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

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.

The UK Department of Transport recently proposed limits for particles on a number basis for diesel vehicles through the UN-ECE GPRE Particle Measurement Programme. Similarly, various regulatory authorities are also seeking number concentration based information on ambient NPs for designing future mitigation strategies in urban areas. The proposed work is believed to be the first study of this kind in the UK involving fast response measurements of NPs in vehicle wakes. It will provide a novel data set on NP distribution and dynamics in the vehicle wake regions. The scientific publications from this research will benefit the UK scientific community and regulatory authorities directly because it will develop better understanding of an important class of pollution, thus allowing targeted mitigation strategies, rather than (for example) blanket bans on certain classes of vehicle (such as the current clean air zone in London). Better understanding of the science behind the NP dynamics involved in the tailpipe-to-road region would fill the gap between studies targeting either roadside or engine measurements, separately. The investigator, in his PhD research, was one of the first to use the DM500 for measurements of ambient NPs, because this instrument was originally designed for the measurements of engine emissions. Air quality management and scientific communities are looking for standard guidelines and instrumentation to measure ambient NPs number and size distributions; several publications from the investigator's past work have provided fundamental knowledge in this area. The present work proposes the use of a newly commercialised particle spectrometer (DMS50), the portable version of the DMS500 with some modifications, for measurements of ambient NPs in vehicle wake. As this instrument has not yet been used for such applications, the investigator will be the first to deploy it in this manner. Comparisons will be made with the DMS500 to make sure that both instruments produce similar results and with the FFID (in the wind tunnel) to clarify precise sampling parameters. These exercises will introduce an alternative solution to the scientific community for the measurement of ambient NPs. We will inform the regulatory authorities about the potential of advanced methods for field measurements. Finally, as mentioned above, this study should reveal many interesting aspects of NP dynamics during their transport from tail pipe to the roadside, information that will help modellers to design appropriate algorithms for the complex aerosol dynamics after emissions from tail pipe.