| dc.description.abstract | In Taiwan, many studies have been carried out over the past to survey the reasons of visibility degradation, but there still lack of the comprehensive analysis in aerosol physical-chemical characteristics. This study established a high time-resolution monitoring system and seted the trailer station – IMPACT at the campus of Tunghai University in Taichung during October 2017 to August 2019. Measurement items include the mass concentration, extinction coefficient(bext), size distribution, water-soluble inorganic ions, heavy metal elements, tracer gas and meteorological data in two years. In this study, to evaluate the reasons of visibility degradations, we used the revised IMPROVE algorithm and the stage variation of events to estimate the chemical components and extinction coefficient (bext) of fine particulate matter (PM2.5).
During the observation period, the extinction mechanism is dominated by the scattering particles. The low visibility events (high bext) are not only caused by high PM2.5 concentrations but also affected by high mass extinction efficiency (MEE). In seasonal variations, the bext is lower in summer and autumn than in winter and spring. Based on the wind-roses analysis, lower wind speed (WS < 1 m/s) would cause higher bext, single scattering Albedo (SSA), and mass scattering efficiency (MSE) during each season. In the IMPROVE chemical scattering of event analysis, the main contributor to bext is Nitrate (43%), followed by OM (32%) and Sulfate (18%). Nitrate would get a higher proportion (12% to 43%) in visibility degradation than clean case. Furthermore, the particle size range would become larger and concentrated, Nitrate is 200 to 500 nm, OM is 100 to 300 nm and Sulfate is 400 to 600 nm. While the increasing particle size make higher scattering effect and lower visibility. In stage 1 and stage 2 of event, wind speed is lower (< 1 m/s) and the temperature is higher than the clean case. The different relative humidity (RH) would correspond to those Nitrate formation-reaction. Low RH (70%) during the daytime is suitable for gas-phase reactions, and high RH (90%) during the nighttime is appropriate for heterogeneous reactions. Surprisingly, daytime and nighttime mechanisms are different, but the final situation cause visibility degradation and the same produce - nitrate. | en_US |