| 摘要: | 大氣中氣膠水溶性無機離子對太陽輻射的散射，對氣候變遷和大氣能見度有很大的影響，本文於2019年3月10日至4月10日在鹿林山大氣背景觀測站(2,862 m a.s.l.)，利用半自動監測儀器觀測高時間解析度的PM2.5水溶性無機離子，並結合觀測站相關監測資料進行數據分析；此外，為了觀察都市環境短時間水溶性無機離子變化，於2019年4月17日至4月30日選擇鄰近台中交流道及工業區的環保署西屯測站進行觀測分析。
春季是中南半島生質燃燒(Biomass Burning, BB)旺季，燃燒煙團藉由盛行西風經常傳輸至東亞，2019年鹿林山春季受傳輸BB煙團影響時，PM2.5質量濃度高達26.9 ± 9.0 μg m-3，SO42-、NO3-、NH4+濃度分別提升至5.0 ± 1.7、1.2 ± 0.9、0.8 ± 0.8 μg m-3，BB氣膠指標成分之一的K+濃度增加為0.3 ± 0.2 μg m-3，氣膠數目濃度分布集中在90-300 nm粒徑區間。在三月下旬一個BB事件，當有太陽輻射且相對濕度低的環境下，發現20-50 nm粒徑區間有新微粒形成現象。
在台中市西屯測站監測期間，PM2.5平均質量濃度為22.6 ± 7.8 μg m-3， SO42-、NO3-、NH4+濃度分別為4.8 ± 2.0、2.5 ± 2.8、4.7 ± 2.0 μg m-3。相較於高山背景環境，都市環境的監測常發現NO2-，這可能是夜晚較高相對濕度的環境下，HONO、NO2氣體在氣膠潮濕表面發生異質反應而產生NO2-。
本文透過氣膠光學分類法，連結氣膠化學成分與大氣光學特性，發現鹿林山生質燃燒氣膠屬於中度或微幅吸光型，西屯測站氣膠屬於非吸光型，前述類型的歸類在高PM2.5濃度才具有一致性，低PM2.5濃度時則較分歧。轉移到氣膠光學厚度(Aerosol Optical Depth, AOD)，本文發現在低相對濕度、非靜風且現址無雲霧及降雨的環境條件下，鹿林山及西屯測站大氣氣膠化學特性和大氣AOD相關性良好，並且發現AOD有隨著NO3-在總水溶性無機離子占比提升而增加的跡象，這可能與NO3-較強的吸濕作用有關。針對氣膠酸度，本文進行ISORROPIAⅡ熱力平衡模式模擬，顯示鹿林山生質燃燒事件主要為酸性氣膠，而都市地區因有過剩NH4+而較不酸。
總結來說，本文發現在生質燃燒煙團長程傳輸的影響下，PM2.5和O3濃度在高山背景站甚至高於都市測站，都市測站的較高CO、NO3-、NO2-濃度則反映了交通污染排放的影響。
;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. |
