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

最近的研究表明，纳米粒子(NPs)对人体健康、城市能见度和全球气候变化的负面影响可能比更粗的粒子(PM10或PM2.5，即空气动力学直径分别为10微米或2.5微米的粒子的质量浓度)更大。在这里，NPs被称为尺寸为300 nm的颗粒物，因为这个尺寸范围包括城市环境中几乎所有的颗粒物(&gt；99%的总浓度)。道路车辆在此大小范围内发射大多数粒子。目前的空气质量法规是基于PM10和PM2.5的，因此没有控制颗粒物的数量浓度(PNC)。相比之下，超细纳米粒子(&lt；100 nm)对粒子质量浓度的贡献很小，但在总PNC中的贡献显著(~80%)。这意味着，现有的空气质量法规对控制道路车辆颗粒物排放的主要部分无效。最近，联合国欧洲经委会颗粒测量方案向前迈进了一步，对轻型和重型柴油车提出了多个颗粒物(覆盖10-300纳米尺寸范围)的排放限制；这些排放标准已被纳入欧5和欧6的排放标准。环境核动力源也需要这样的举措，这将使监管当局能够设计有效的缓解战略，以在一定数量的基础上控制城市核动力源。然而，这一进展受到了阻碍，(部分)原因是缺乏衡量NPs的标准指南和仪器，对其在各种空间尺度上的扩散的了解有限，以及所涉及的复杂的粒子动力学。今天，全球近一半的人口生活在城市地区，在这些地区，人类接触车辆排放的高浓度PNC的可能性要高得多。因此，在现实世界的驾驶和稀释条件下检查每辆汽车的排放是一个公众和科学关注的问题。然而，当考虑精细空间尺度研究时(例如在车辆尾迹中)，情况变得复杂。这是因为汽车尾气排放后，由于一系列转变(即凝聚、冷凝、沉积和成核)和稀释过程的相互竞争影响，NPs的分布发生了快速变化。关于这些快速过程的时间尺度的信息对于尾气到道路区域的NPs的建模至关重要，但由于现有仪器的采样频率不足而无法获得。拟议的工作旨在部署最近商业化的快速响应差分迁移率光谱仪(DMS50)，以10赫兹的采样频率测量5-560 nm尺寸范围内的颗粒数量和粒度分布。DMS50还没有被应用于环境测量。其目的是研究由于稀释和转化过程对从尾管到路边的行驶时间的竞争影响而引起的NP分布的变化，并在精细空间尺度上(即移动车辆的近尾迹区域和主/远尾迹区域)模拟这些颗粒的去向。这些目标将通过(I)通过使用DMS50在车辆(柴油发动机轿车和货车)以不同速度移动的尾流中进行NP数量和尺寸分布的现场测量来实现，(Ii)通过使用风洞模拟来模拟现场实验以研究车辆尾流区的流动和扩散特性，以及(Iii)通过分析从现场实验和风洞模拟获得的数据，为预测车辆尾流中的NP浓度奠定基础。这项工作的结果将有助于科学界和监管当局更好地了解尾气与道路之间涉及的NP动力学背后的科学，并在研究之间建立联系，分别针对路边或发动机的测量。

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.