## 3 Results and Discussion
### Trends and Meteorological Adjustment for Visibility and PM\({}_{10}\)  
Station CB has a significant positive (increasing) trend in visibility in the P1 period (0.8 km dec\({}^{-1}\), where \"dec\" denotes decade); however, its magnitude becomes smaller and also insignificant in the P2 period (0.3 km dec\({}^{-1}\)). Visibility at LC declines significantly in the P2 period (\(-\)1.7 km dec\({}^{-1}\)) while the trends at ST significantly decline in the P1 period (\(-\)2.1 km dec\({}^{-1}\)); however, it becomes persistently low in the P2 period. Although the trends at RY and SK are positive, they are not significant. At both CT and PB, the trends are once significantly positive (0.6 km dec\({}^{-1}\)); however, they become negative over recent years (significantly in the case of PB, \(-\)1.0 km dec\({}^{-1}\)) (Figure 9). All of the four stations with the long-term visibility data display either a negative shift of trend (from the P1 to P2 periods) or persistent low visibility due to their proximity to urban and/or industrial areas. Station LC has the fastest decline of visibility over the recent period. The insignificant trend in visibility over SK could be due to a rural background and not much change in anthropogenic activities. Since the time span for trend calculation is limited (nine years) at RY, not much can be established from it. The results of trend and meteorological adjustment for CB, LC, and RY are shown in Figure 10. The final fitted GLM explained 41.2%, 59.7% and 51.9% of total variance in visibility for CB, LC and RY, respectively. For PM\({}_{10}\), the final GLM fit explained 62.7%, 53.9% and 49.7% of total variance. All the fitted models were found to be acceptable in terms of residual diagnostics. Notice that the adjusted trends tend to fluctuate less year-to-year than the original trends, which is due to the fact that substantial inter-annual variability resulting from meteorology was already removed. Here, our discussion focuses specifically on the adjusted trends (as better reflecting the anthropogenic signature or influence than the original ones, and more policy-relevant). At CB, PM\({}_{10}\) level decreases significantly at the rate of \(-\)12 \(\mu\)g m\({}^{-3}\) dec\({}^{-1}\), suggestive of local primary emissions being reduced over time. However, the visibility trend is insignificant (0.2 km dec\({}^{-1}\)). At LC, adjusted visibility and PM\({}_{10}\) level show contrasting trends. Visibility has decreased sharply at \(-\)0.7 km dec\({}^{-1}\), while PM\({}_{10}\) shows an insignificant decreasing trend (\(-\)8.6 \(\mu\)g m\({}^{-3}\) dec\({}^{-1}\)). The potential cause and linkage that may help explain such contrast, at least in part, is the formation of secondary aerosols. In the previous section (Section 3.5), we found secondary aerosols to be associated with visibility. Such secondary aerosols are in fact fine particles to which visibility or optical extinction is sensitive, and they are not readily represented by PM\({}_{10}\). Contrasting trends in visibility and PM\({}_{10}\) have also been reported in some other  
Figure 8: (a) PM\({}_{10}\)/CO (\(\mu\)g m\({}^{-3}\) ppm\({}^{-1}\)); (b) SO\({}_{2}\)/CO (ppb ppm\({}^{-1}\)); (c) NO\({}_{x}\)/CO (ppb ppm\({}^{-1}\)); and (d) NMHC/CO (ppbc ppm\({}^{-1}\)) by visibility class at stations CB, LC, and RY. studies [10; 15]. At RY, the time coverage is relatively short (nine years, generally not very adequate for trend estimation). We found that both visibility and PM\({}_{10}\) level do not have any significant trends. Based on the adjusted trend results discussed, none of the visibility trends are significantly positive, while some are in contrast with those of PM\({}_{10}\). These results indicate that meteorology affects trends in both visibility and PM\({}_{10}\). The results also suggest that the trend in fine particulate matter (PM\({}_{2.5}\)) could correlate better with the trend in visibility.