Article pubs.acs.org/est
Effect of Traffic and Driving Characteristics on Morphology of Atmospheric Soot Particles at Freeway On-Ramps Swarup China,*,† Neila Salvadori,‡,§ and Claudio Mazzoleni†,‡ †
Atmospheric Sciences Program, Michigan Technological University, Houghton, Michigan 49931, United States Physics Department, Michigan Technological University, Houghton, Michigan 49931, United States § Department of Civil and Environmental Engineering, University of Trento, Italy ‡
S Supporting Information *
ABSTRACT: Vehicles represent a major source of soot in urban environments. Knowledge of the morphology and mixing of soot particles is fundamental to understand their potential health and climatic impacts. We investigate 5738 single particles collected at six different cloverleaf freeway onramps in Southern Michigan, using 2D images from scanning electron microscopy. Of those, 3364 particles are soot. We present an analysis of the morphological and mixing properties of those soot particles. The relative abundance of soot particles shows a positive association with traffic density (number of vehicles per minute). A classification of the mixing state of freshly emitted soot particles shows that most of them are bare (or thinly coated) (72%) and some are partly coated (22%). We find that the fractal dimension of soot particles (one of the most relevant morphological descriptors) varies from site to site, and increases with increasing vehicle specific power that represents the driving/engine load conditions, and with increasing percentage of vehicles older than 15 years. Our results suggest that driving conditions, and vehicle age and type have significant influence on the morphology of soot particles.
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INTRODUCTION Particles suspended in the atmosphere (aerosols) impact the climate by affecting Earth’s radiation balance, clouds properties, and atmospheric chemistry;1,2 in addition, particles reduce visibility.3 Traffic related pollution in urban areas can adversely affect human respiratory system and cause various diseases.4,5 Carbonaceous soot aggregates (often also termed as black carbon) or ns-soot (nanosphere soot)6,7 consist of many spherical monomers ranging from 20 to 50 nm in diameter composed of graphite carbon, and coated with polycyclic aromatic compounds, hydrocarbons, lubricating oil, sulfate layers, and/or inorganic materials.6,8−11 Soot particles are ubiquitous in the atmosphere12 and are generated during combustion processes such as in internal combustion engines, power plants, biomass burning13,14 and domestic heating.15 Vehicles are one of the major sources of soot in the atmosphere.16 In 2012 the International Agency for Research on Cancer (IARC) and the World Health Organization (WHO) classified diesel exhaust emissions as “carcinogenic to humans” (group 1).17 In addition, soot is an efficient light absorber and it has been suggested to be the second most important anthropogenic climate forcer with a total positive radiative forcing up to 1.1W/m2 with high uncertainty.18 Chemical composition, size, mixing, and morphology of soot particles determine their properties and their effects on the environment. For example, the mixing state and morphology of © 2014 American Chemical Society
soot affect its light scattering and absorption cross sections due to the potential enhancement of these properties upon coating6,19 or their reduction upon soot aggregate restructuring. In addition, soot morphology can significantly affect their deposition in human respiratory system.20 Due to scale invariance of soot particles, their structure can be described using fractal formalism introducing Mandelbrot’s concept of fractal dimension and following the scaling law:21 ⎛ 2R g ⎞ Df ⎟⎟ N = kg ⎜⎜ ⎝ dp ⎠
(1)
Where N is the number of monomers per aggregate, Rg is the radius of gyration, dp is the monomer diameter, kg is the fractal proportionality constant (also called fractal prefactor or structural coefficient), and Df is the mass fractal dimension. The radius of gyration is the root-mean-square distance from the center of each monomer to the aggregate center of mass. The prefactor is an important parameter as it is related to the cluster mass, atmospheric transport processing, and optical properties.22 The value of the prefactor kg, represents the level Received: Revised: Accepted: Published: 3128
November 20, 2013 February 11, 2014 February 21, 2014 February 21, 2014 dx.doi.org/10.1021/es405178n | Environ. Sci. Technol. 2014, 48, 3128−3135
Environmental Science & Technology
Article
The license plate database was matched with the Vehicle Identification Number (VIN) database of the Michigan Department of Motor Vehicle (DMV) providing information on model year, manufacturer and country, body-style, vehicle type (MOBILE 6 classification), and fuel type. Vehicle speed and acceleration were measured using two diode lasers and two detectors at opposite sides of the road, set up at about 10 cm above the road pavement. Wind speed, wind direction, relative humidity and temperature were measured using a wireless meteorological station (6162 Vantage Pro2 plus by Davis Instruments, CA). Additional information about sampling sites, experimental setup, and data collection is provided in the SI. Particle Types and Classification. Examples of different types of particles with various shapes found on the filters are shown in Figure 1. The most abundant are soot particles but we
of compactness, the smaller the prefactor value the lower the packing density for a given Df. Densely packed or compacted soot particles have higher Df than chain-like branched clusters or open soot structures.23 Df for a soot particle reflects its history and is controlled by the particle source, generating conditions, mixing, and aging processes.24 For example, Dye et al.25 studied the morphology of urban roadside and background aerosol and observed various agglomerate particles having several morphologies with and without coating. They found a relatively higher fraction of agglomerate compared to nonagglomerate particles in roadside aerosol (94%) with respect to background aerosol samples (89%). The perimeter-based fractal dimension was significantly greater at the roadside than at the background site, especially for the 120−220 nm size range. The authors suggested that the differences between roadside and background aerosols were due to inclusion of particles from other sources and generation of new agglomerates away from the roadside under dilute atmospheric conditions. Similarly, Barone and Zhu26 studied changes in atmospheric particle morphology, with increasing distance from a roadside near two major Los Angeles freeways. They found that the fraction of agglomerates was greater near a freeway than 90 m downwind, while an opposite scenario was observed for multiple-inclusions type particles which contained smaller solid and/or liquid particles inside or on the edge. Collisions of particles increase the faction of multiple-inclusions particle downwind from the freeway. The authors suggested that the decrease in the fraction of agglomerated particles away from the freeway was due to secondary aerosol formation. In this study we report morphological properties of road-side soot particles collected on 28 filters at 6 different cloverleaf freeway on-ramps in Southern Michigan. We study the morphology of soot particles using 2D projected images from scanning electron microscopy (SEM) for various on-road driving situations. In this study we investigate single soot particle characteristics to address the following questions: (1) What is the prominent type of particles and how does the particle type relate to traffic density? (2) What are the mixing states of soot? (3) What is the effect of on-road driving conditions on soot morphology? (4) Does soot morphology correlate with vehicle type and model year?
Figure 1. Examples of different particle morphologies encountered in this study: (a) Particles mixed with some fiber-like material possibly of biological nature; (b) mineral dust; (c) open-soot; (d) collapsed soot; (e) particle with multiple inclusions; (f) coated soot.
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also found several smaller spherical particles. Typically, vehiclegenerated nanoparticles (diameter
