The interest in smellscapes increased during the last decade among urbanism scholars with the employment of advanced technologies and various analysis methods. The improvement of multisensory perception in urban spaced led to the significance of sensory perceptions other than visual. Although the smell is not visible, it is a significant factor in urban spaces since individuals perceive and react to olfactory elements. Urban areas are no perceived only as a built environment but based on the impact of sensory and climatic events on these spaces (Thibaud, 2014). Smellscapes were initially defined by Porteus (Porteous, 2016) to emphasize the relationship between the space and memory, and in following years, Drobnick (Drobnick, 2010) discussed smellscapes within the concept of place. Smellspaces were described as an environment shaped by experience and behavior (Henshaw, 2011; Kabat-Zinn, 2005), as affected by individual’s experiences (Xiao J, 2018), as a part of life (McLean, 2019), and as an environment formed by an object-based and temporal perception process (Young, 2020).
Although it is not expressed significantly with this term in urban spaces, smellscape is mentioned in planning and design stages as preferred and non-preferred smells. However, the mentioned smells in these stages are often limited to plant and/or sewage, waste and industrial zone smells. There are various smells induced by diverse activities in urban areas. Urban smells induced by various occupancy types include domestic waste smells in residential areas, sewage smell due to wastewater and solid and liquid waste, emission and waste smells in industrial zones, solid and liquid waste facility smells, smells dispersed by restaurants, ovens, etc., vegetables, fruit and animal smells in commercial zones such as markets. These smell sources are observed in all cities, regions and countries, but smell type could vary based the source (Muallim Mubina Auwal, 2019). Similarly, the Environmental Protection Agency (EPA) classified the smell sources as industrial sources such as sewage, treatment plant, animal processing plant, slaughterhouse, landfill, and composting facilities, and local smell sources such as store and restaurant smells. It could be observed that various smell sources affect human behavior in urban spaces. In land use analysis, which is a significant component of urban planning, it was demonstrated that smell is a factor in the preference of residential areas based on their relationship with industrial zones (Batalhone, 2002), and the preference of clean and non-noisy routes that are far away from intense traffic and without exhaust emissions by bicycle riders (Gössling et al., 2019; Simpson, 2018). Quercia et al.(2016) emphasized that urban administrators do not consider olfactory opportunities fully and consider smell a negative factor; however, smellscapes could be developed by controlling the air flow at transportation stops and pedestrian streets with plant arrangements. Various smell control applications have been developed in urban areas and Henshaw(2011) listed these applications as masking, removal, separation and odorization. In these studies, removal aims to completely remove the smell and is different from the other techniques and generally used to remove exhaust smell and pedestrianization is recommended. Other control studies focused on the promotion of other smell sources that would mask or isolate the undesired smells.
A smell is characterized by factors such as climate conditions, density, duration, volume and volatility, and temperature fluctuations, wind speed, relative humidity and/or atmospheric pressure significantly affect the character and the perception of the smell (Badach et al., 2018). The wind significantly affects the smell cloud dissipated at the smell source (Le et al., 2015). There is an inverse proportion between the wind speed and the size and permanence of the smell cloud. The smell is dispersed to a larger area at high wind speeds, while the intensity of the smell is lower (Ayan Çeven, 2020; Lin et al., 2006; Song & Wu, 2022). The wind transports and disperses the smell based on the wind direction (Conchou et al., 2019). In a smell perception experiment conducted during a walk, it was reported that the wind dispersed the smell and smell was not percieved (Bouchard Natalie, 2013). The temperature affects the dispersion of the smell. Since temperature leads to convection, the smell moves at a higher rate in the air based on the temperature increase. Thus, in warm weather (22.5oC or above), the smell is dispersed at a higher rate. Furthermore, the smell could stay at ground level based on the temperature (Lin et al., 2006) In an indoors cigarette smoke study, the smell intensity increased at 25.5oC (Cain et al., 1983). Cold and freezing temperatures reduce the strength of smells. Snow cover and freezing temperatures reduce smell perception. In a smell emission study conducted with pig manure, it was determined that a reduction in temperature led to a decrease in smell dispersion [15]. Another environmental factor, humidity changes smell intensity. As humidity increases, smell intensity increases (Bottcher, 2001). In other words, humidity emphasizes the presence of smells. For example, the smell of wet soil after the rain blurs the smell of minerals and emphasizes plant smells (Le et al., 2015). Bouchard (Bouchard Natalie, 2013) reported that humidity emphasizes smells and Cain et al. (Cain et al., 1983) stated that 70% humidity increases the intensity of the smell. As the humidity rate increases, the air becomes heavier, increasing the perception of the smell (Bouchard Natalie, 2013). The perception of the smell could be improved with humidification (Pearce, 2006) based on water/temperature/evaporation relationship and formation of humid environments (Bouchard Natalie, 2013).
The main aim of the present study was to determine the integration of the variations in smell sources in urban landscape based on climatic and spatial factors that influence urban experiences and preferences with landscape planning and design processes and the spatial constructs of smell sources that could be emphasized in the whole urban center. These analyses would allow the development of smell maps with human and mechanical smell measurements and employ these maps in smell dispersion models and land use analysis. Furthermore, it should also be remembered that new touristic routes could be developed based on the smell maps, which in turn could contribute to urban tourism.
Smell Dispersion Modeling
Dispersion models are particularly employed in air studies to mediate certain regulations. In air quality studies, these models interpret short and long-term atmospheric movements to develop future projections. Dispersion models are employed to analyze the correlations between human health and air pollution based on the analysis of PM10 and NO2 that affect air quality based on traffic density (Gulliver & Briggs, 2011). These dispersion models support air quality monitoring networks, allowing the development of temporal and spatial contexts (Munir et al., 2020).
The discomfort due to smell increases due to urban land use and the number of analyses conducted on related regulations has increased in the last decade (Nicell, 2009). Several studies have been conducted on smell measurement and analysis methods and standardization (Wojnarowska et al., 2021). Dispersion models have been developed with various software to determine the environmental effects of undesired smells particularly generated in urban industrial zones and the smell sources measured with various techniques and the distance between smell source and residential areas (Wu et al., 2019). The most common software is the AERMOD, which is employed in air quality models developed by the Environmental Protection Agency. These models are employed to determine national smell effect criteria for smell-based discomfort (Wu et al., 2019). Another software, CALPUFF employs the Gaussian cloud model (Fallah-Shorshani et al., 2017). Both AERMOD and CALPUFF software are complex systems based on topographic and climatic factors and used in the United States and Europe (Daly & Zannetti, 2007). Another computer software, Along with basic data management and mapping tools, ArcGIS includes analytical techniques that have long served as a foundation for GIS. The spatial statistics toolbox includes a set of methods for describing and modeling spatial information. Besides that, they expand to assess spatial pattern, processing, trends, distributions, and relationships. The methodology of the study was applied in three steps; Inverse distance weighted (IDW) geostatistical technical analysis, creation and validation of regression models. IDW Inverse distance weighting (IDW) is the most common operation in GIS. IDW is a special interpolator, so its statistics values are well thought out (Jumaah vd., 2019).
GIS produces smell dispersion maps based on statistical interpolation formulas using smell source measurements obtained with inverse distance weighed (IDW) and Kriging methods (Ramirez & Intarakosit, 2007; Sówka et al., 2017; Wojnarowska et al., 2021). Sόwka et al, (2017) reported that a higher number pf measurement points would lead to a more accurate interpolation based on the number of points and the area size. GIS provides more data processing options when compared to AERMOD (Maantay et al., 2009). Thus, dispersion was modeled with GIS software that integrates the area bases in the study.