I.D. Longley, J.R. Dorsey, M.W. Gallagher, J.D. Allan, M.R. Alfarra, H.Coe Physics Department, UMIST, Manchester, U.K. Exposure to ultrafine particles from traffic in city streets and the urban atmosphere
PM 10 Particles monitored as PM 10 (< 10 m) across Europe PM 10 is epidemiologically linked to mortality and morbidity
Urban particle sources and sizes Vehicle emissions, combustion Long-range transport, secondary particles Dust, wear products, biological particles, minerals Measured in Princess Street, Manchester (Atmos. Environ. 37, 1563-71) PM 10
Ultrafine Particles – mass and number Above: mass size distribution from Manchester street canyon Above: number size distribution from Manchester street canyon Typical UK urban concentrations: PM 10 :~ 20 g m -3 Most particles are ultrafine, but have tiny contribution to PM 10 PM 0.1 :~ 1 g m -3
Ultrafine Particles in the body Ultrafines (UFP) efficiently deposit to alveolar walls Overload can cause chronic inflammation and irreversible damage to tissues and defences Inflammatory response triggers systemic reaction in cardiovascular system – increases in blood viscosity, formation and disruption of plaques, heart rate variability. Can lead to arrythmia, ischaemia and heart attack (immediately or in future) Effects seen in ‘non-toxic’ particles – is toxicity in size, surface area or composition???
Spatial variation in urban PM 10 (1) 2005 predicted annual mean PM 10 / g m -3 NOTE scale divisions Variation from 9.25 – 10.50 g m -3 across most of city Prediction dominated by ~ 8 g m -3 regional background
Dominated by city-wide episodes linked to meteorology. Temporal variation in PM 10 PM 10 is dominated by extra-urban sources and processes. Implies that little can be achieved by local intervention
Urban Ultrafine Particles Toxicity lies in ultrafine fraction Traffic is the dominant urban source Not routinely monitored Much stronger spatial gradients and temporal variability Manchester from UMIST Building
SASUA diurnal particle number flux (Oct/Nov) Below: sensible surface heat flux Diurnal cycle in urban ventilation related to heat flux cycle Morning concentration peak related to reduced ventilation
SASUA diurnal particle number flux (May) Below: sensible surface heat flux Higher heat flux, but slightly lower urban ventilation flux, due to lower emission rates Earlier sunrise and stronger heating means morning concentration peak is diminished. This effect dependent upon latitude.
Estimating background N 0.1 from NO x SCAR site Network monitor Predicts background range of 4 000 – 9 000 cm -3
Ultrafines number size distribution in channelled flow
Ultrafines number size distribution in recirculating flow Recirculation caused by perpendicular approach flow (>40 from canyon axis) Extra particles in ‘fresh exhaust’ size range
Effect of sheltering on concentration enhancement
Statistical variation in ultra-fine concentrations Roughly log-normal concentration distributions
Comparison with background N 0.1 N 0.1 / cm -3 Street canyonBackgroundRatio mean26 5006 0004.5 Rush-hour mean35 0007 3004.8 98.5 th percentile (1 hr) 122 00012 7509.5
Influence of average daily exposure 23 hours background of 6 000 cm -3 1 hour (peak traffic period) canyon exposure at 35 000 cm -3 Assume no other significant residential or occupational exposures Background: 80 % Canyon: 20 % average breathing rates are 2 – 3 times higher in streets, deposition deeper Exposure ratio street canyon:background may be 10 –20
Long-term exposure to UFP is dominated by a well-mixed urban background concentration. Exposure is enhanced by short-term peaks at traffic- influenced locations. Total personal exposure influenced by duration of proximity to traffic especially in street canyons. Diurnal and longer-term variations in concentrations controlled by the ventilation rate of the city and hence by the thermal climate. High morning peaks in UFP can occur when rush-hour begins before significant urban thermal emission, more likely at high latitudes in winter. Within ~ 100 m of traffic sources meandering plumes make a small contribution to long-term means but a large contribution to upper percentile concentrations Conclusions
Exposure in street canyons dominated by fresh plumes. Ventilation of streets is reduced by flow isolation. Short, high doses may make a small contribution to total exposure, but can be highly significant in triggering ill-health. Significance of street exposure greater for occupational exposure or leisure activities (e.g. street cafes). PM 10 does not capture the strong gradients in UFP, or the short-term variability. Interventions (e.g. street closure) may have minor effect on modelled PM 10, but potentially major reductions in UFP. Conclusions
With thanks to Cassella Stanger, London, UK Centre for Ecology and Hydrology, Edinburgh, UK CERC Ltd., Cambridge, UK Acknowledgements
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