| 英文摘要 |
In Taiwan, monitoring of atmospheric visibility lacks objective methods and continuous real-time measurement data, which result in a gap between measurement data and public perception. This work proceeded with visibility measurements at the Banqiao, Xitun, and Xiaogang air quality monitoring sites of Environmental Protection Administration from March 2019 to June 2020. In addition, the different visibility measurement methods were compared to provide valid observation data.
This work set up the visibility monitoring technology equipped with Long Path Visibility Transmissometers (LPV-4) and Open-Air Integrating Nephelometers (NGN-2a). Within 95% confidence interval with ANOVA statistical analysis, the simultaneous measurements from three LPV-4 instruments were regarded as the same. There was also not statistically significant difference among three NGN-2a simultaneous measurements. After verifying the measurement consistency of the instrument, monitoring stations and instruments were installed at the Banqiao, Xitun, and Xiaogang sites and conducted to investigate continuous hourly extinction and scattering coefficient. The linear regression analysis showed extinction coefficient correlated well with scattering coefficient at the Banqiao, Xitun, and Xiaogang sites, respectively (R2 = 0.86, 0.84 and 0.93), which indicated that the instruments operated normally and stably. However, the results of T-test for visibility measurements from LPV-4, Central Weather Bureau (CWB) human observation, and Visibility ＆ Present Weather Sensor (SWS-250) revealed three methods were significantly different during most measurement periods. The possible reasons were the individual vision objective and the differences of observed environment.
So far, the domestic standards and indicators of visibility has been not established. To evaluate of visibility reduction by PM2.5 (an aerodynamic diameter equal to or less than 2.5 μm) concentration and relative humidity (RH), this work distinguished the characteristics and occurrence frequency by visual range (VR) and RH at different visibility event. Additionally, visibility indicator (deciviews) developed by Interagency Monitoring of Protected Visual Environments (IMPROVE) was used to evaluate visual air quality and effect factors at the Banqiao, Xitun, and Xiaogang sites. Reducing concentration of PM2.5 is beneficial to decrease frequencies of visibility events ( VR < 10 km) and improve visual air quality. Under the same PM2.5 concentration condition, the extinction coefficient was proportional to the RH. In addition, the extinction mass efficiency also increased with the RH at the same extinction coefficient. It indicated that the hours of visibility events caused by the deliquescence of PM2.5 were more than those caused by excessive PM2.5 concentration.
The PM2.5 chemical compositions were applied to the revised IMPROVE regression to estimate the extinction coefficient. The estimated extinction coefficient was correlated well to measured extinction coefficient by LPV-4 with R2 of 0.96, 0.96, and 0.98 at Banqiao, Zhongming, and Xiaogang, respectively. It indicated that the estimated extinction coefficient could be used to evaluate the contributions of chemical compositions reduction in visibility improvement. At the Banqiao site, the highest contributive composition to the light extinction coefficient was sulfate, and the contributive proportion of each composition did not change apparently throughout the seasons. Nitrate significantly affected visibility at Zhongming and Xiaogang in winter. Regarding the contributions of PM2.5 chemical compositions in extinction coefficient, sulfate and nitrate are important contribution sources of extinction coefficient at each site. According to the relation between PM2.5 mass extinction efficiency and chemical compositions, it was found that the increase in the proportion of NO3- in PM2.5 mass would lead to the decrease in visibility and the increase in mass extinction efficiency. Although, the high proportion of sulfate in PM2.5 is an obvious characteristic, the mass extinction efficiency was not ascended as increase in the mass proportion of sulfate. The results indicated that avoiding the emission and formation of nitrate is one of the primary solution to improve visibility.
For source apportionment of pollution, positive matrix factorization (PMF) was used at the Banqiao, Zhongming, and Xiaogang sites, and this work reconstructed the LPV-4 extinction coefficient with PMF factors and modified with relative humidity factor to understand the contribution of the sources in the real atmospheric environment. The results showed that the extinction coefficient of each source did not show the obvious seasonal variation at Banqiao site. The extinction coefficient at Zhongming was higher in winter and spring, which mainly contributed by sources of nitrate (RH), coal-burning, and chlorine-rich (RH). Reducing of the extinction coefficient from these three contribution sources was the primary recommendation to improve visibility. At Xiaogang site, the nitrate (RH) and vehicle emissions were the main contribution sources of extinction coefficient, which caused the high extinction coefficient in autumn and winter. Especially in winter, nitrate (RH) affected the 56.1% of extinction coefficient. As a result, reducing the source of nitrate (RH) might be one of the important solutions for improving the visibility.
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