| dc.description.abstract | The water-soluble inorganic ions (WSIIs) of atmospheric aerosol influences climate change and atmospheric visibility significantly by scattering solar radiation. This study used a semi-continuous monitoring instrument to observe high time-resolved WSIIs of PM2.5 and analyzed the data in companion with the related monitoring data at the Lulin atmospheric background station (LABS, 2,862 m a.s.l.) from March 10 to April 10, 2019. Additionally, the Xitun Monitoring Station of the Environmental Protection Administration (Xitun Station) near the Taichung Interchange and the industrial park was selected for the observation of WSIIs in the urban environment from April 17 to April 30, 2019.
Spring is the extensive biomass burning (BB) season in the Indochina Peninsula; the BB smoke was frequently transported to East Asia by the prevailing westerly. The PM2.5 mass concentration increased to 26.9 ± 9.0 μg m-3 under the influence of the transported BB smoke at LABS in the spring of 2019. The mean values of SO42-, NO3-, and NH4+ increased to 5.0 ± 1.7, 1.2 ± 0.9, and 0.8 ± 0.8 μg m-3, respectively. The concentration of K+, one of the BB tracers, increased to 0.3 ± 0.2 μg m-3. Meanwhile, aerosol number concentration was found to distribute in the 90-300 nm size range. Notably, a phenomenon of new particle formation appeared in the 20-50 nm size range under solar radiation and low relative humidity (RH) in a BB event of late March.
During the monitoring period at Xitun Station in Taichung City, the mean value of PM2.5 mass concentration was 22.6 ± 7.8 μg m-3 along with SO42-, NO3-, and NH4+ concentrations at 4.8 ± 2.0, 2.5 ± 2.8, and 4.7 ± 2.0 μg m-3, respectively. In contrast to the high altitude environment, the monitoring at the urban station discovered the presence of NO2- frequently. The formation of NO2- might be originated from the heterogeneous reaction of HONO and NO2 gases on the wet surfaces of aerosols under high RH at night.
In this study, an optical classification method was applied to connect aerosol chemical components with their atmospheric optical properties. The results showed that the aerosol was moderately or slightly absorbing at LABS and that at the Xitun Station was non-absorbing. However, the above classification was only applicable to high PM2.5 concentration and was divided under low PM2.5 concentration. Switching to Aerosol Optical Depth (AOD), this study found aerosol chemical components correlated with AOD well under low RH, non-calm wind, and without cloud and rain conditions at both LABS and Xitun station. Also, AOD values tended to increase with NO3- share in total WSIIs, indicating stronger hygroscopicity of NO3-. For the aerosol acidity, the simulation of ISORROPIA II, a thermodynamic equillibrium model, indicated the dominance of acidic aerosol in the BB smoke at LABS in contrast to less acidic in the urban environment because of the excess NH4+.
For a summary, this study found that PM2.5 and O3 concentrations at the high altitude station even exceeded that of the urban station under the influence of BB smoke from long-range transport. In contrast, higher CO, NO3-, and NO2- concentrations in the urban station reflected the effects of traffic pollution emissions.
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