Trend and Interrelationship of PM2.5, Gaseous Pollutants and Meteorological Factors in Kuala Terengganu, Malaysia

Outdoor air pollution has become a leading environmental health issue in low, middle – and high-income countries where WHO estimated in the year 2019 that about 4.2 million premature deaths worldwide, which caused by exposure to fine particulate matter. The exposure eventually causes cardiovascular disease, respiratory disease, and cancer [1]. Particulate matter 2.5 microns (PM_2.5) is air particulates that possess has aerodynamic diameter equal to less than 2.5mm. Its tiny size made it easier to penetrate the human respiratory system and become one major cause of mortality and morbidity in humans [2]. Previous studies found that an increment in risk of arrhythmia, atherosclerosis, hypertension, myocardial infarction (MI), stroke, thrombosis, and heart failure in susceptible individuals within several hours to days of exposure to PM_2.5 [3-6]. According to the Environmental Quality Report 2020, growing numbers of air pollutants point of sources have resulted in air pollution in Malaysia. The main sources of the air pollutants emission were from operation of power plants, industrial activities, motor vehicles and other activities such as residential, commercial, and agricultural activities [4]. Industrial activities have caused PM_2.5 to be one of the main air pollutants which violate air quality in Southern Peninsular Malaysia [5].

Othman et al. [5] emphasized that air pollution might be affected by numerous factors and variability of air pollutants also can be affected by various sources of pollution. Nonetheless, Wu and Zhang [6] highlighted that the concentration of PM_2.5 is strongly affected by the concentration of air pollutants and conditions of the meteorological parameters in a non-linear pattern. Thus, this study was conducted to investigate the trend of the particulate matter, gaseous pollutants, and meteorological factors from the year 2018 until 2022 and to investigate the relationship of PM_2.5, gaseous pollutants, and meteorological factors at Kuala Terengganu. The outcome of this study shall be to help towards a better understanding of the variation of the air pollutants concentrations and meteorological factors over time and their function and external factors that affect the concentration of PM_2.5 concentrations which may be linked to the air pollution, and sources of the air pollutants at the area. Comprehending the relationship between PM_2.5 with meteorological factors and gaseous pollutants can help prevent and diagnose related conditions, as well as drive the development of better techniques and technologies to treat PM_2.5-induced illnesses [7-9].

3.1 Trend of air pollutants and meteorological factors

Results of the descriptive analysis of PM_2.5, PM_10, SO_2, NO_2, O_3, CO, WS, RH, and AT are tabulated in Table 1. The trend of the air pollutants and meteorological factors from the year 2018 until 2022 was also illustrated in Figure 1, Figure 2, and Figure 3. Based on Table 1, the highest average concentration of PM₁₀ was 29.445 µg/m³ in the year 2019 which is higher than the concentration of PM_2.5 (20.507 µg/m³) in the same year. The maximum concentrations of PM_2.5 and PM₁₀ are 206.285 µg/m³ and 216.649 µg/m³, respectively, while the minimum concentration values of PM_2.5 and PM₁₀ are 0.08 (2022) and 265 µg/m³ (2021). Maximum value of PM_2.5 and PM₁₀ are 206.285 µg/m³ (2021) and 216.649 µg/m³ (2021). The yearly mean of PM_2.5 is exceeded from the New Malaysian Ambient Air Quality Standard (NMAAQS) which is 40 µg/m³ (PM₁₀) and 15 µg/m³ (PM_2.5). Table 1 shows only the year 2020 complied with the standard for PM_2.5 with a value of 14.726 µg/m³, while all yearly mean for PM₁₀ comply with the standards. In Table 1, results showed that the highest value of yearly mean for the concentration of the gaseous pollutant is CO (0.646 ppm) at the year 2018 followed by O₃ (0.018 ppm) (2018), NO₂ (0.006 ppm) (2018) and SO₂ (0.001 ppm) (2018-2022). The maximum concentration of SO₂ (0.007 ppm) (2019 and 2022), NO₂ (0.030 ppm) (2019), O₃ (0.690 ppm) (2018), and CO (2.658ppm) (2018) while the minimum concentration of SO₂, NO₂, and O₃ is zero and the minimum concentration of the CO is 0.073 ppm (2021).  

In Table 1, the highest yearly mean for wind speed is 1.372 m/s (2021), relative humidity (84.292%) (2022) and ambient air temperature (27.595℃) (2021). Maximum wind speed is 11.805 m/s (2020), relative humidity (99.283%) (2020) and ambient temperature (38.945℃) (2021) and minimum wind speed recorded is zero, relative humidity (44.634%) (2019) and ambient temperature (17.996℃) (2018).

Table 1. Descriptive statistics of hourly air pollutants and meteorological parameters in Kuala Terengganu from 2018-2022

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Figure 1. Trend of PM_2.5 and PM₁₀ in Kuala Terengganu

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Figure 2. (a) Trend of sulfur dioxide (SO₂); (b) Trend of nitrogen dioxide (NO₂); (c) Trend of ozone (O₃); (d) Trend of carbon monoxide (CO)

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Figure 3. (a) Trend of wind speed; (b) Trend of relative humidity; (c) Trend of ambient temperature

Figure 1 shows that there is a similar trend of change in the concentration of PM_2.5 and PM₁₀ over the period of five years. This is in line with the result of the study done by Mura et al. [8] that had identified that there is a strong positive correlation between PM_2.5 and PM₁₀. The trend of high concentrations of particulate matter between August and September 2019 and 2022 is most likely due to dry weather conditions in Malaysia as concluded by Othman et al. [5]. The overall concentration of particulate matter in 2020 was slightly lower compared to the previous years. Particulate matter (PM) emissions in Malaysia decreased from the previous year to approximately 22.6 thousand metric tons in 2020 [15]. That year saw the lowest level of PM pollution in the nation due to the Movement Control Order (MCO) enforced by the Government of Malaysia during the pandemic of COVID-19. Abdullah et al. [9] found that there was a reduction of up to 58.4% in the concentration of PM_2.5 during implementation of the MCO. Air quality has been impacted by rapid development and urbanization, which has sparked interest in researching PM_2.5 causes and impacts. While Khan et al. [16] discovered that motor vehicle emissions, secondary inorganic aerosol, and coal-fired power plants are the main sources of PM_2.5, Rusmili et al. [15] and Sinkemani et al. [17] suggested that PM_2.5 originates from fuel burning, vehicular exhaust, and certain industrial activities. According to studies [18-20], industrial and intensive commercial activities are the source of PM₁₀ and PM_2.5 as well. According to Li et al. [21], airborne dust occurrences may have played a role in the high PM_2.5 concentrations observed in Middle Eastern nations like Saudi Arabia, Kuwait, Iraq, and Iran [22, 23]. The uncontrolled burning of Indonesian forests has resulted in Southeast Asian haze occurrences, which have been connected to high concentrations of PM_2.5 in Malaysia [22-25]. These patterns demonstrate that PM_2.5 is a serious issue that must be addressed immediately with regulations and strategies to solve the issue on a worldwide scale [26-28]. As a developing nation, Malaysia must implement the measurement of PM_2.5 for its inhabitants, which requires a robust air quality monitoring system [29-31]. Incorporating continuous PM_2.5 measurement into the national environmental monitoring program is a new enhancement to the air quality monitoring network of the Malaysian Department of Environment (DOE) [19, 20, 32]. In mid-2017, PM_2.5 standards and guidelines were introduced. Compared to PM₁₀, PM_2.5 monitoring better captures the real condition of high particulate matter concentration from combustion, such as from burning biomass and vehicle emissions [24-28]. This explains why PM_2.5 has decreased somewhat in 2019 and 2020 because of MCO [9].

Figure 2(a) shows that distant peaks of the SO₂ were recorded, where the concentration of the SO₂ exceeds 0.007 ppm. Exceeds SO₂ in this study is supported by Mansor et al. [10] that concentrations of SO₂ are most likely due to industrial activities, where the combustion of sulfur compounds and fossil fuels (coals and heavy oils) and biomass burning take place. Besides, Mansor et al. [10], and Hashim et al. [11] also stated that the increment of NO₂ as shown in Figure 2(b) is usually caused by the emission of NO₂ from motor vehicles in the surrounding area. The transportation industry, the combustion of coal and gasoline and the energy sector which processes natural gas are the main contributors to the pollution of NO₂. The concentration of O₃ was at its highest peak, almost reaching 0.07 ppm in September 2018 due to dry weather and the kind of weather substantially positively correlated with the concentration of O₃ in the air as had been explained by Ramli et al. [12]. Mohd Napi et al. [13] suspected that concentrations of the O₃ were produced from the surrounding human activities such as open burning, mobile sources, and others. Literature from Mansor et al. [10] supported concentrations of CO as illustrated are believed emitted from motor vehicles as the CO is produced from the incomplete combustion of fuels.

Figure 3(a) shows a uniform trend of wind speed and there was an upward trend of the wind speed from October to December observed over the years. Upwards of wind speed between October to December is suspected due to the northeast monsoon season when the east coast area receives strong wind speed and heavy rain as described by Nizamani et al. [14]. The trend of relative humidity shows a downward trend from January to June/July and an upward trend from July/August to December. However, the trend of ambient temperature shows an upward trend from January to June/July and shows downward from July/August to December. According to the Meteorological Department, Malaysia, the highest temperature in east coast states of Malaysia is in April and May, and Terengganu receives minimum relative humidity in March [33].

3.2 Variation effect of air pollutants and meteorological factors on concentration of PM_2.5

The result of the correlation analysis is tabulated in Table 2. The result shows that there is a significant strong positive correlation between PM_2.5 with PM₁₀ (r = 0.900, p < 0.05), which is similar to the result of a study done by He and Lu [34], where the concentration of PM_2.5 with PM₁₀ within same monitoring network were strongly correlated with each other. Besides, PM_2.5 is also significantly strongly correlated with CO (r = 0.500, p< 0.05). Ucheje et al. [35] explained the contribution of CO to the concentration of PM_2.5 is most likely due to the surrounding traffic volume [29, 32], which is supported by a statement from a study done by Roslan et al. [36] that CO is emitted into the ambient air from incomplete combustion when there is partial oxidation of carbon in the fuel. Meanwhile, there is no correlation between PM_2.5 with O₃ but CO and O₃ has high correlation (r= 0.949, p<0.05).

Table 2. Correlation analysis between air pollutants and meteorological parameters

* Correlation is significant at the 0.05 level (2-tailed).

** Correlation is significant at the 0.01 level (2-tailed).

Results of the analysis also indicate that there is a significant weak correlation between the PM_2.5 with wind speed (r = -0.035, p <0.05), and relative humidity (r = -0.020, p < 0.05). The direction of the correlation of the variables is similar to the result of analysis done by Amnuaylojaroen et al. [37] in north Thailand where PM_2.5 is positively correlated with temperature and negatively correlated with wind speed and relative humidity. As cited by Nguyen et al. [38], complicated interactions between the variables are the main challenge to deepening understanding of the effect of the meteorological factors on concentration of the PM_2.5.

Anthropogenic emissions from industries, vehicles, power plants, burning biomass, and other sources are primarily responsible for atmospheric PM_2.5. One of the main sources of PM_2.5 is coal burning, especially in Asia during the wet season [26, 27]. But as more and more cars are driven through metropolitan areas of megacities, their contribution to air pollution, including NOx and particulates, is growing [26]. Furthermore, during the post-harvest months of May to June and October to November, burning biomass is a significant source of PM_2.5 [28]. Pollution episodes are often brought on by the build-up of human contaminants in sluggish weather conditions. Furthermore, the gas-to-particle conversions of CO, NOx, and VOCs also play a significant role in the generation of PM_2.5 pollution, especially in conditions of high relative humidity. Approximately 77% of the total mass of PM_2.5 is composed of the secondary inorganic species $\mathrm{NO}_3^{-}, \mathrm{NH}_4^{+}$and $\mathrm{SO}_4^{2-}$ and this showed the relationship between PM_2.5 with CO in this study [26-29].