The effects of compact urban development on air pollution

Abstract

The influence of the compact city on sustainability is still not sufficiently verified. And especially there remains a room for debate on the relationship between the compact city and air pollution. Researchers, both for and against the compact city are hampered by the absence of cogent reasons to support the effects of changes in density on air pollution because they have not been taking into account ambient air pollution path which pollutants are dispersed or diluted in the air. This study aims to investigate the air pollutant concentration distribution brought about by cumulative of emission sources empirically and to explore the effects of urban compactness on air quality, along with an aspect of dispersion and thus to guide sustainable urban development from the perspective of the atmospheric environment. One of the weak points of evaluating the effects of urban compactness on air pollution in planning literature is that it is still to approach short-sighted way. When taking into consideration that environmental damage from urban development is long-term and cumulative, air pollution problems should be analyzed as a time-series approach. A model accounting for intra- and inter-regional characteristics is required since changes in urban characteristics make a difference for air pollution and the scope of influence varies by time and spatial variability. Unique and unobservable properties of urban need to be employed as well because the advances in air pollution technologies and policies for air pollution mitigation can potentially influence air pollution in the mid- to long-term. The panel data model allows for optimal modeling results not only by regulating estimate errors that arise from the time-series process and regional unit data but also by giving proper treatment to omitted unobservable variables that have a significant effect on air pollution difference econometrically. This study attempted to identify that high-density development causes the spatial concentration of emission sources, which may result in increase of air pollution. The distribution of pollutant concentrations according to the distance from the CBD was conducted at the local level, at the metropolitan level, and at the interurban level. The results showed that the concentration distribution of air pollutants was high or low when getting closer to the CBD. In certain cases there are pollutants that had an even distribution depending on the distance from the CBD. PM10 emissions were concentrated in the CBD at the local level, while the opposite was true with PM10 at the metropolitan level. The distribution of O3 concentrations was low in the CBD at the local and metropolitan level, while NO2 and CO concentration values appeared high at the metropolitan level. With regards to the spatial distribution of pollution levels at the interurban level, the distribution of CO concentrations appeared significantly higher closer to the CBD but NO2 concentrations had a low or an even distribution in the CBD. The pollution level distribution of PM10 was low in the CBD only from 2006 to 2008. As a result, it cannot be determined that air pollution is aggravated by the spatial concentration of emission sources, suggesting that the pollution levels are influenced by the extent and magnitude of dispersion, which may vary according to urban characteristics and the diverse conditions they exist in. Therefore, there is a need to differentiate whether the emission sources are concentrated at the local or regional level and to establish air pollution control strategies appropriate for such conditions. The main concern of this study was to investigate the effects of urban compactness on air pollution by controlling for factors which affect air quality. The definition of urban compactness here encompasses the high-density built form with a proportion of green land within a standard spatial unit. Two meanings involved in this definition were presented as follows. High-density brings the spatial concentration of emission sources, which lead to an increase in pollution, while green land secured from high-density development can encourage dispersion and dilution, resulting in a reduction of air pollution. The relative magnitudes of opposing effects determine air quality. The panel data model showed that NO2 and CO concentrations significantly increased with a rise in net density, while SO2 and CO decreased with increase in proportion of green land and more importantly, green land was relatively more effective at decreasing CO compared to SO2. Although the results are confined to only certain pollutants, urban compactness had two dimensions to air pollution. The dispersion and dilution of pollutants may hold the answer. The results suggest that high-density developments that secure enough green land can enlarge dispersion and contribute to reduced pollution levels. Meanwhile, PM10 and O3 emission values were irrelevant to urban compactness and PM10 increased with population growth, implying that PM10 has a potential to increase as the urban size increases. This study may offer a clue to the debate on whether the compact city improves air quality or not. A number of studies have not considered that air pollution concentrations are determined by the dispersion and dilution process varying with regard to time and space. Therefore, urban air pollution problems may not only require an understanding of spatial and temporal differences in urban characteristics but a comprehension of the dispersion mechanism, which undergoes complex diffusion in the atmosphere. It is imperative to develop an integrated management system, which minimizes local-to citywide emissions and thus regulates total urban emissions. It is essential to set historical emission trends as it can be used to guide appropriate antipollution measures. Preferential controls and optional management strategies need to be followed to respond to changes in the pollution levels, especially the maximum concentration at a certain period.

Key words: urban compactness, high-density development, green land, air pollution, dispersion, sustainability