Industrialization and urbanization are the main reasons to burn more fossil fuels in developing countries like Iran to face with a high rate of air pollution (Atkinson et al., 2012; Fathollahi-Fard et al., 2020a). There is no doubt that this high rate is the greatest environmental hazard to mankind health (Barwise and Kumar, 2020). Moreover, it damages the people’s health and is associated to the cardiopulmonary, morbidity and mortality (Calderón-Garcidueñas et al., 2002; Pope and Dockery, 2006). This motivates the concept of green city as it can contribute to a healthier air and air pollution removal (Nowak and Heisler, 2010; Nowak et al., 2006).
As an introduction to the green city, recognition of vegetation characteristics and their relationships to environmental impacts are two of the several factors leading to a great reduction in the air pollutions (Klingberg et al., 2017; Magee et al., 2008; Xing and Brimblecombe, 2019). It goes without saying that urbanization and industrialization are two main green space and ecosystems (Wan et al., 2015; White and Greer, 2006; Zhou et al., 2016; Fathollahi-Fard et al., 2020b). Therefore, they have a direct impact on the life quality in large cities like Mashhad in Iran. Since vegetation and urban greening absorb sun radiations and affecting heat islands, they are able to reduce air pollution and to clean the air (Dimoudi and Nikolopoulou, 2003; Perini and Magliocco, 2014; Susca et al., 2011, Yu et al., 2021). This highlights the need of detection of change values and direction to determine their effects on human lives (Pettorelli et al., 2005). Satellite imagery and remote sensing technology provide the data on a region past. They present many changes which can be detected and compared to determination of changes and their effect on environment (Gao et al., 2020; Mojtahedi et al., 2021).
Vegetation indexes are of important algorithms which are able to extract canopy conditions by means of remote sensing (Salas and Henebry, 2014). Reflective spectrum of sun radiation is used to measure vegetation conditions, as some wavelength are adsorbed and others are reflected (Berger et al., 2019). The most common used index in this research area is the Normalized Difference Vegetation Index (NDVI) (Ren et al., 2018). This metric is an important spatial indicator of vegetation quality (Thenkabail and Lyon, 2016) and is derived from the Red and near-infrared reflectance caused by leaves that is associated to canopy greenness (Brantley et al., 2011). The NDVI is linearly correlated to the canopy and is related to the vegetation photosynthesis and energy adsorption (García-Gómez and Maestre, 2011). The NDVI is sensitive to soil properties and may be influenced in sparse vegetation and dark backgrounds like dry sandy soils (Fern et al., 2018).
In order to diminish NDVI deficiency, there are some other metrics to improve the evaluation. One of them is the Enhanced Vegetation Index (EVI) which was developed to improve NDVI by increasing sensitivity to canopy variation and reducing atmospheric and soil reflectance impacts (Huete et al., 2002). As indicated in the literature review, the EVI is used in most crop-mapping studies (Wardlow and Egbert, 2010). Another metric is the Optimized Soil-Adjusted Vegetation Index (OSAVI) which was developed to decrease the background effect. This metric is an extension to the Soil-Adjusted Vegetation Index (SAVI) and Transformed Soil-Adjusted Vegetation Index (TSAVI). One merit of OSAVI in comparison with SAVI and TSAVI is that it is simpler and doesn’t require prior knowledge about soil (Rondeaux et al., 1996; Steven, 1998). In addition, it has been found that it is more efficient than NDVI in reduction of background effects of soil type and to further determine the vegetation (Fern et al., 2018; Liu et al., 2012). However, the EVI is more efficient to reduce the aerosols disorder (Liu et al., 2012).
To study the relevant studies in the implementation of green city concept using aforementioned metrics, one of the earliest studies is Rafiee et al., (2009) who investigated the green space changes and patterns of Mashhad city in Iran from 1987 to 2006. Most notably, they used the satellite imagery. Next year in another study, Richardson and Mitchell (2010) evaluated the relationship between green lands and the people’s health via considering their gender in the United Kingdom using an ecological approach. Following, the implementation of green city in Toronto, Ontario in Canada was studied by Villeneuve et al., (2012). They calculated the mortality rate and the green space relationship using satellite imagery and NDVI metric.
Selmi et al., (2016) estimated the air removal role of trees in Strasbourg city in France. They used the i-Tree Eco model in this regard. Xing and Brimblecombe (2019) analyzed the role of green spaces on the air pollution distribution along parks using computational fluid dynamics. De Carvalho and Szlafsztein (2019) evaluated the vegetation loss and its impact on the air quality and air pollution using NDVI. At last but not least, Jaafari et al., (2020) evaluated the green lands effect on the air pollution diminishment and mortality rate of respiratory diseases in Tehran, Iran. They applied the structural equation modeling and the partial least squares method to the green city concept.
With regards to aforementioned literature and to the best of our knowledge, although the role of urban vegetation on air quality and pollution was repeatedly studied, no study contributed the concept of green city correlation to the clean, healthy and unhealthy days. This study aims to fill this research gap. All in all, the main contributions are summarized as below:
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This study aims to investigate the green space area and its changes in Mashahd, Iran during 2013 to 2019.
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The clean, healthy and unhealthy days are contributed and studied for the first time.
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This paper studies the relationship between clean, healthy and unhealthy days with green space area.
The rest of this paper is organized as follows: Sect. 2 provides the materials and methods for this research including the case study, the research methodology, the logic of metrics and statistical calculations. Section 3 does the computations and discussed the results. Finally, Sect. 4 is the summary of this study with findings and recommendations.