2.1. Green Investment and Air Quality
Most researchers consider green investments as a type of investment that raises environmental standards and considers social responsibility (Eyraud et al. 2013; Karásek and Pavlica 2016). Xu et al. (2017), on the other hand, for an investment to be considered a green investment, it must reduce the proportion of fossil fuels in total energy consumption. In this context, green investments will support the transition to renewable energy, and this will help reduce the rate of carbon released to the environment. When the literature is examined; Wan and Sheng (2022), Chen and Ma (2021), Shen et al. (2021), Musibau et al. (2021), and Ren et al. (2020) have determined that green investments have a significant contribution to reducing carbon emissions. Despite that; Zahan and Shuai (2021), Hammoudeh et al. (2020), and Charfeddine and Kahia (2019), on the other hand, could not detect a significant relationship between green finance investments and carbon emissions. Nevertheless, the hypothesis regarding the assumption that green investments will have a positive effect on carbon emissions is set up as follows;
H: There is a statistically positive relationship between green investments and air quality.
2.2. Economic Growth and Air Quality
The relationship between carbon emissions and economic growth can sometimes be confusing. The Environmental Kuznets Curve (EKC) hypothesis explains this confusion as follows: The curve showing the carbon emission on the graph moves up until it reaches a certain income threshold, while it moves down after the threshold is crossed, forming an inverted “u” shaped graph (Arouri et al.,2012). In other words, while increasing income increases carbon emissions in the short term, it decreases them in the long term. The relationship between economic growth and carbon emissions has been studied extensively, especially in the last 40 years. However, the results of empirical studies have been controversial, showing contradictory effects. For this reason, a definite policy cannot be suggested for its implementation between countries (Acaravcı and Öztürk, 2010). When the relevant literature is examined; Wang et al. (2012), Wang et al. (2013), Paul and Bhattacharya (2004), Elif et al. (2011), Acaravcı and Öztürk (2010), Al-mulali et al. (2013) have found that economic growth increases carbon emissions. Heidari et al. (2015), Narayan and Pop (2012), and Jaunky (2010) found that while economic growth increases carbon emissions in the short run, as the EKC hypothesis suggests, it decreases it in the long run. Despite that; Kalimeris et al. (2014), Bruns et al (2014), Hossain (2011), and Soytaş et al. (2009) concluded that there is no clear relationship between economic growth and carbon emissions.
2.3. Urbanization and Air Quality
Considering that carbon emissions have a positive relationship with energy consumption (Ramzan et al. 2021), it can be said that urbanization will increase energy consumption and energy consumption will increase CO2 emissions. In other words, as urbanization increases, more energy will be consumed and carbon emissions will increase accordingly. In this case, it can be expected that there will be a positive relationship between urbanization and carbon emissions. Reviewing the literature, Zambrano-Monserrate et al. (2018), Nasreen et al. (2017), Rahman (2017), Alama et al. (2016), Karakaş (2016), Bozkurt and Okumuş (2015), Ohlan (2015), Onafowora and Owoye (2014), Prose and Rehman (2011), etc. They concluded that there is a positive relationship between these two variables. In this context, it is expected that urbanization will increase carbon emissions and this situation will decrease air quality.
2.4. Energy consumption and Air Quality
In parallel with the progress of modern industrial civilization and the excessive increase in production, energy consumption has increased considerably. This situation has brought about the use of energy sources such as fossil fuels that are harmful to the environment and increase CO2 emissions. This situation emerges as a major obstacle in achieving the goal of a sustainable world. Based on the fact that energy consumption increases CO2 emissions, the use of renewable energy sources and ensuring sustainability have become the common goal of countries (Lia et al. 2011). When the literature is examined for the relationship between energy consumption and CO2 emission; According to Ramzan et al. (2021), Adedoin et al. (2021), and Mahalik et al. (2021). Cetin and Yuksel (2018), Sun et al. (2017), Bautabba (2014), Shahbaz et al. (2013), Hamit-Haggar (2012), and Pao et al. (2011) found in their studies that energy consumption increases carbon emissions. In contrast, Adebayi et al. (2022), Kırıkkale and Adebayo (2021), Ding et al. (2021), Ibrahim and Ajide (2021), Yuping et al. (2021), Umar et al. (2020) found a negative relationship between renewable energy use and CO2 emissions in their studies. In this case, if an evaluation is made by looking at the literature, it is seen that the use of non-renewable energy increases carbon emissions, while the use of renewable energy sources reduces carbon emissions. In this context, it is expected that the increase in energy consumption did not decrease the air quality.
2.5. Trade Openness and Air Quality
Since the beginning of the 2000s, whether there is a relationship between trade openness and environmental pollution has become a debated and researched the issue. This discussion emerged after the positive relationship between trade openness and economic growth was understood (Shahbaz et al. 2017). The biggest factor in the impact of trade openness on the environment is the hypothesis called a pollution haven. In short, the pollution haven hypothesis is that large or high-polluting companies invest in countries where environmental regulations are not strictly enforced and make large-scale production there, resulting in huge CO2 emissions (Dauda et al., 2021). When the literature is reviewed, Sarkodie and Strezov (2019), Liu et al. (2018), and Chai et al. (2018) determined the correctness of the pollution haven hypothesis in their studies and showed that countries with trade openness increase CO2 emissions. Besides, Chen et al (2019), Balsalobre-Lorente et al. (2018), and Shahbaz et al. (2017) have shown in their studies that trade openness negatively affects the environment. According to Essandoh et al (2020), trade openness reduces CO2 emissions in developed countries while increasing them in less developed countries. According to Wang and Zang (2021), trade openness reduces carbon emissions in high-income countries. Kim et al. According to (2019), while southern trade reduces carbon emissions, northern trade increases. Wang and Wang (2021) found that trade openness reduces carbon emissions in high-income and low-middle-income groups, but increases it in the upper-middle-income group. In contrast, Kearsley and Riddel (2010), Ang (2007), and Soytas et al. (2007) could not conclude whether trade openness has an environmental impact or not.