都會區大氣懸浮微粒的特性及PM2.5貝他計的測值誤差探討

Abstract

本研究於環保署新莊、中山及竹東空品測站使用雙通道採樣器(Dichot, Andersen Model SA241, Andersen Inc., Georgia, USA)及微孔均勻沉積衝擊器(Micro-orifice Uniform Deposit Impator ,MOUDI)採集大氣中PM10、PM2.5及PM0.1的微粒樣本，並分析三個測站各個PM的質量濃度及水溶性離子成分。此外，本研究也發現Dichot的PM2.5採樣結果(PM2.5,D)和空品測站貝他計(Bata Attenuation Monitor, BAM)測值(PM2.5,B)間有系統性誤差，在新莊、中山及竹東測站，前者分別較後者低了15、42及26%，因此本研究也將針對此問題作深入探討。 分析MOUDI所測得之質量粒徑分布的結果發現，三個測站均呈現典型交通排放源的雙峰分佈。水溶性離子分析結果顯示，離子佔質量濃度的比例以新莊測站最低，竹東測站最高。離子中的主要成分皆以衍生性氣膠SO42-、NO3-及NH4+為主。在PM2.5,D及PM2.5,B兩者誤差探討方面，本研究發現使用ISOROPIA-II模式所計算出之微粒含水量高於兩者之誤差，此結果顯示除了含水量造成的正向異常生成物外，還有一些由沉積在BAM採樣入口WINS分徑器內的微粒以及發性物質在採樣過程中揮發等所造成之負向異常生成物。研究結果顯示當RH>75%時，WINS衝擊杯內的負荷量造成PM2.5,B降低的情況較RH<75%時明顯。 本研究並使用多元回歸分析找出PM2.5,B (μg/m3)與PM2.5,D (μg/m3)、微粒含水量(Wat, μg/m3)及微粒負荷量(L, μg)間的關係，所求得的經驗式為PM2.5,B = αPM2.5,D + βWat + γL + δ，其中當RH < 75 %與RH > 75 %時，各參數分別為α = 1.215與1.236、β = 0.119與-0.163、γ= -0.001與-0.001及δ= 3.863與2.229。整體來說，微粒含水量在所有造成PM2.5,B與PM2.5,D的誤差因子中扮演著最重要的角色。相較於低RH的情況下，當大氣RH較高時，BAM分徑器WINS內的微粒負荷量對其測值有較顯著的影響，因此本研究建議提高WINS衝擊杯維護的頻率，或是改成微粒負荷量較佳的VSCC作為新的PM2.5分徑器，以減少微粒負荷對量測誤差造成的影響。

PM10, PM2.5 and PM0.1 samples were collected by a Dichotomous sampler (Dichot, Andersen Model SA241, Andersen Inc., Georgia, USA) and a Micro-orifice Uniform Deposit Impactor (MOUDI) at Sinjhuang, Jhongshan and Judong air monitoring stations, Taiwan. The collected samples were further analyzed for mass concentration and water-soluble ion compounds. A systematic difference between the PM2.5 measured by the Dichot (PM2.5,D) and those by the Beta Attenuation Monitor (BAM) (PM2.5,B) was found, where the PM2.5,B are higher than the PM2.5,D by 15、42 and 26%, respectively, at Sinjhuang, Jhongshan and Judong stations. Therefore, the present study will also focus on this issue. Results of the mass size distribution measured by the MOUDI show that all the three station performed the bi-model distribution which is influence by typical traffic emission. The results from water-soluble ion analysis show that the Sinjhuang station has the highest contribution of ionic species to total PM while the Judong station has the lowest. The main component of ion is the secondary aerosol such as the SO42-, NO3- and NH4+. Calculated water contents by ISOROPIA-II model were found to be higher than the differences between 24-h average PM2.5,B and PM2.5,D concentrations, which indicates that besides positive artifacts due to aerosol water content, deposited particles in the WINS (Well Impactor Ninety-Six), the PM2.5 inlet of the BAM, and particle evaporation loss during sampling and detection of the BAM also result in negative artifacts in PM2.5. This study showed that the loaded particle mass in the WINS had more influence on the reduction of the PM2.5,B at high relative humidity (RH≧75%) conditions than that at low RH conditions (RH< 75 %). Multi-linear regression was used to relate 24-h average PM2.5,B (μg/m3) concentrations to PM2.5,D concentrations (μg/m3), aerosol water content (Wat, μg/m3) and loaded particle mass (L, μg) as PM2.5,B = αPM2.5,D + βWat + γL + δ, where α = 1.215 and 1.236, β = 0.119 and -0.163, γ= -0.001 and -0.001, δ = 3.863 and 2.229 for RH less or greater than 75%, respectively. Overall, aerosol water content plays the most dominant role on the measurement differences of PM2.5,D and PM2.5,B among all factors. At high RH conditions, loaded particle mass in the WINS of the BAM shows more influence on the measurement differences that at low RH conditions. Therefore, more frequent cleaning of the WINS or replacement of the inlet to the VSCC (very sharp cut cyclone) is important to reduce the effect of loaded particle mass on PM2.5,B concentrations.