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

Lead Research Organisation: University of Surrey
Department Name: Civil and Environmental Engineering


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.

Planned Impact

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.


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Description An overview of significant past research work relevant to modelling the dispersion of pollutants in the wake of vehicles, with a particular focus on nanoparticle dispersion, was carried out as a preliminary step towards the project objectives (see Carpentieri et al., 2011a for details). Literature related to vehicle wakes and nanoparticle dispersion was reviewed, taking into account field measurements, wind tunnel experiments and mathematical modelling approaches. The results from this preliminary study can be summarised as follows:

1. Field measurements and nanoparticle dynamics modelling studies highlighted the very short time scales associated with nanoparticle transformations in the earliest stages after emission. These transformations are strongly dependent on the flow and turbulence conditions immediately behind the emitting vehicle, hence the need to characterise these in some detail, especially in the near wake. This is particularly important when attention is focussed on short range (i.e. street scale) dispersion. Very few studies have addresses this topic and those that do are generally applicable only in the far wake. A simple approach derived from building wake studies has also been proposed. Given the lack of experimental data that might be analysed to determine suitable parameterisations, further research is needed in the areas of wake dispersion and nanoparticle dynamics.

2. Field experiments purposely designed for measuring nanoparticle evolution in the wakes of vehicles are rare; there is a clear research need involving fast response measurements that can provide detailed information on transformation processes occurring just after the release of emissions as a result of interactions with the near wake flow and turbulence.

3. Wind tunnel experiments for characterising vehicle wakes have successfully been used in the past in the few studies on this topic that can be found in the literature. The methodologies highlighted in those works may be further improved and applied specifically for the development of nanoparticle dispersion models in the wake of vehicles. One of the main findings of previous wind tunnel studies is the strong dependence of the near wake characteristics on the vehicle shape, so there is a clear necessity to study different geometries in order to generalise the results to be used for model development. None of the previous laboratory studies on pollutant dispersion in vehicle wakes involved the use of a rolling floor; this technique is widely used in aerodynamic studies and it is essential to reproduce realistic flow and turbulence fields in the wind tunnel. Nanoparticle concentration measurements in the wind tunnel have never been attempted; these would greatly help characterise the dispersion process, though care must be given to the key similarity criteria in reduced scale models.

In the first phase of the project, two sets of experimental campaigns were carried out using a fast response particle spectrometer to measure number and size distributions of nanoparticles in the wake of a moving diesel car (see Carpentieri and Kumar, 2011). Both ground-fixed and on-board setups were used. This experimental strategy allowed us to take into account the effect of the vehicle wake on the nanoparticle dispersion process, providing also new insight for the interpretation of results from previous studies by other authors.

Temporal changes in results were divided into three main stages (pre-evolution, evolution and post-evolution) after the release of exhaust emissions from the tailpipe. The evolution stage is of most interest where the main changes to particle number and size distribution occurred. Up to four evolution sub-stages were observed, each showing distinct evolution patterns of particle size distributions, depending on the particular experimental run. In agreement with previous studies, the measurements confirmed the dilution process as a main driver for controlling the transformation of nanoparticles in the emitted plume of a diesel car throughout all the evolution stages. Results for the first evolution phase, immediately after emission, agreed with previous literature and confirmed the presence of a first phase in which PNCs and PNDs change rapidly due to nucleation processes driven by rapidly increasing dilution. This stage is over after a few tenths of second, when condensation processes start to become effective in increasing the size of the particles, with a fast (but slower than in the first phase) increase of accumulation mode particles.

What happens next is less clear due to complex interactions between the flows in the vehicle wake and exhaust emissions that start to affect the dispersion and transformation of nanoparticles. The combined analysis of ground-fixed measurements, on-board measurements and past literature clearly showed the presence of two different groups of nanoparticles in the volume immediately adjacent to the back of the moving vehicle: new particles, freshly emitted from the tailpipe, and relatively aged particles in the flow recirculation wake of the vehicle. The two groups have experienced different transformation processes with different time scales, strongly linked with the dilution levels they are subject to. While at immediate distance immediately downstream of the tailpipe the freshly emitted group of new nanoparticles is abundantly dominant, in most of the volume above the tailpipe level and close to the back of the car only the relatively aged group can be found. Other zones, further away from the vehicle, contain a mixture of both and particular care should be taken in interpreting the results from measurement campaigns. This aspect related to the vehicle wake was often overlooked in past studies.

In the second phase of the project, wind tunnel experiments were carried out in the wake of small scale models of passenger cars (see Carpentieri et al., 2011b, 2011c). Both velocity and concentration fields were measured by means of a LDA and a FFID. The results showed the influence of the vehicle wake on the dispersion process, while this aspect is usually neglected by standard mathematical dispersion models. Since the effects of the wake on pollutant dilution cause complex interactions with other transformation processes affecting nanoparticles, operational dispersion models for nanoparticles will need to acknowledge these effects, in order to correctly reproduce the physical phenomena.

The experiments allowed a complete characterisation of the near wake, and part of the far wake, behind a small scale model of the same car used during the field measurements. The tests were conducted in a large environmental wind tunnel and a number of configuration adjustments were carried out in order to reduce any unrealistic boundary layer growth on the floor, as discussed above. While these adjustments were able to minimise the unwanted effect, they could not completely remove it. Preliminary tests were then conducted in the smaller rolling road wind tunnel, able to completely remove the boundary layer in the vicinity of the model. A limited experimental data set was produced, and these tests showed great potential for future studies.

The high resolution experimental database obtained from the wind tunnel experiments, in conjunction with the large nanoparticle concentration data set obtained from the field measurements, can be used as a first step for deriving mathematical parameterisations to be used with operational nanoparticle dispersion models.

As evident from our preliminary literature review (Carpentieri et al., 2011a; Kumar et al., 2011a), none of the current dispersion models is able to reliably calculate the dispersion of nanoparticle in the wake of a vehicle, especially in the near-wake, where most of the transformation processes occur. A modeling framework has been proposed during this project, using existing building wake models as a starting point. However, adapting building wake models to vehicle wakes is not trivial, and several problems must be solved in the future. The recommendations and insight resulting form this project, and documented in the listed publications, can be used to identify future research needs for developing adequate mathematical models for nanoparticle dispersion in vehicle wakes. A first step will likely involve the comparison of our experimental results with existing and well established building wake models (for example, ADMS-BUILD, a model routinely used for environmental impact assessments) in order to gather information about the parameters involved and how they might need to be modified.

Further analysis of the dataset obtained from this project is being conducted as the basis of a simple mathematical parameterisation (Carpentieri et al., 2011b). The project also identified a number of questions to address in future. In particular, similar experiments must be carried out on other test cases having different vehicle geometries for formulating generalised mathematical parameterisations that could then be embedded in existing dispersion models. The novel findings as well as the high resolution database obtained through a integrated experimental methodology developed during this project, however, offer a solid starting point for future developments, and provides valuable information for further studies on this topic.
Exploitation Route Conference presentations

Journal Publications

Invited talks/lectures
Sectors Environment,Transport

URL http://www2.surrey.ac.uk/cee/people/prashant_kumar/#publications
Description The publications arisen from this grant has been cited 100+ times and the approach established is helping numerous numerical studies.
First Year Of Impact 2012
Sector Environment,Transport
Impact Types Societal,Policy & public services