Understanding dispersion of nanoparticles in vehicle wake combining fast response measurements and wind tunnel simulations.

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Recent studies have indicated that nanoparticles (NPs) may have greater negative impacts than coarser particles (PM10 or PM2.5 i.e. mass concentrations of particles with aerodynamic diameters <10 or 2.5 um, respectively) on human health, urban visibility and global climate change. Here, NPs are referred to as particles of size <300 nm as this size range includes nearly all particles (>99% of total number concentrations) in the urban environment. Road vehicles emit most particles within this size range. Current air quality regulations are based on PM10 and PM2.5 and therefore do not control particle number concentrations (PNCs). In contrast, ultrafine fraction (<100 nm) of NPs contribute little to particle mass concentrations but significantly higher (~80%) to total PNCs. It means that existing air quality regulations are ineffective to control a major part of road vehicle particle emissions. Recently, the UN-ECE Particle Measurement Programme has taken a step forward by proposing emission limits for particles (covering 10-300 nm size range) on a number basis for light and heavy duty diesel vehicles; these have been included in Euro 5 and 6 emission standards. Such initiatives are also required for ambient NPs that will allow regulatory authorities to design effective mitigation strategies for controlling urban NPs on a number basis. However, this progress has been hampered due (in part) to the lack of standard guidelines and instrumentation to measure NPs, the limited knowledge of their dispersion at various spatial scales and the complex particle dynamics involved. Today nearly half of the global population lives in urban areas where probability of human exposure to vehicle-emitted high PNCs is considerably higher. Therefore, it is a matter of public and scientific concern to examine emissions from individual vehicles under real world driving and dilution conditions. However, the situation becomes complex when fine spatial scale studies are contemplated (e.g. in a vehicle wake). This is because the distribution of NPs changes rapidly after emission from the tailpipe in the wake of a moving vehicle due to the competing influences of a number of transformation (i.e. coagulation, condensation, deposition and nucleation) and dilution processes. Information on the time scales for these rapid processes is essential for the modelling of NPs in the tailpipe-to-road region but is not available because of the inadequate sampling frequencies of available instruments.The proposed work aims to deploy a recently commercialised fast response differential mobility spectrometer (DMS50) for measuring particle number and size distributions in the 5-560 nm size range at a sampling frequency of 10 Hz. The DMS50 has not been applied ever for ambient measurements yet. The objectives are to study the change in NP distributions due to competing influences of dilution and transformation processes over the travel time from tailpipe to roadside and to model the fate of these particles at a fine spatial scale (i.e. the near and the main/far wake regions of a moving vehicle). These objectives will be achieved (i) by performing field measurements of NP number and size distributions using a DMS50 in the wake of vehicles (a diesel-engined car and a van) moving at various speeds, (ii) by mimicking the field experiments using wind tunnel simulations for investigating the flow and dispersion characteristics in the wake regions of vehicles, and (iii) by analysing the data obtained from field experiments and wind tunnel simulations to develop the basis for predicting NP concentrations in vehicle wakes.Findings from this work will assist the scientific community and regulatory authorities in better understanding the science behind the NP dynamics involved in the tailpipe-to-road region and in doing so provide a link between studies targeting either roadside or engine measurements separately.