Energy and Environment Research Laboratory

Near-Road Air Pollution Research at EERL

Populations near roads are exposed to a mixture of traffic-related primary and secondary pollutants. Approximately 30–45% of urban populations in the United States are likely exposed to elevated pollution levels near major roadways. In many countries with densely populated urban areas, this figure is likely higher. Emission reduction programs implemented by government agencies throughout the world have significantly reduced emission rates of air pollutants from motor vehicles. In spite of these reductions, motor vehicles still significantly contribute to pollution in urban areas, often due to large increases in vehicle use offsetting per vehicle emission reductions.

Comprehensive Turbulent Aerosol Dynamics and Gas Chemistry Model (CTAG)

Our modeling framework is called Comprehensive Turbulent Aerosol Dynamics and Gas Chemistry (CTAG), a computational fluid dynamics (CFD)-based environmental turbulent reacting flow model designed to simulate the transport and transformation of multiple air pollutants on and near roadways. Figure 1 describes the structure and the components in CTAG. A brief introduction is given below. On the pollutant transport side, CTAG is designed to capture the major turbulentmixing processes in the roadway environments: vehicle-induced turbulence (VIT) and road-induced turbulence (RIT) and atmospheric boundary layer turbulence (ABLT). RIT includes turbulence due to roadway configuration, road surface properties and roadside structures. On the transformation side, currently CTAG has incorporated NOx chemistry and aerosol processes such as nucleation, condensation/evaporation, and coagulation. Additional chemical mechanisms such as detailed photochemical reactions of volatile organic carbon VOC) can be added on. The on-road and near-road simulations are linked by implements a multi-scale structure in CTAG.

In addition to numerical simulations, researchers at EERL also conducted field measurements charactering the air pollution gradients near the intersection of I-81 and I-690 in downtown Syracuse, and the Impact of local traffic exclusion on near-road air quality in New York City.

Figure 1: CTAG framework and components

CTAG implements a multi-scale structure (Figure 2) based upon the mechanistic roadway air quality modeling framework proposed by Zhang and Wexler (2004). CTAG resolves aerosol dynamics and gas-phase chemistry in the on-road domain (i.e., “tailpipe-to-road”) and near-road domain (i.e., “road-to-ambient”), respectively. The turbulent mixing process on the on-road domain is dominated by VIT, and is dominated by RIT and ABLT for the near-road domain. CTAG first resolves the plume dynamics behind individual vehicles, and the subsequent interactions of multiple plumes in the on-road domain. The processed on-road emissions will serve as inputs to the near-road domain simulations, where CTAG resolves the dynamics of roadway plumes in the roadside micro-environments. The processed roadway plume profiles on the near-road domain can be used as inputs to regional-scale air quality simulations. This multi-scale structure enables CTAG to serve as an advanced analytical tool for a variety of applications.

Figure 2: The multi-scale structure in CTAG

An advantage of the CTAG model is the capability to generate 3-D, size-resolved particle concentrations in the micro-environment. CTAG is capable of quantifying the contributions of each highway to a given receptor location on the domain. In the intersection micro-environment, I-490 and I-590 contribute to most of the particles (above 95%) because of their large traffic volumes. The contour of particle number concentrations in the near-road domain on 11/27/2006 is shown in Figure 5, representing the spatial variations of particulate pollution. The highest on-road particle concentration was found on the roadway of I-590 under parallel wind condition. (226.1°), shown as Circle 1 in Figure 5. The parallel wind leads to accumulation of tailpipe emissions on the highway. High particle concentrations can also occur at the locations that had insufficient ventilation, i.e., surrounding the embankment where I-490 intersects with I-590 (Circle 2). A flow recirculation zone formed behind and beneath the embankment of the intersection, caused the flow recirculation and therefore hampered the dispersion of pollutants. The presence of vegetation canopy in this intersection micro-environment allowed the examination of its effects on CO and particle concentrations. Figure 5 indicates that, when the vegetation canopy is on the upwind side of I-590, it decreased the ambient wind velocity, hindering it from dispersing pollutants from the roadway (Circle 3). Therefore, the vegetation barrier on the upwind side of the roadway might exacerbate particulate air pollution. In contrast, if the vegetation canopy was on the downwind side, it tended to mitigate the particulate air pollution. We investigated the scenario on 07/23/2005 to explain this phenomenon, described as follows.

Figure 3: Contour plot of Aerosol particle number concentration at an intersection in Rochester

Highway Building Environment

Highway-building environments are prevalent in metropolitan areas. We employ and improve the Comprehensive Turbulent Aerosol Dynamics and Gas Chemistry (CTAG) model to simulate the spatial variations of black carbon (BC) concentrations near highway I-87 and an urban school in the South Bronx, New York. The results of CTAG simulations are evaluated against and agree adequately with the measurements of wind speed, wind directions and BC concentrations. Our analysis suggests that the BC concentration at the measurement point of the urban school could decrease by 43-54% if roadside buildings were absent. Furthermore, we characterize two generalized conditions in a highway-building environment, i.e., highway-building canyon and highway viaduct-building. The former refers to the canyon between solid highway embankment and roadside buildings, where the spatial profiles of BC depend on the equivalent canyon aspect ratio and flow recirculation. The latter refers to the area between a highway viaduct (i.e., elevated highway with open space underneath) and roadside buildings, where strong flow recirculation is absent and the spatial profiles of BC are determined by the relative heights of the highway and buildings. The two configurations may occur at different locations or in the same location with different wind directions when highway geometry is complex. Our study demonstrates the importance of incorporating highway-building interaction into the assessment of human exposure to near-road air pollution. It also calls for active roles of building and highway designs in mitigating near-road exposure of urban population.
Graph Figure 4: Highway-building Environment in South Bronx, NY

Vegetation Modeling

Vegetation in urban environments can create complicated flow patterns which are difficult to take into account for many models. However, the potentially large effect on air quality due to vegetation necessitates accurate modeling of this common urban feature. The CTAG model is able to incorporate the various effects such as aerodynamic resistance and particle and gas deposition. The model has been validated by comparison with experimental data from a field study in Chapel Hill, NC (Figure 5). In the future, simulation and experiment will be performed to examine the best ways to use roadside structures to reduce pollutant levels in urban communities. Graph Figure 5: Vegetation site in Chapel Hill, NC