Search results

(1 - 2 of 2)
CHARACTERIZATION OF DISPERSION AND ULTRAFINE PARTICLE EMISSION FACTORS USING NEAR ROADWAY FIELD MEASUREMENTS
Title
CHARACTERIZATION OF DISPERSION AND ULTRAFINE PARTICLE EMISSION FACTORS USING NEAR ROADWAY FIELD MEASUREMENTS
Creator
Xiang, Sheng
Date
2019
Description
Recent epidemiology evidence suggests that vehicle emissions are major contributors to poor urban air quality. Human exposure to elevated...
Show moreRecent epidemiology evidence suggests that vehicle emissions are major contributors to poor urban air quality. Human exposure to elevated concentration of traffic emissions has been associated with increased risk factors for a range of negative health outcomes. Evaluation of human exposure to vehicle emissions (e.g. ultrafine particles) mainly relies on dispersion models. Consequently, dispersion models need to comply with constantly increasing requirements to provide predictions of pollutant concentration. The dynamic of near roadway dispersion process needs to be investigated since most of the existing models does not account traffic condition variability (e.g. vehicle type and mode of operation) for dispersion. A five-year long field study was conducted to characterize dispersion near roadway with various vehicle mode of operation and vehicle type. To better understand the dispersion process near roadway, the impact of different ambient background categories (e.g. remote, lake, urban, industrial) on ultrafine particles (UFPs) need to be evaluated. Results demonstrate that each category has a different average ambient background concentration (pt cm–3) as follows: remote, 2,700; lake, 6,000; industrial 12,000 and urban 11,000. The large variations exist in ambient background concentration will result in significant variations in near roadway concentrations. The total near roadway measurements are generally near 20,000 pt cm–3 and reach to 60,000 pt cm–3 depending on the background and traffic emission. The dispersion near the roadway is also investigated in this study. A roadway restricted to light-duty vehicles (LDVs) was selected to conducted near roadway field measurement. Results indicate that the dispersion induced by vehicles is a two-stage process. When under the unsteady-state condition with small number of operating vehicles, the rate of dispersion near roadway increased from 2 m2 s-1 to 6 m2 s-1 as the number of vehicles increased. For steady-state condition, the rate of dispersion was constant near 6 m2 s-1 and not increased with additional vehicles. For a roadway mixed with both LDVs and heavy duty vehicles (HDVs), similar results were found. Dispersion increased from 6 to 18 m2 s-1 as total vehicle flow rate increased to 10,000 veh h-1 and HDV flow rate increased to 1000 veh h-1. Finally, the calculated dispersion near roadway is used to estimate the UFP emission factors. The UFP emission factors were ranged from 0.5 × 1013 to 1.5 × 1013 pt km-1 veh-1 and from 7 × 1014 to 20 × 1014 pt km-1 veh-1 for LDVs and HDVs, respectively. The variations in UFP emission factors are due to change in vehicle mode of operation.The results from this study will be critical for parameterization of dispersion near roadway and provide important emission inventory for interdisciplinary partnership among different fields (e.g. air quality, transportation design and urban planning) in solving transportation air quality problem.
Show less
Parameterization of vertical dispersion coefficient (σz) near roadway: vehicle wake, density and types
Title
Parameterization of vertical dispersion coefficient (σz) near roadway: vehicle wake, density and types
Creator
Yu, Yu-Ting
Date
2020
Description
Pollutants emitted by motor vehicles is one of the major public concern in modern society since pollutants (e.g. ultrafine particles and CO2)...
Show morePollutants emitted by motor vehicles is one of the major public concern in modern society since pollutants (e.g. ultrafine particles and CO2) have been related to adverse health effect and climate change. Currently, air quality dispersion models (e.g. CALINE4 and AERMOD) are widely used to predict concentration near roadway. However, both air quality dispersion models are not account for the vehicle variability (vehicle types), causing the inaccuracy in predicting pollutant concentration near roadway. To better understand the dynamic of dispersive process, it is important to evaluate the variation of vertical dispersion coefficient (σz) for different vehicle types (e.g. LDV and HDV). A “wake area model” is developed based on “effective wake area” and vehicle density to reveal the dynamic of dispersion process that related to vehicles. Results from this thesis demonstrate that the near roadway σz that calculated from field measurements increases as vehicle density increases and HDV can generate higher value of σz compare to LDV. σz increase from 1.7 m to 4 m with LDV density increase from 0.005 m-1 to 0.18 m-1; while σz starts at 2 m and reach up to 6 m with HDV density increase from 0.001 m-1 to 0.01 m-1. The significant differences of σz are due to vehicle density and vehicle types. The effective wake area contributed by one HDV (397 m2) is much larger than effective wake area contributed by one LDV (13 m2), which indicates that one HDV contributes 31 times higher than one LDV in terms of near roadway σz. The significant differences of “effective wake area” are due to vehicle wake structure and vehicle dimension.Compared to the air quality dispersion models results, both CALINE4 and AERMOD failed to respond the variation of σz as vehicle density increases for LDVs and HDVs. CALINE4 shows the value of σz is near 2.8 m and 2.4 m for LDV and for vehicle fleet (mixture of LDV and HDV), respectively. AERMOD shows the value of σz is near 1.2 m and 1.3 m for LDV and for vehicle fleet, respectively. Because both air quality dispersion models did not correctly predict σz near roadway, a new equation that parameterize σz near roadway need to be developed to better predict the σz in real world condition. For a given road segment, the parameterization of σz is a result from dimension of “effective wake area” for different vehicle types and number of “effective wake area” for a vehicle fleet. Comparison of measured and parameterized σz near roadway dispersion coefficients show good agreement (slope=0.99, R2=0.85) with a range between 1 and 4 m for LDVs and 2 to 6 m for HDVs. Also, Comparison of measured and simulated CO2 show good agreement (slope=1, R2=0.90) with a range between 0.01 g m-3 to 0.12 g m-3 for simulated value and 0.01 g m-3 to 0.14 g m-3 for measured value. For UFPs, the results also show good agreement (slope=1.14, R2=0.71) with a range between 600 pt cm-3 to 11,000 pt cm-3 for simulated value and 600 pt cm-3 to 17,000 pt cm-3 for measured value.The “wake area model” provides a new perspective for σz that related to vehicles and bridges vehicle wake to σz. The results from this study will be critical for improving predictions of pollutant concentration from air quality dispersion near roadway.
Show less