台灣北部三個空品測站大氣超細微粒的特性

Abstract

本研究使用1台微孔均勻沉降衝擊器(Micro-Orifice Uniform Deposit Impactors, MOUDI, Model 110, MSP Corp., MN, USA)、1台雙頻道虛擬衝擊器(Dichotomous, Model SA-241, Andersen Inc., Georgia, USA)及1台掃描式電移動度粒徑分析儀(Scanning Mobility Particle Sizer, Model 3936, TSI Incorporated, St. Paul, MN, USA, SMPS)於台灣北部的中山、新莊及竹東空氣品質監測站(Taiwan Air Monitor Station, TAMS)量測大氣微粒質量濃度及數目濃度，並計算超細微粒的有效密度(ρeff)及動力形狀係數(χ)。由MOUDI量測的質量濃度轉換成數目濃度，探討超細微粒對環境的影響。 由微粒逆軌跡運算分析結果顯示，台灣北部夏季的大氣微粒分別源自於菲律賓西方的中國南海、菲律賓東方的菲律賓海及西太平洋，其餘三個季節的微粒主要源自於中國或北方的俄羅斯及內蒙古。由於秋末春初東北季風吹拂大陸沿岸或內陸黃沙污染物至台灣北部的影響，PM2.5濃度會高於盛行西南及太平洋海風的夏季。春末夏季時PM0.1濃度比其他三季節稍微高；中山測站量測的PM0.1濃度最高，本研究推測中山測站緊鄰交通繁忙的新生高架道路易受車輛排放出的新生微粒影響而使微粒濃度增高(Betha et al., 2014)。 結果顯示微粒的水溶性離子佔PM2.5的比率較PM0.1多，推測此現象主要原因為較大的PM2.5微粒因滯留在大氣的時間較長，因此微粒表面會有凝結的水分並與大氣中的無機氣體反應抑或是受到光化反應產生二次氣膠，使微粒成分中無機鹽類所佔比例較高(Cao et al., 2013)。 微粒質量濃度分佈在中山、新莊及竹東測站呈現典型的都會區雙峰分佈，粗微粒模式中間氣動粒徑介於4-8.5 μm，累積模式則介於200-800 nm。若採樣時受到內蒙古沙塵事件或颱風侵襲，會使得原本典型的雙峰分佈轉變為單峰分佈，單峰模式的中間氣動粒徑介於0.84-3.26 μm。 本研究在2014年於中山測站及沙鹿測站利用SMPS量測得的微粒數目濃度與MOUDI測得的質量濃度來計算微粒有效密度，並引用Hu et al. (2012)量測的塊材密度計算動力形狀係數，中山測站的PM0.1及PM0.1-0.18的平均有效密度分別為0.68±0.16及1.06±0.32 g/cm3，平均動力形狀係數為2.06±0.19及1.45±0.16；沙鹿測站的PM0.1平均有效密度分別為0.62±0.07及0.93±0.16 g/cm3，平均動力形狀係數為2.30±0.21及1.45±0.11。2011-14年在中山、新莊及竹東測站以MOUDI量測奈米微粒(PM0.1)佔細微粒(PM2.5)比率的研究結果顯示，質量濃度比率為7.9±4.4 %，數目濃度比率達89.0±5.5 %，表面積比率為42.1±12.8 %。由此結果可知，若以數目濃度作為奈米微粒空氣品質評估標準，PM0.1在大氣氣膠中仍佔大多數的比率，對人體健康的影響不容忽視。

In this study, a MOUDI (Micro-Orifice Uniform Deposit Impactors, Model 110, MSP Corp., MN, USA), a Dichotomous (Model SA-241, Andersen Inc., Georgia, USA), and a Scanning Mobility Particle Sizer (Model 3936, TSI Incorporated, St. Paul, MN, USA, SMPS) were deployed in Jongshan, Sinjhuang, and Jhudong site to measure atmospheric aerosol mass and number concentrations. The effective density (ρeff) and dynamic shape factor (χ) calculated based on the measured number and mass concentrations were used to convert mass concentrations into number concentrations to investigate the effect of ultrafine particles (UPs) on environment. The results of back trajectory analysis showed that the atmospheric aerosols in northern Taiwan in summer came from South China Sea, Philippine Sea, and western Pacific Ocean. In the other seasons, the atmospheric aerosols mainly came from China, Russia, and Inner Mongolia autonomous region. PM2.5 concentrations in Autumn, Winter, and Spring were higher than those in summer because of the influence of dust from mainland China transported by prevailing north-eastern monsoon. The highest PM0.1 number concentration was observed in Jongshan site due to the traffic emissions from Xinsheng highway (Betha et al., 2014). The particle water soluble ions contributed higher fractions to PM2.5 than to PM0.1 because PM2.5 are aged aerosols in which large fraction of condensed water on the surface of particles may absorb inorganic salts. PM2.5 are also secondary aged particles generated by gas-to-particle partitioning or photochemical reaction (Cao et al., 2013). Mass concentration distributions in Jongshan, Sinjhuang, and Jhudong site shows bimodal distribution, in which the aerodynamic mean diameter was in the range of 4-8.5 μm, the accumulation mode was in the range of 200-800 nm. When the dust or typhoon event occurred, the bimodal distribution was changed to single modal distribution and the aerodynamic mean diameter in the range of 0.84-3.26 μm. The ρeff and χ were calculated based on the number and mass concentration measured by SMPS and MOUDI, respectively, in Jongshan and Shalu site. The bulk density obtained by Hu et al. (2012) was applied to calculate dynamic shape factor. The average ρeff of PM0.1 and PM0.1-0.18 were 0.68±0.16 and 1.06±0.32 g/cm3, respectively, the average χ were 2.06±0.19及1.45±0.16, respectively, in Jongshan site. The average ρeff of PM0.1 and PM0.1-0.18 were 0.62±0.07 and 0.93±0.16 g/cm3, respectively, the average χ were 2.30±0.21 and 1.45±0.11, respectively, in Shalu site. Based on the number and mass concentration measured in the sampling as the aforementioned, the effective density was calculated to convert the mass concentration measured by MOUDI into number concentration and surface concentration. The results showed that although the mass ratio of PM0.1/PM2.5 of 7.9±4.4 % was low, the number concentration ratio of PM0.1/PM2.5 was 89.0±5.5 %, and the surface concentration ratio was 42.1±12.8 %. Therefore, we should pay attention to the adverse effect of PM0.1 on human health.