Presentation on theme: " If it is assumed that w is made up of a contribution from the mean wind and a contribution from the traffic, then it can be suggested that Introduction."— Presentation transcript:
If it is assumed that w is made up of a contribution from the mean wind and a contribution from the traffic, then it can be suggested that Introduction Vehicle emissions of particles and pollutant gases are causing increasing concern due to their adverse effects upon the health of the public. The greatest numbers of people are exposed to the highest concentrations of such pollutants where the sources are most concentrated - in urban street canyons. In such canyons dispersion is constrained by the buildings and complicated by recirculation and the enhanced and highly localised turbulence associated with such a complex space. Regulation of urban air quality increasingly relies upon simple dispersion models such as ADMS-Urban (McHugh et al, 1997). Such models are based upon a simplified empirical transport scheme treating aerosol purely as a PM 10 metric and do not describe the differing behaviour of different size ranges or number concentrations. Recent concerns over the appropriateness of the PM 10 metric for regulation purposes (e.g. EPAQS, 2000, Harrison & Yin, 2000) has, in part, led to a need to understand the size-segregated behaviour of aerosols better in campaigns such as this one. Objectives The experimental campaign was named Street Canyon Aerosol Research, or SCAR. The principal aim of SCAR was to obtain the emission velocity and ventilation fluxes of aerosol from the canyon space. It was intended that these fluxes could then be parameterised with regard to controlling factors. The factors expected to have most influence over these fluxes were wind speed and direction and traffic flow both directly and indirectly through thermally and mechanically produced turbulence. Measurements of Aerosol Pollutant Transport in a Manchester Street Canyon I.D. Longley, M. Flynn, J.D. Dorsey, P.I. Williams, M.W. Gallagher, J.R. Allan, M.R. Alfarra, H. Coe. University Of Manchester Institute of Science and Technology, Manchester, UK Activities A measurement campaign has been conducted in an asymmetric street canyon with busy one-way traffic (Princess Street) in central Manchester. The principal experimental period (SCAR-4) covered two weeks (Monday to Friday, 24-hour operation) in October 2001, preceded by three week-long preparatory experiments in February, April and May 2001. The eddy correlation method was used to determine fluxes of size segregated accumulation mode aerosol. Measurements of accumulation mode aerosol and trace gases at a static location were made concurrently with measurements on a platform lift giving vertical profiles. Size segregated measurements of ultra-fine and coarse particle concentrations were also made simultaneously at various heights. Also, a small mobile system has made measurements of turbulence at various pavement locations within the canyon. Table 1. Instruments deployed during SCAR-4 discussed in this poster: Mean aerosol number concentrations (SMPS data, 4.7nm<D p <157nm) As expected for a road-vehicle-dominated environment, number concentrations were dominated by ultra-fine particles. Average total number concentrations at the lowest level of 4m were approximately 36 000 cm -3. The total number concentration of particles in the range measured by the SMPS was found to exhibit some correlation with traffic activity in the street. There was a clear vertical gradient in number concentration. Mean ultra-fine aerosol number size distributions The SMPS revealed variations in particle number distributions at different heights within the canyon. Concentrations are reduced with height in the canyon, until the roof level is reached. At this level (17m in this case), the particle spectrum is very slightly broadened towards larger particles. The modal size increases by a few nanometres. It is believed that this is due to the mixing of the canyon-generated aerosol with aerosol from beyond the subject canyon. Between day and night-time the concentrations vary considerably with the traffic level, but the shape of the number distribution spectrum remains the same. Aerosol Chemistry An Aerosol Mass Spectrometer (AMS) was operated approximately 30m above street level at the UMIST building in central Manchester in January 2002 (campaign described by Allan, J, in preparation). There is a persistent accumulation mode at around 400-500nm, which is manifested in nitrate, sulphate and organics. There was a mode in the mass spectra at around 100-200nm consisting of aliphatic organic chemicals. This compares well with results obtained by Kleeman et al. , in which different engine types produced a mode at 100-200nm, consisting mainly of organic carbon (OC). Organic activity within the Manchester sampling periods, and particularly in the sub 200nm mode, correlates well with NO x activity. We conclude that the Aitken mode consists mainly of motor vehicle emissions. This also agrees with the findings of Williams et al. , who found that Manchester ambient particle number concentrations of the size range 100-500nm to be linked to traffic activity. Concentrations increase at times of low temperature and wind speed. This implies that there is reduced mixing and pollutants are being concentrated in a surface layer and not being dissipated by the wind. In such conditions, the sampled aerosol are essentially being aged in situ and so observed changes of mass loadings and distributions with time may be explained to some extent by processing, though changes in emissions may also impact the aerosol. Mean aerosol vertical fluxes The average total number flux in the range 100nm<D p <3 m ranged from around 100 cm -2 s -1 at night to 1600 cm -2 s -1 in the middle of the day. The flux can be seen to be related to both source strength (traffic flow) and heat flux. The contribution of wind-dependent re-suspension of larger particles, thermal stability and the different behaviour of different size sub- ranges is under investigation for future publication. Emission velocity An emission velocity v e can be defined such that, i.e. flux/number concentration. The emission velocity was calculated for the data from the ASASP-X (100nm<D p <3 m) using the flux and concentration for the total size range. This emission velocity was found to range from around 0.3 to 3 cms -1 at a height of 5m, with an average of around 1.5 cms -1. The emission velocity showed a clear diurnal cycle, peaking around noon. Mean Air Flow Means of wind speed and direction from the anemometers in the canyon show that the wind was mostly channelled along the axis of the canyon. Evidence of vortex motion can be seen in the sign of vertical wind angle which peaks when ambient winds are perpendicular to the canyon. The mobile sonic recorded almost no downward winds when sited on the north pavement (2 to 4m above the road). This seems to indicate that the vortex did not penetrate this deep into the canyon, or was overwhelmed by other flows. Stress Positive values of u’w’ co-variance were recorded quite frequently (e.g. 28% of data for one of the anemometers). u’w’ co-variance was found to be very dependent upon wind direction, with large values in perpendicular winds, positive with winds approaching over the shorter wall of the canyon and larger negative values when the wind approached over the taller canyon wall. u’w’/U 2 (or u * /U) converged when U > 1.5 ms -1, allowing a vertical profile to be constructed, showing enhanced shear near the roof level. Turbulent variances Due to the positive stresses and dependence of stress upon wind direction it was found that the variances were best presented as standard deviations normalised by local sonic wind speed, i.e u,v,w /U. w /U was well-relate to sonic wind speed U, with increased values when U < 1.5 ms -1, indicating the relative importance of other sources, such as thermal and traffic-induced turbulence, when wind speeds are low. A vertical profile of w /U (when U > 1.5 ms -1 ) shows a weak positive gradient, but with a layer of greatly enhanced turbulence in the lower 2m or so. v /U followed a similar pattern to w /U, but u /U was different in having larger values. ‘Hot-spots’ of w /U occurred wherever U/U roof fell below about 0.3. This was more likely to happen in the ‘sheltered zone’ at the downwind end of the canyon. This zone extended over a greater length of the canyon in more perpendicular winds, and when the wind blew over the higher canyon wall. ‘Hot spots’ also occurred where winds blowing from opposite ends of the canyon met. Conclusions The flux and emission velocity of fine mode aerosol (100nm<D p <3 m) have been measured in a city centre street canyon with busy traffic. The total number flux was of the order of 1000 cm -2 s -1, whereas the emission velocity was between 0 and 3 cms -1. A vertical gradient was found in ultrafine aerosol number concentrations below roof level. The modal particle size was within 25 – 30 nm. At roof level ultrafine number concentrations were slightly raised in comparison to below, and the spectrum was slightly broadened with the number mode occurring at a slightly higher particle size. A weak positive vertical gradient was seen in turbulent variances. However, greatly enhanced variances were seen at the bottom of the canyon (2m above floor). A parameterisation has been derived for w based upon local wind speed and traffic flow rate. Evidence of a mean vortex motion within the canyon was observed, with upward flow on the lee wall of the canyon. However, downward flow on the downwind wall was only observed on the higher canyon wall, highlighting the significance of the canyon’s asymmetry. Further analysis is to be published on the detail of the fluxes and turbulence, and how they relate to each other. References Allan, J, 2002: [in preparation] Expert Panel on Air Quality Standards (EPAQS), 2000: What is the appropriate measurement on which to base a standard ? Harrison, RM., Jianxin Yin, 2000: Particulate matter in the atmosphere: which particle properties are important for its effects on health ?, Sci. of Total Env. Vol. 249 pp. 85-101 Kleeman, MJ, Schauer, JJ, Cass, G.R, 2000: Size and composition distribution of fine particulate matter emitted from motor vehicles, Enviro. Science & Tech., vol. 34, no.7, pp.1132-1141 McHugh, CA, Carruthers, DJ, Edmunds, HA, 1997: ADMS-Urban: an air quality management system for traffic, domestic and industrial pollution, Intl. Jl. Of Env. & Poll. Vol.8, nos.3-6, pp.666-674 Williams PI, Gallagher MW, Choularton TW, Coe H, Bower KN, McFiggans G, 2000: Aerosol development and interaction in an urban plume, Aerosol Science & Tech., vol.32, no.2, pp.120-126 Above: The experimental canyon, showing definitions of wind directions used in analysis. NOTE asymmetry of the canyon Right: equipment on site showing platform lift and instrument trailer InstrumentDetailsMeasuring SMPS – ultrafine mode TSI Model 3936number concentrations, 4.7nm<D p <157nm OPC – accumulation mode PMS ASASP-XAerosol number spectra & fluxes, 100nm<D p <3 m Ultrasonic anemometers RM Young 81000 3D wind speed, temperature Ultrasonic anemometer (fixed) Gill Solent R23D wind speed, temperature Above: plan view contours of a) wind speed ratio U/U roof, and b) w /U on one SCAR period Above: plan view contours of c) wind speed U, and d) w /U on a different day a) b) c) d) From Solent data, was taken to be 0.16 at a height of 3.5m. It is possible that a rises with height – this is still being investigated. wt was found to follow the relationship wt = 0.0135T 1/2 at 3.5m, where T is traffic flow rate in vehicles per hour. The correlation between wt and traffic becomes poorer with increased height.