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

最近的研究表明，纳米颗粒（NP）可能比粗颗粒（PM10或PM2.5，即空气动力学直径<10或2.5 μ m的颗粒的质量浓度）对人类健康，城市能见度和全球气候变化产生更大的负面影响。这里，NP被称为尺寸<300 nm的颗粒，因为该尺寸范围包括城市环境中几乎所有的颗粒（>99%的总数浓度）。道路车辆排放的大多数颗粒物都在这一尺寸范围内。目前的空气质量法规基于PM10和PM2.5，因此不控制颗粒数浓度（PNC）。相比之下，纳米颗粒的超细级分（<100 nm）对颗粒质量浓度的贡献很小，但对总PNC的贡献显著较高（约80%）。这意味着现有的空气质量法规对控制大部分道路车辆颗粒物排放无效。最近，联合国欧洲经济委员会颗粒测量计划向前迈出了一步，提出了轻型和重型柴油车辆颗粒（涵盖10-300纳米尺寸范围）的排放限值;这些限值已被纳入欧5和欧6排放标准。对于环境NP也需要此类举措，这将使监管当局能够设计有效的缓解战略，以在数量上控制城市NP。然而，这一进展一直受到阻碍，由于（部分）缺乏标准的指导方针和仪器来衡量纳米粒子，有限的知识，他们的分散在各种空间尺度和复杂的粒子动力学。今天，全球近一半的人口生活在城市地区，人类暴露于车辆排放的高PNC的可能性相当高。因此，在真实的世界驾驶和稀释条件下检查单个车辆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，在车辆尾流中），情况变得复杂。这是因为在移动车辆的尾流中从排气管排放后，由于许多转化（即凝结、冷凝、沉积和成核）和稀释过程的竞争影响，NP的分布迅速变化。有关这些快速过程的时间尺度的信息对于模拟排气管到道路区域的颗粒物是必不可少的，但由于现有仪器的采样频率不足，因此无法获得。拟议的工作旨在部署最近商业化的快速响应微分迁移率光谱仪（DMS 50），用于测量5- 10年内的颗粒物数量和粒径分布。560 nm的尺寸范围，采样频率为10 Hz。DMS 50还没有应用于环境测量。我们的目标是研究NP分布的变化，由于竞争的影响，稀释和转换过程的旅行时间从排气管到路边，并模拟这些颗粒的命运在一个精细的空间尺度（即近和主要/远尾区的移动车辆）。这些目标将通过以下方式实现：（i）使用DMS 50在车辆尾流中对NP数量和粒度分布进行现场测量（柴油发动机汽车和货车）以不同速度移动，（ii）通过使用风洞模拟来模拟现场实验以研究车辆尾流区域中的流动和分散特性，以及（iii）通过分析从现场实验和风洞模拟获得的数据，建立预测车辆尾流中NP浓度的基础。这项工作的结果将有助于科学界和监管机构更好地了解NP动力学背后的科学，排气管到道路区域，并在这样做时提供了分别针对路边或发动机测量的研究之间的联系。

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.