# Long-Term and Seasonal Changes in Sources of Urban Atmospheric Particulates in the Western Pacific
## 3 Results and Discussion
### Overall Trends of the Western Pacific  
Oil and coal account for more than 60% of the world's primary energy production, around 30% of which was consumed in the Western Pacific countries in 2019 [23]. Therefore, this region has a strong effect on global atmospheric conditions. Figure 5 shows long-term trends of summer and winter [P\({}_{\rm c}\)] in the Western Pacific from 1997 to 2018. [P\({}_{\rm c}\)] levels in Sapporo, Kanazawa, Tokyo, Sagamihara, Shanghai, Kitakyushu, and Busan (Figure 5A) were low. Their first-order linear regression equations, based on the least squares method, show negative slopes (summer \(-\)0.117 and winter \(-\)0.282) and the correlation coefficients  
Figure 4: Sources of Atmospheric Total Suspended Particulate (TSP) and Combustion-Derived Particulates (P\({}_{\rm c}\)) in Shanghai. P\({}_{\rm c}\), P\({}_{\rm n}\), P\({}_{\rm h}\), and P\({}_{\rm I}\); The same as in Figure 1 caption.  
(r) (\(-0.636\) and \(-0.547\), respectively) mean that the correlations were negative. [P\({}_{\rm c}\)] levels in Beijing, Shenyang, and Vladivostok (Figure 5B) were high. The first-order linear regression equation for winter shows a negative slope (\(-0.3643\)). The r value (\(-0.522\)) with \(p\) values smaller than 0.05 means that the correlation was negative. However, the summer equation has a very small r value (\(=0.023\)) and \(p\) values larger than 0.05. In these cities, [P\({}_{\rm l}\)] was often relatively high even in summer (Table S2), suggesting that coal combustion such as from factories may decrease the correlation. These results suggest that [P\({}_{\rm c}\)] was decreasing in this region.  
Coal and gasoline were considered to be the main combustion sources of P\({}_{\rm c}\) in this region in the last two decades [10]. Figure 6 shows the long-term trends of summer and winter [P\({}_{\rm h}\)]/[P\({}_{\rm c}\)] ratios of the 10 cities from 1997 to 2018. Figure 6A,B contain the same cities as Figure 5. Figure 6A shows first-order linear regression equations with negative slopes (summer \(-0.0201\) and winter \(-0.0237\)) and negative r values (\(-0.487\) and \(-0.546\), respectively) with \(p\) values smaller than 0.05. Figure 6B shows the equations with negative slopes (summer \(-0.028\) and winter \(-0.0072\)), although the correlations are not so strong as those in Figure 6A, and \(p\) values are larger than 0.05. These results indicate that P\({}_{\rm c}\) emission from vehicles was decreasing, but P\({}_{\rm c}\) emission from other sources, such as coal combustion, was not.  
This is the first study to estimate the long-term and seasonal contributions of combustion sources to TSP in the Western Pacific cities. Many gasoline and diesel engine vehicles will be replaced by electric and hydrogen engines vehicles which do not emit P\({}_{\rm c}\), but consumption of fuels for air and sea transportation is increasing [26]. Although there is a global trend to replace coal with natural gas and renewable energies, a quick shift may not be easy. More improvements of combustion systems for fuels will be needed to reduce emissions of P\({}_{\rm c}\) in this region.  
Figure 5: Long-Term Trends of Summer and Winter [P\({}_{\rm c}\)] in the Western Pacific. (**A**) Sapporo, Kanazawa, Tokyo, Sagamihara, Shanghai, Kitakyushu, Busan; (**B**) Beijing, Shenyang, Vladivostok. Season: Winter, \(\bullet\) Summer.