1. Introduction
Ambient air pollution is one of the biggest environmental threats to public health, resulting in around 4.2 million global deaths yearly [
1,
2]. Rapid urbanization and swift industrialization are boosting the global economy, resulting in the cost of environmental pollution [
3,
4]. Infrastructural damage to ecological imbalance is happening at an alarming rate because of uncontrolled air pollution worldwide, especially in South and East Asian cities. Besides, air pollution is accused of a significant amount and economic cost in developing countries [
4,
5]. Furthermore, air pollution is also the fifth leading risk factor for mortality worldwide, accounting for more deaths than many better-known risk factors such as malnutrition, drug addiction, and obesity [
6]. The average air quality index is very alarming in some major cities of Bangladesh [
7,
8,
9]. The air pollution level in Dhaka and its adjacent areas are very severe as it is ranked second in the world’s most polluted cities [
10,
11,
12]. Dhaka is also considered one of the most polluted urban cities in the world, where 82 µg/m
3 annual average PM
2.5 concentration from a wide variety of pollution sources [
13,
14,
15].
PM
2.5 (particulate matter aerodynamic diameter less than 2.5 μm) is one of the major air pollutants in the city area, which is a significant threat to human health and all living organisms [
16,
17]. It is revealed that the key reasons for this upsetting air quality in Dhaka and its adjacent areas are mainly unplanned urbanization, industrialization, and motorization. A large share (almost 58% of total PM
2.5) of Dhaka’s air pollutants is covered by the brick kiln operated in and around Dhaka and also followed by motor vehicles (10.4%), road dust (7.70%), fugitive Pb (7.63%), soil dust (7.57%), biomass burning (7.37%), and sea salt (1.33%) [
7]. Furthermore, the fuel used by brick kilns operating in this area is mainly coal, while wood is used as a secondary fuel which ultimately contributes to the concentration of almost two third of PM
2.5 found in the air of Dhaka [
7,
18,
19]. However, western countries suggest reducing the level of PM
2.5 concentration on a both daily and annual basis [
20]. In contrast, developing countries like Bangladesh still emit higher levels of PM
2.5 concentration in the atmosphere. Moreover, every year more than 1.59 billion US dollars, equivalent to 134 billion Bangladeshi takas the cost of the capital alone in terms of loss of human health and life [
21].
Many researchers have completed research on the relationship between PM
2.5 and land use. [
22] conducted a sampling-based study to determine the atmospheric PM
2.5 concentration in the Gazipur and Mymensingh districts in Bangladesh, where they found an increased level of pollutants in February 2019 because of different factors such as industrial activities, vehicular emissions, construction, and others. The study’s main limitation was that it used a small number of sample points that did not represent the whole study area. [
23] conducted a spatiotemporal analysis of PM
2.5 concentration and quantified the relationship between vegetation cover and air pollution in greater Dhaka, Bangladesh. Their results showed that the winter season experienced the highest concentration of PM
2.5, and the amount increased over time. His studies revealed that vegetation cover and PM
2.5 concentration strongly correlate negatively (
r = -0.75). The lack of proper land use information and the limited number of sample points did not give an appropriate relationship, which is the opposite of our paper. On the other hand, [
24] concluded research that found that artificial surfaces and desert land have positive effects on PM
2.5 concentration, while forest, grassland, and barren land have negative effects on PM
2.5 concentration.
Climatic variables have an important role in assessing PM
2.5 in rural and urban areas. [
25] conducted research on the relationship between PM
2.5 and seasonal meteorological factors in Dhaka, Bangladesh, where they found that rainfall and temperature had a negative association with PM
2.5. Rainfall was also negative in Dhaka [
11]. Long-term PM
2.5 links with temperature, surface pressure, and relative humidity were studied by [
19] in Dhaka, Bangladesh using temporal air pollutant data from 2003-2019. Their results show that Pearson’s correlations were significantly associated with surface pressure and relative humidity, while there was a positive correlation with surface temperature. Their key findings also revealed that vehicular emissions, road dust, soil dust, biomass burning, and industrial emissions contributed to PM
2.5. Temperature, wind speed, and wind direction significantly predict PM
2.5 in Dhaka, Bangladesh. [
26] completed research to investigate the statistical relationship between PM
2.5 and temperature, wind speed, and wind direction. They found that these factors accounted for 94% of the total variability.
Based on the literature review above, most of the studies used a limited number of sample points of PM2.5 with a few climatic variables. In addition, most of the research used small geographic areas. As a result, the relationship between PM2.5 with land use and several climatic variables in larger geographic areas is still unknown. To fill this knowledge gap, this paper has conducted this study using a series of multi-date PM2.5 data, land use, and eight climatic variables in large geographic areas (6,043 square kilometers). Finally, this paper aims to investigate the relationship between PM2.5 and land use and climatic variables and to identify the riskiest areas and population groups using Geographic Information Systems and statistical analyses.
5. Discussion
Estimating the spatiotemporal concentration of PM
2.5 is a critical issue for local and regional atmospheric pollution research and public health concern. This study used a set of PM
2.5 concentration data to map a hotspot area and analyze the statistical relationship between land use and eight climatic variables. In addition, the derived PM
2.5 data was used to find out the most affected people and areas. Due to a similar urbanization pattern between China and Bangladesh, the average PM
2.5 value in 2021 was 82 μg/m
3 and 77 μg/m
3 in China and Bangladesh, respectively. In Bangladesh and its mega-cities, about 35% of ambient PM
10 and 15% of PM
2.5 are generated from brick kiln emissions and transportation systems [
8,
44,
45]. Even emissions from diverse kinds of diesel and petrol vehicles and poorly maintained automobiles are generating air pollution due to PM
2.5 pollutants in urban areas of Bangladesh [
46,
47]. The concentration of PM
2.5 in the atmosphere depends on several anthropogenic factors such as transportation (vehicle movements), industrial (manufacturing plants and mining), cooking and heating activities [
48], and some meteorological factors like wind speed, air relative humidity, cloud cover, and ambient temperature [
49]. The result of this study revealed that the areas, i.e., Dhaka, Narayanganj, and Gazipur districts have more anthropogenic sources like manufacturing factories, high traffic congestion, and other combustion activities, ultimately leading these districts with relatively higher annual PM
2.5 concentration, which is similar with the PM
2.5 concentration in India, Tanzania, and Iran [
50,
51,
52]. In contrast, the other two study areas, Narshingdi and Munshiganj, have a relatively lower level of PM
2.5 concentration regarding their pollution sources than in Europe [
53]. However, the incorporation of meteorological factors and seasonal variations could give more precise information about the concentration of PM
2.5 fluctuation instead of depending on annual average concentration, which could sometimes be misleading in describing short-term anthropogenic activities or weather conditions [
54].
Land use has an important role in changing the nature and pattern of PM
2.5. This paper has explored that the highest level of PM
2.5 concentration and their annual pattern has been increased over barren lands, forests, cropland, and urban areas between 2002-2021 because of urbanization, huge construction sites, road networks, industrial activities, agricultural practices, huge traffic movement, impervious surface, and permeable pavement. The relationship between PM
2.5 and different land use patterns is complex, comprehensive, and dynamic. [
22] mentioned that vehicle emissions, brick kilns emissions, and industrial smoke are the main key factors for environmental problems and public health risks, particularly PM
2.5 pollution in Ghazipur and Mymensingh districts in Bangladesh. [
55] also indicated that the dominant factor affecting PM
2.5 pollution was the traffic condition found using a land use regression (LUR) model and statistical analysis to explore the effect of land use on PM
2.5 pollution in the Nanchang urban area, China. Urban areas are more vulnerable to atmospheric inversion, which may trap different air pollutants close to the ground and increase their density or concentration over time. The combination of these factors, the high population density, and their energy consumption are the vital triggering factors for influencing PM
2.5 in many ways. On the other hand, forest/vegetation can play a crucial role in producing and reducing PM
2.5 in the local atmosphere. Some specific trees or vegetation can directly absorb PM
2.5 and other particulate matter, even if they filter the air naturally by releasing good air. Often trees and vegetation reduce wind direction which can help the circulation of PM
2.5 from one area to another. [
23] mentioned that the vegetation cover and PM
2.5 concentration have a strong negative correlation (r
2 = -0.75). It means that the higher vegetation will reduce the level of PM
2.5 concentration in Bangladesh. This is also observed by [
56] that the forest experienced PM
2.5 of 35–50 μgm
−3 (lower than other land cover types), likely due to the potential filtering and absorption function of the forests and vegetation.
The dispersion and transportation of PM
2.5 are affected by local, and regional climatic factors. The local and regional climatic factors such as air pressure, air temperature, evaporation, ground heat, humidity, rainfall, water vapor, and wind speed have a daily, monthly, and annual contribution in reducing or increasing PM
2.5. [
26] mentioned that wind speed and direction did not significantly influence PM
2.5, although other wind parameters have the highest variability, which is opposite to our paper. Our paper found that wind speed has a positive correlation (r2=0.34) while air pressure has a negative (r2=-0.24) correlation. [
25] found that the Pearson correlation coefficient (r) between the PM
2.5 and meteorological variables was negative with rainfall (r2=- 0.62) and humidity r2= (- 0.82) but positive with wind speed (r2=0.09) and temperature (r2=- 0.73) in Dhaka, Bangladesh. In addition, a Pearson correlation revealed a significant association among the pollutants, while a significant correlation was observed between PM
2.5 and surface temperature, which is similar to our paper’s result. [
19] mentioned that surface temperature is signified because of vehicular emissions, road/soil dust, biomass burning, and industrial emissions in Dhaka, Bangladesh. [
57] also argued that meteorology parameters such as temperature, relative humidity (RH), and precipitation are important predictors for PM
2.5 variability all over the USA.
The higher concentration of PM
2.5 and its adverse effects on urban communities and inhabitants are exposed as a common public health problem in Bangladesh. Most public health concerns are pulmonary, cardiovascular, cancer, diabetics, chronic respiratory, low birth weight, and premature death [
58]. In this study, a huge number of populations ages 0-5 (1,948,029) and 50-69 (485,407) are at risk due to the higher level of PM
2.5. In China, 341,701 and 67,325 premature deaths were recorded due to stroke and lower respiratory infection, respectively [
59]. Even about 25 million populations are at air pollution risk in Delhi, India, due to different human, societal, developmental, and industrial reasons [
60]. These reasons are identified as similar problems for this study area too.