Examining the effects of local urban morphological features on air quality

Abstract

Air quality has become a significant issue of concern in recent years, especially in developing countries such as India. Some of our tier-I cities, like Delhi, Mumbai, Chennai, and Bangalore, are already dealing with the effects. According to various literature reviews, the primary reasons for degrading air quality are the city's spatial structure and function (socioeconomic activities). Rapid urbanization, industrialization, increased congestion, vehicle ownership, reduced transit capacity, and a lack of effective pollution control policies all contribute. Second, as these cities rise in size, commute distances increase, causing increased road congestion. As a result, the city significantly contributes to the degradation of air quality. This is commonly referred to as city-centric pollution. To conclude, a city's spatial structure impacts anthropogenic activities and, therefore, a significant source of urban air pollution, which defines cities as the central focus. Present programs in India include the “National Clean Air Program (NCAP),” “Forty-Two Action Points,” the “National Air Quality Monitoring Program (NAMP),” and others. Recently, tier II and III cities have come under scrutiny, and a list of 122 non-attainment cities is compiled where Maharashtra's cities predominate. Existing literature specifies two ways to deal with urban pollution; one is by environmental engineering where the source apportionment for the city is done, and the quantification and monitoring of pollutant are done and the second way is the study of dispersion, which only a few papers have focused upon, where it is believed that any pollutant is released from its source is carried away by the wind environment. If the wind speeds are good enough, the dilution of these pollutants will be maximum. This research "examining the effects of local urban morphological features on air quality" is a research hypothesis that focuses more on the "dispersion" aspect in coping with air pollution. The relationship comes out as the urban microclimate, particularly the wind environment, is the primary link between urban air pollution and urban form. The quality of urban space directly influences the health and comfort of people. The wind condition, which is affected by the urban microclimate, determines the dispersion of air pollutants. The key factors influencing the urban wind climate are the urban structure and density. This unique spatial form gives and unique wind environment within the city. By identifying the city's urban morphological features that are liable for air quality deterioration, this study tests the significance. Lastly, it suggests some recommendations for mitigation to counter urban-centric pollution strategies at a local spatial scale. The study area for the research is Nagpur because it comes under non-attainment criteria, and source apportionment for the city has not yet been accomplished. The study was conducted on two levels. The first level focuses on the city level analysis, where the baseline scenario about air quality and urban morphology was established, and questions such as: - What are the pollution levels, what pollutant exceeds more than the “NAAQS” levels, what season the pollution situation is worst, and what are the sources of pollution (Anthropogenic) were addressed. The urban form was also articulated at the city level, defining the shape, size, compactness ratio, and sprawl. Thus, a city-level analysis gave a brief overview of the city and pollution. Different levels of concentration within the city were now evaluated to trickle down on the local spatial scale. These concentration levels further were comprehended concerning densities and local climate zones. The urban densities provided information about the population's exposure to various pollution levels, while local climate zones concentrate on the physical structure and are used for urban climatic studies. The microclimate is essential for assessing the air pollution scenario because it defines the relationship between the built structure and the wind environment. The second part of the study now deals with the local levels, where four different criteria about pollution, people, and urban form were established, namely the High Pollution-High Density, High Pollution-Low Density, Low Pollution-High Density, and Low Pollution-Low Density, where the sources of pollution were marked, and the urban structure was quantified based on three indicators, namely, Canyon aspect ratio, Building volume density, and Plan area fraction, and lastly, The simulations were carried out to explain the trapping of wind between the morphology depicting the trapping of air pollution, thus preventing dispersion. It was found through correlations and regressions that the higher the values of urban form indicators, the lower the ventilation conditions. The tools used to map current pollution levels are kriging and inverse distance weightage, which provided details on the distribution of emissions within the city for various pollutants. The urban morphology tool, Metropolitan Form Analysis toolbox, and many similar spatial analysis tools compatible with ArcGIS were used to quantify urban structure, which talks about the scale, shape, sprawl, fragmentation, street network, socioeconomic characteristics, and so on, at a city level, and for dispersion, Autodesk Flow design was used. Finally, the hypothesis that “air quality is affected by urban morphological parameters at local spatial scale” was tested for significance using statistical and computational fluid dynamics analysis, which indicates several suggestions aimed at reducing the blind expansion of urban land and optimizing urban morphological features. It also suggests that when designing the street layout, the microclimate, especially wind speed and direction, be considered.