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Characterizing Potential Urban Street Canyons in Downtown Vancouver

Abstract

Urban street canyon formation has become a growing concern over the past decade due to the prevalence of high-rise, high density residential and commercial development in the downtown core. Since this phenomenon poses significant implications on health, the purpose of our study is to identify the areas in downtown Vancouver where street canyons are most likely to occur. By combining the effects of aspect ratio and wind direction, our results indicate that there is high potential for street canyon formation along the vast majority of streets in our study area, with some streets having multiple ideal locations. Three locations with the highest potential of formation were identified using multi-criteria analysis and hot spot analysis.

Background

To understand the importance of correctly identifying urban street canyons, we must first recognize the implications that this modeling will have on the interplay between urban air pollution and its associated health effects.

Air pollution has been a significant environmental and health concern for centuries. This exposure is widespread and important for all populations since it is unavoidable. The Global Burden of Disease 2010 estimated that 3.1, 3.5, and 0.2 million deaths occurred annually as a result of exposures to ambient particulate matter, household solid fuels and ambient ozone pollution, respectively. (1) With rapid urbanization of the world population, air quality is anticipated to be on the decline as sources of pollution aggregate. In highly populated cities, energy consumption and human activities (e.g. power generation and vehicle use) must rise to keep up with the demands of growing regions.

In a dense urban environment, such as downtown Vancouver (our chosen study area), the main contribution to spatially varying pollutants is traffic-related air pollution (TRAP). These spatial gradients are largely explained by road traffic density and intensity. Motor vehicle emissions are produced in combustion processes leading to the formation of nitrogen oxides and carbon monoxide. Sulfur dioxides can arise from fossil fuel combustion in industrial processes. In metropolitan areas, non-exhaust emissions are possible from mechanical abrasion (a source of coarse particulate matter) of brakes, tires and road surfaces. (2) An important secondary pollutant that contributes largely to climate change that is formed in the photochemical reaction between volatile organic compounds (VOCs) and nitrogen monoxide (NO) is tropospheric ground level ozone (O3).

Traditional air quality monitoring networks are the groundwork for understanding pollution trends (temporal and spatial patterns), compliance evaluations, health effects research and assessment of air quality management programs. Currently, the networks in place not only measure limited surrogate air pollutants, but are also limited in capturing important neighborhood-scale spatial patterns, despite having fine temporal resolution. To supplement these discrete monitoring sites, a number of smaller and more portable devices have been used to capture pollutant variability in 2-dimensions.

Detailed spatial information has important implications for health - for example, numerous studies have reported association between TRAP and birth outcomes (low birth weight and pre-term births), (3) cardiovascular effects, (4) childhood asthma and respiratory disease (bronchiolitis and otitis media). (5-8)

Because various adverse health effects have been linked to TRAP recently, there is a push to better resolve pollution gradients related to traffic sources. Two-dimensional models fail to include the vertical gradients that exists in air pollution. Although previous models may be sufficient for estimating exposures of occupants in small residential buildings where single households reside, errors and uncertainty in approximating personal exposures for individuals living in high-rise residential developments may be considerable.


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Overview of Street Canyons

Aspect ratios are determined by the ratio of building heights to street width. The likelihood of street canyon formation increases with aspect ratio; with 0.7 being the level beyond which we can assume there is a risk of pollutant accumulation - as relatively stagnant air is present near the base of the canyon. Within the street canyon, air recirculation is poor. (9)

Prevailing wind direction also plays a critical role in street canyon formation. If wind direction runs perpendicular to the length of the street, the likelihood of there being a canyon increases as the airflow above building tops is unable to adequately exchange with air trapped within the canyons. Therefore, aspect ratio and wind direction are taken into account in our multi-criteria analysis of potential urban street canyons.


Our Project

The aim of this project is to develop a simple 3-dimensional model using readily available geospatial information to assist in the identification of potential street canyons within a section of downtown Vancouver.

Eventually, this model will be modified to cover the entire downtown area so a few street canyon locations (and some non-canyon locations) can be selected for vertical dispersion sampling. Small autonomous battery-powered air quality monitors will be used to measure air pollution along a vertical transect to derive vertical profiles for air pollutants and test the validity of the proposed model. These equations can later be used in parallel with existing land use regression models to assign exposures to Vancouver residents living at a range of heights.


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