Characterization of on-road vehicle emissions and air quality in densely-built environment

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Characterization of on-road vehicle emissions and air quality in densely-built environment


Author: Lau, Cheuk-yin Jason
Title: Characterization of on-road vehicle emissions and air quality in densely-built environment
Degree: Ph.D.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Automobiles -- Motors -- Exhaust gas
Automobiles -- Pollution control devices -- Measurement
Department: Dept. of Civil and Structural Engineering
Pages: vii, 189 leaves : ill. ; 30 cm.
InnoPac Record:
Abstract: Vehicle emission is one of the major contributors of gaseous pollutants such as carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC) in the urban area. This work examines on-road gaseous emissions from vehicles of different vehicle and fuel types in order to gain a better understanding of emissions from vehicles operating under urban driving conditions and their effect on air quality in a city with a high density of high-rise buildings. The work described in this thesis has made four important contributions to the current understanding of vehicle gaseous emissions and their effects on the adjacent urban environment: 1. fostered a better understanding of the current methods of developing vehicle emission factors through a thorough review and critique exercise; 2. fostered a better understanding of the vehicle emission characteristics in relation to engine operating modes and developed instantaneous vehicle emission factors through intensive on-road emission measurements of four taxis and four light goods vehicles with a sophisticated portable emission measurement system (PEMS); 3. developed initial co-relations between two instantaneous vehicle emission data sets one collected from on-road PEMS and the other from roadside remote sensing (RS) system so as to improve the confidence level of using the RS system in monitoring on-road vehicle emissions; and 4. discovered long memory effects of vehicle emissions in the microenvironment of the urban areas. Measurement of on-road gaseous emissions from taxis and light goods vehicles were carried out using a portable emission measurement system (PEMS). This investigation was carried out to gain a better understanding of on-road emission characteristics of LPG taxis and diesel light goods vehicles (LGVs) under urban driving conditions. Emission indices and rates exhibit similar patterns in relation to speed while emission factors are inversely related to vehicle speed. Test vehicles emit lower amounts of gaseous pollutants when idling and higher levels of gaseous pollutants when undergoing hard acceleration. Emission level of one taxi is extraordinarily high when travelling at low speed. This is likely due to malfunction of control of fuel supply to the engine. Gaseous emissions from LGVs equipped with a catalytic converter are lower than those without one. Emissions from LGVs not equipped with a catalytic converter appear to depend mainly on engine size and power. Over 250,000 vehicles emission measurements recorded by remote sensing techniques between 2007 and 2008 are examined to gain a better understanding of emission characteristics of the vehicle fleet in Hong Kong. Older model vehicles emit more gaseous pollutants compared to newer vehicles, possibly due to the cleaner engines of newer vehicles. Emissions from vehicles of the same vehicle type operating on different types of fuel are compared so as to examine fuel effects on gaseous emissions from vehicles. Light-duty diesel vehicles produce higher amounts of NO and lower amounts of CO and HC compared to both petrol and LPG vehicles, while LPG vehicles emit higher levels of gaseous pollutants than petrol vehicles. Older LPG commercial vehicles produce higher levels of gaseous emissions compared to non-commercial petrol and diesel vehicles of similar age, likely due to deterioration from the vehicles' extended use. Emissions from light-duty vehicles, fuelled by various fuel types, under a number of speed and acceleration modes are examined. The results show that emission factors and vehicle speed are inversely related. High levels of gaseous pollutants are emitted when vehicles undergo heavy acceleration.
Emission factors derived from different measurement methods often vary greatly due to factors such as traffic conditions and emission variability of individual vehicles. To bridge the discrepancy between emission factors derived from different measurement methods, emission factors of LPG taxis and diesel light goods vehicles derived from on-road and remote sensing measurements are compared. The results show that emission factors are generally higher when measurements are collected on-board. This discrepancy is likely owing to the dilution and reaction of vehicle emission in the atmosphere. By establishing correlation with emission factors derived from on-road (PEMS) method that provides accurate measurements, capabilities of roadside remote sensing method which measures emissions from a large number of vehicles can be improved. The effect vehicle emission has on air quality in different regions within a city is examined by comparing variations in traffic volume and air quality in built-up areas within Hong Kong. This comparison reveals that daily patterns in air quality at various parts of the city follow variations in traffic volume closely, indicating that vehicle emission has a strong effect on air quality in built-up areas. The effect vehicle emission has on urban air quality is illustrated by studying the correlation between air quality measurements at roadside and background monitoring stations, with the aid of two statistical techniques - principal component analysis and cluster analysis. The results show that vehicle emissions strongly influence variations in NO₂ concentration in the urban area and its effect on PM₁₀ is smaller, due to the presence of other urban sources. The discrepancies in the station clusters identified by the two methods indicate that variations in urban background air quality depend largely on local sources. Examination of variations in air quality in relation to variations in traffic indicates that dispersion of vehicle emission is hindered by tall buildings within the urban area. Vehicle emission's impact on urban air quality is also greatly affected by the level of development within the city, which can influence the effectiveness of the dispersion process. In addition, traffic conditions in the urban area can further contribute to deterioration of urban air quality, as frequent stops and acceleration results in higher levels of vehicle emissions.

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