## 2 Literature Review  
### Air Pollution Studies  
Many studies have been conducted on air pollution in the cities of Saudi Arabia. Meanwhile, studies also indicate that air pollution leads to human health problems, such as respiratory diseases and heart diseases. They have also discussed the sources of pollution and methods for assessing and improving air quality. The authors of [15] studied the relationship between pollution parameters and atmospheric factors, including temperature and wind speed, in the city of Al-Qurayvat in the Kingdom of Saudi Arabia, and satellite data were used instead of observed data from ground stations to detect the concentration rate of air pollutants, including NO\({}_{2}\), SO\({}_{2}\), and CO. During the data collection period, this region experienced frequent dust storms, which were the primary contributors to the rising levels of environmental pollutants [16]. These storms posed significant health risks and were linked to numerous fatalities each year [17]. The World Health Organization reported that Riyadh and Jubail rank among the ten most polluted cities globally, holding the fourth and fifth positions, respectively. Their study also examined the relationship between air pollutants and atmospheric parameters, revealing a negative correlation between air pollutants, temperature, and wind speed. On the other hand, relative humidity demonstrated a positive correlation with air pollutants. The study attributed the high concentration of air pollutants primarily to traffic emissions and the burning of fossil fuels for electricity generation. The authors believe that the results of this study are useful for decision-makers, helping them to assess the levels of air pollutant concentrations in the study area. In [18], the authors discussed the air quality parameters in Makkah, Madinah, and Jeddah by analyzing the atmospheric parameters, including temperature and relative humidity, and their relationship with air pollution from June to September 2019 and 2020. While air quality is generally affected during the Haji season, during 2020, it showed a decrease in pollutant concentrations, and the COVID-19 pandemic had a positive impact due to the reduction of human activities [19]. In [20], the researchers performed current and future air quality analysis in Riyadh using the Air Quality Index. Six major air pollutants were considered, namely NO\({}_{2}\), SO\({}_{2}\), PM, O\({}_{3}\), CO, and CO\({}_{2}\). The air quality in Al Riyadh was compared to the local, regional, and international levels. Factors such as seasons and working hours were considered. The main sources of pollutants were identified as industrial emissions, fuel combustion, and wind-borne dust. Overall, the air quality components were below the required levels. Of all the pollutants studied, PM particles posed the greatest threat to human health in this city.  
As mentioned previously, Al Riyadh and Al Jubail have exceptionally high levels of air pollution, which is expected due to the high population and urbanization of Al Riyadh and the high industrial activities in Al Jubail. In this study, we aimed to focus more on these two cities and compare them to an agricultural city, Najran, which has a low population density. Moreover, we wanted to avoid the period of the COVID-19 pandemic, as its effect has been studied extensively in many places around the world. Instead, we were more concerned with studying the effect of the normal activities of these three cities, all of which are undergoing continuous development to achieve the goals of Vision 2030. Hence, our study aimed to collect data from these three cities for two years, 2022 and 2023, to facilitate the comparison of all months during these two years, which allows for studying the effect of different weather factors on these three cities with different characteristics.