Between 1998 and 2003 the greatest change in the annual mean concentration of primary PM 10 occurred at sites adjacent to busy roads: Marylebone Road (7.0±2.6 μg m −3), Wandsworth 4 (6.0±1.1 μg m −3) and Ealing 2 (4.5±1.4 μg m −3). The smallest change in the concentration of PM 10 from primary sources occurred at background locations in towns outside London. It is therefore likely that the additional primary PM 10 arose from road traffic. These increases in PM 10 from primary sources contradict emissions inventory estimates that indicate reductions in primary PM 10. The annual mean concentration of PM 10 from secondary and natural sources declined by 2.0±3.0 μg m −3 or 11±16% between 1997 and 2003. The annual mean concentration of PM 10 from these sources reduced until the year ended 1st July 2001 and then increased during the remainder of 2001, 2002 and 2003.

The implications of tapered element oscillating microbalance (TEOM) software configuration on particulate matter measurements in the UK and Europe

The tapered element oscillating microbalance (TEOM) is used to measure ambient particulate matter concentrations worldwide. The instrument is configured with a correction factor required by the US Environmental Protection Agency (EPA) to account for the difference between the TEOM and the gravimetric method. However, numerous studies have shown that this factor does not fully account for these differences, consequently, the European Commission (EC) insists on an independent assessment of the differences. The US EPA correction was found to contribute between 3 and 5 μg m −3 to the daily mean PM 10 and PM 2.5 concentrations at Marylebone Road, London, between 1 January 2001 and 31 December 2004. The TEOM is also configured to express measurements to a standard temperature (25 °C) and a standard pressure (1 atm). This differs from the US EPA requirement of ambient reporting conditions for both PM 10 and PM 2.5. It also differs from the European position, which also requires reporting at ambient conditions. When compared to ambient reporting conditions, the TEOM standard temperature and pressure correction was found to contribute between −1 and 7 μg m −3 to the daily mean PM 10 and PM 2.5 concentrations, and 2 μg m −3 to the annual mean concentrations. The standard temperature and pressure correction was also demonstrated to vary the annual mean concentration measured using the TEOM by 2–8% between European capital cities, with higher values in colder northern cities. The standard temperature and pressure correction to individual daily mean concentrations was shown to be up to 20%. It is recommended that the US EPA correction factor is removed from TEOM measurements and that all measurements are reported at ambient temperature and pressure.

Pedestrian exposure to air pollution along a major road in Central London, UK

Pedestrian exposure to PM 2.5, the loss of reflectance (‘blackness’) of the PM 2.5 filters, ultrafine particle counts (particle range: 0.02–1 μm) and carbon monoxide (CO) was investigated along a major road running through the DAPPLE study site in Central London, UK. During an intensive 12-day exposure measurement campaign, groups of four volunteers sampled twice in the morning and twice in the afternoon along Marylebone Road. They were randomly designated a walking direction, walking position (kerbside or buildingside) and a side of the major road to walk along. PM 2.5 was sampled using high-flow gravimetric personal samplers, ultrafine particle counts were measured using TSI P-TRAKs and Langans were used to measure CO exposure. PM 2.5 samples were analysed gravimetrically and reflectance was measured using a smoke stain reflectometer to obtain a measure of ‘black smoke’. In total 603 acceptable samples were obtained—155 PM 2.5 and reflectance, 120 ultrafine particle count and 173 CO. The average pedestrian exposure along the road was 37.7 μg/m 3, 12.1 m −1×10 −5, 80 009 pt/cm 3 and 1.3 ppm for PM 2.5, loss of reflectance, ultrafine particle counts and CO, respectively. PM 2.5 exposure in the morning was significantly higher than in the afternoon, and there was a significant difference in exposure on the different sides of the road. For both reflectance and ultrafine particle counts, the exposure was significantly different both between the two walking positions on the pavement and the two sides of the street canyon. However there was no significant difference in CO exposure based on walking position, walking direction, canyon side or timing. Filter reflectance was significantly but weakly correlated with PM 2.5 exposure ( r = 0.3 , N = 155 ), CO exposure ( r = 0.2 , N = 154 ) and ultrafine particle count exposure ( r = 0.7 , N = 108 ). PM 2.5 and CO personal exposure measurements were much higher than those recorded at both the local background fixed monitoring station (FMS) and a kerbside FMS, but PM 2.5 personal exposure was significantly correlated with the PM 2.5 concentrations at the background FMS ( r = 0.6 , N = 155 ).

Interpretation of particulate elemental and organic carbon concentrations at rural, urban and kerbside sites

Concentration measurements are reported for particulate organic and elemental carbon, measured using an R&P 5400 ambient particulate carbon monitor at four sites in the United Kingdom: one roadside (London, Marylebone Road), two urban (London, North Kensington, and Belfast, Centre) and one rural (Harwell). The measurements were collected on a continuous three hourly average basis between January 2002 and mid-2004. The concentrations show no obvious seasonal cycle, except for an increase in OC/EC ratio at London, North Kensington, during the summer months consistent with a possibly greater relative contribution of secondary organic aerosol. Perhaps surprisingly this is not, however, seen at the rural Harwell site. At Belfast, both organic and elemental carbon show elevated winter concentrations, consistent with important local primary sources. Only at the roadside Marylebone Road site is there a high correlation ( r 2 = 0.59 ) between organic carbon and elemental carbon concentrations which persists when simultaneously measured urban background concentrations from the nearby North Kensington site are subtracted; the OC/EC ratio in the traffic-related concentration increment is 0.88, well below the ratios typical of the urban background. Directional analysis of the data from the Marylebone Road street canyon shows that whilst elemental carbon concentrations are determined primarily by on-road traffic emissions, both organic carbon and PM 10 derive primarily from inputs from outside the street canyon. It therefore appears that at all times of year non-traffic sources of particulate organic carbon, be they primary or secondary, are dominant over traffic emissions in the urban background. Organic compounds account for about 22% of aerosol mass, irrespective of site.