Title: Physicochemical evolution of air pollutants from traffic emissions to roadside in complex built environments
Authors: Rakowska, Agata Barbara (0)
Abstract: Background: Even though, the Environmental Protection Department reports the negative trend in local pollution the air quality remains poor. The majority of air pollutants originate from motor vehicles, which combined with the high population and vehicle densities in Hong Kong, has great influence on the public exposure. This exposure may result in acute health problems, respiratory and cardiovascular diseases and increased mortality rates. Additionally, such impacts may be amplified by the complex built environment that tends to place the public close to the emission sources and heavily polluted zones. Hong Kong is built on a small area near a busy harbor right up to a mountainous terrain, with a busy roadway network. Street canyons are common with high skyscrapers, narrow paved roadways and sparse greenery. The vibrant lifestyle of the city finds citizens visiting streets with selling stalls, open-door shops and restaurants which enable the vehicle generated pollutants to penetrate indoor air and affect people. In order to better understand public exposure to these air pollutants, it is critical to quantify their physicochemical characteristics in specific microenvironments in urban settings. However, the regulatory monitoring network has very limited coverage of the highly heterogeneous spatial profile of air pollutants, and it also fails to capture the processes that occur after emissions enter the urban atmosphere.
Objectives: The main objectives of the dissertation investigations were:
1. to show how urban built environment influences the distribution of air pollution;
2. to understand the physical and chemical processes that occur between the source emission and ambient environment;
3. to find out how different fleets have different contribution to local air pollution conditions.
Based on the objectives, the investigation included two hypotheses:
1. Difference in built environment can affect distribution of traffic-related pollution concentrations;
2. Prioritization of the fleet based traffic control can effectively improve roadside air quality in urban settings.
Approaches: The investigations presented in the dissertation have three integrated components.
First, a novel vehicle plume chasing approach was used to characterize the primary emissions from various vehicle fleets in Hong Kong. Both gaseous and particle emissions were normalized by carbon mass balance method and statistically analyzed by vehicle fleet, model year and emission standards to determine the nature of contributors to the roadside air pollutants.
Second, a field campaign was carried out in an urban street canyon representing typical built environment with urban settings to understand the relative contribution of primary emissions from different vehicle fleets to roadside air quality. In this approach, a comparative investigation was done on primary and secondary air pollutants in on-road and roadside environments to understand the processes that may occur during dispersion from tailpipe emission to ambient in street canyon.
Third, a monitoring campaign was carried out in the vicinity of an open highway representing the built environment with free dispersion commonly found in urban roadway network. The measurements were performed both on-road and roadside to capture the transformation of pollutant concentrations and their physicochemical characteristics during dispersion from the source. The investigation was designed to establish the profile of primary and secondary pollutants with different distances from the highway and to compare this with the profile for urban street canyon scenario, while the highway and to compare this with the profile for urban street canyon scenario, while The investigations were carried out using a mobile monitoring platform equipped with multiple analyzers to provide high resolution gas and particle pollutants measurement and capture the temporal and spatial variations during their atmospheric processing. This approach was augmented by portable packs of monitors designed to capture the physicochemical characteristics of pollutants near roadways.
Results and outcomes:
The findings from on-road vehicle chasing protocols demonstrated that the emission from diesel vehicles depends strongly on vehicle type and also upon maintenance condition. The trend of reduction of emissions from diesel franchised buses appeared to be in line with government regulations and had higher NOx emission factors (EF) than diesel goods vehicles but much lower black carbon and PM2.5 emission factors. One noteworthy finding was that 16% of the largest emitters of black carbon contributed to 50% of the total emissions. This pattern was different for NOx. Consequently, the inspection/maintenance programs should focus much more on BC than on NOx emission. In general the results suggest that franchised buses had much better bus service and maintenance.
The link between the fleets and emission observed during on-road chasing study was reflected and reinforced by emission measurements in city busy street canyon conditions. The on-road measurements with traffic counts and classification clearly demonstrated source related impacts on roadside air quality. They suggested that diesel vehicles (especially buses) are responsible for high concentration of black carbon and ultrafine particles on certain streets of city center in Hong Kong. The highly bus-trafficked particular street had extreme 'hot spots' with a resultant potential for very high exposure to the pedestrians that have activities along the streets. Additionally, the effect of reduced pollution dispersion in a street canyon with trapped high concentrations of pollutants enabled the formation of secondary pollutants.
The concurrent monitoring of on-road and roadside conditions in the city street canyons indicated two particle pollutants, roadside black carbon and ultrafine particles correlated very well with on-road results. Higher ultrafine levels were found in the roadside studies, especially, indicated accumulation of primary emission and also suggested secondary PM formation.
By contrast in ambient/rural conditions dispersion occurred much faster with pollutants' concentrations decreasing rapidly within few hundred meters from the source. The dispersion process was highly influenced by wind conditions with increased concentrations for downwind cases. The PAH/BC ratio decreased with the distance while UFP/BC ratio increased suggesting the presence of evaporation and nucleation process, respectively.
Long term and wider impact: One of the novelties of these investigations lie in the new monitoring approaches adopted that may be complemental to traditional stationary air monitoring since it provides the limited temporal and spatial coverage. The application of the mobile platform enabled to study the atmospheric processes. It demonstrates an efficient means to observe the distribution of air pollutants in urban traffic impacted locations. This study gives an initial picture of how complex urban environment can affect the processing of traffic related pollutants in the atmosphere and result in the formation of secondary pollutants during the physical and chemical transformations. Therefore, the distribution of air pollution poses new challenges for the air monitoring and public health protection.
Additionally, complex urban built environments that inhibit and facilitate certain processes which play an important role in overall picture. The key conclusion is that regulatory-based fixed air monitoring might need additional novel measurement approaches in order to capture concentration gradients or to estimate the population exposure to traffic emissions in built urban environment.
The disproportionality of the contribution from different fleets identified the need for novel transport policy that would control the traffic composition in the critical areas of the cities.
Since it is known that the urban design of those 'hot spots' increases the emission concentration and human exposure the special attention should be drawn by policy makers to limit the threat they pose to public. The new transport design depending on city areas may significantly improve the roadside air quality.
Finally, knowing the high concentration of the emission along the roads and highways Finally, knowing the high concentration of the emission along the roads and highways exposure especially in those areas. Such attention should be drawn to the bus and mini bus stops conveniently positioned close to kerbside in direct vicinity of busy roads. The people's exposure to pollutants as well as the consequences are not yet well examined and need further in depth examination. The results may have great influence on bus stops design and city design in general.
Notes: CityU Call Number: TD886.5 .R34 2015; xvii, 118 pages : illustrations (some color) 30 cm; Thesis (Ph.D.)--City University of Hong Kong, 2015.; Includes bibliographical references (pages 110-118)
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