## 4 Results and Discussion
### Data Summary  
In the Zhongli dataset, moderate is the most frequent AQI level in any given month (Figure 5a). _Unhealthy_ occurs more frequently in December through April, indicating that peak pollution usually occurs in winter and spring. The year-based grouping (Figure 5b) clearly shows a general drop in pollution levels from 2014 to 2018, with a small uptick in 2016. In general, the moderate class accounts for 51% cases while good and unhealthy, respectively, account for 28% and 21%.  
Similar to the Zhongli AQI pattern, pollution in Changhua peaks in March (Figure 6a). However, the degree of air pollution is more severe in Changhua, with unhealthy accounting for 59% of March readings, as opposed to 39% for Zhongli. Like Zhongli, higher AQI levels in Changhua are also clustered in winter and spring, but September, October, and November also featured significant instances of the unhealthy class (respectively 35%, 38%, and 41%). In general, Changhua has poorer air  
\begin{table}
\begin{tabular}{c c c c} \hline
**No** & **Feature** & **Type** & **Description** \\ \hline
1 & O\({}_{3}\) 8-h & Numeric & Calculated based on O\({}_{3}\) average of last 8 h \\
2 & PM\({}_{10}\) moving average & Numeric & Calculated as follows: (0.5x average of PM\({}_{10}\) in the last 12 h)+(0.5x average of PM\({}_{10}\) in the last 4 h) \\
3 & PM\({}_{2.5}\) moving average & Numeric & Calculated using the same rule as the PM\({}_{10}\) \\
4 & CO 8-h & Numeric & moving average \\
5 & AQI index & Numeric & The average concentration for the last 8 h \\  & & & AQI value based on the maximum index between \\  & & & the AQI pollutants (PM\({}_{10}\), PM\({}_{2.5}\), NO\({}_{2}\), SO\({}_{2}\), \\  & & & O\({}_{3}\), and CO) \\ \hline \end{tabular}
\end{table}
Table 1: Other features added to the prediction model.  
Figure 5: Composition of AQI classes in Zhongli: (**a**) Month-based; (**b**) Year-based and Overall-based.  
quality than Zhongli, with more frequent AQI \(>\) 100 incidents both monthly and annually. However, the full-year AQI readings in Figure 6b show that air quality has gradually improved over time, with a 34% drop in instances of unhealthy from 2008 to 2018.  
Southern Taiwan, especially Kaohsiung City, is notorious for its poor air quality due not only to emissions from nearby industrial parks but from particulate matter blowing in from China and Southeast Asia. Figure 7a,b shows significant instances of the unhealthy class (red bars) air quality for most of the year, with reduced pollution levels only in May to September. The worst air quality is concentrated in December and January (respectively 78% and 80% unhealthy).  
The winter spike in air pollution is partly due to seasonal atmospheric phenomena that trap air pollution closer to the ground for extended periods. From October to March, Fengshan air quality readings are _good_ less than 5% of the time. In terms of year-based AQI class composition, not much improvement is seen until in 2014-2015 with a sharply declined _unhealthy_ scores after which levels remain relatively stable. Overall, for the 11 years, the Fengshan dataset is dominated by AQI \(>\) 100 (46%) followed by 51 \(\leq\) AQI \(\leq\) 100 (37%), and AQI \(\leq\) 50 (17%).