## 5 Results and Discussion  
### Descriptive Statistics  
The descriptive statistics of the measured variables that are used in SVR and RF models for all stations in Malaysia are summarized in Table 1. The columnar AOD\({}_{500}\) over the Malaysian sites during 2018-2019 exhibits a mean of 0.69, which is above the median value 0.46 due to many episodic aerosol events with AODs above 2, representing thick smoke plumes from extensive fires in Indonesia and Indochina [10; 133; 10]. The measured PM\({}_{2.5}\) concentrations at the 65 examined sites follow a similar distribution with a higher mean (21.9 \(\upmu\)g m\({}^{-3}\)) than median (17.1 \(\upmu\)g m\({}^{-3}\)) and a maximum value of 230 \(\upmu\)g m\({}^{-3}\) (Table 1, Figure 3). These PM\({}_{2.5}\) levels are similar to those reported at several sites in Southeast Asia [86,134]. NO\({}_{2}\) and CO exhibit means of 5.2 ppb and 0.6 ppm, respectively, while tropospheric O\({}_{3}\) levels (25.2 ppb) are considered rather high with deleterious effects on human health [135,136]. The box-whisker plots for AOD\({}_{500}\) and air pollutants, separated for urban/industrial and suburban/rural sites and for the four seasons, are shown in Figure 3. The analysis showed that the seasonal-mean PM\({}_{2.5}\) at the urban/industrial sites displayed higher values in all seasons. During the dry season, the maximum PM\({}_{2.5}\) levels were found to be 230.3 \(\upmu\)g m\({}^{-3}\) for the urban/industrial and 226.3 \(\upmu\)g m\({}^{-3}\) for the suburban sites, with \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **PM\({}_{2.5}\)** & **AOD** & **SO\({}_{2}\)** & **NO\({}_{2}\)** & **O\({}_{3}\)** & **CO** & **WS** & **RH** & **TEMP** \\  & **(\(\upmu\)g m\({}^{-3}\))** & **ADD** & **(\(\upmu\)g)** & **(\(\upmu\)g)** & **(\(\upmu\)g)** & **(\(\upmu\)g)** & **(\(\upmu\)g)** & **(\(\upmu\)g)** & **(\(\upmu\)g)** \\ \hline Mean & 21.86 & 0.69 & 1.2 & 5.23 & 25.2 & 0.60 & 1.73 & 66.58 & 30.70 \\ \hline Median & 17.07 & 0.46 & 1.0 & 3.81 & 27.3 & 0.56 & 1.60 & 66.85 & 30.88 \\ \hline Stdev & 19.15 & 0.68 & 0.9 & 6.1 & 15.1 & 0.28 & 1.02 & 10.35 & 2.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Statistical values for the measured parameters in all air-pollution monitoring sites. Figure 3: Box plots of measured PM\({}_{2.5}\) concentrations (**a**) Himawari-8 AOD\({}_{500}\) (**b**), CO (**c**), NO\({}_{2}\) (**d**) and O\({}_{3}\) (**e**) at urban/industrial and suburban/rural sites in Malaysia in the dry, wet and inter-monsoon seasons. means of 31.26 \(\upmu\)g m\({}^{-3}\) and 26.38 \(\upmu\)g m\({}^{-3}\), respectively. This is attributed to the prevailing southwest wind carrying biomass-burning aerosols from Indonesia due to extensive forest fires in this season [90]. However, the seasonal mean PM\({}_{2.5}\) concentrations do not notably differ in the other seasons (lying between 17.80 \(\upmu\)g m\({}^{-3}\) and 22.33 \(\upmu\)g m\({}^{-3}\) for both urban/industrial and suburban/rural sites), indicating a year-long PM\({}_{2.5}\) laden atmosphere across Malaysia. Regarding the Himawari-8 AOD, it is higher during the dry season, while slightly lower mean values (\(\sim\)0.6 to 0.8) were observed in the other seasons, with marginal differences between urban/industrial and suburban/rural sites (Figure 3b). The seasonal and site variations of the columnar AODs are in agreement with ground PM\({}_{2.5}\) concentrations, indicating that the industrial and traffic emissions are the main pollution sources for urban centres and surrounding areas. In general, heavy precipitation during the wet season only marginally reduced the aerosol levels since severe pollution episodes with PM\({}_{2.5}\) > 100 \(\upmu\)g m\({}^{-3}\) and AODs > 3 were also present. However, the columnar AOD displayed a very low correlation (R\({}^{2}\) = 0.09) with the surface PM\({}_{2.5}\), indicating (i) a significant aerosol loading aloft and (ii) different sources and temporal variability between surface PM\({}_{2.5}\) and AODs [137].