PM2.5水溶性無機鹽微粒與前驅氣體之PPWD-PILS的驗證與應用

Abstract

本研究利用自動監測系統PPWD-PILS(Parallel Plate Wet Denuder, PPWD；Particle into Liquid Sampler, PILS)及手動採樣器PDS(Porous Metal Denuder Sampler, PDS)於2014年3月至2014年7月在交通大學校園及台北市中山測站進行前驅氣體與PM2.5水溶性無機微粒進行比對及觀測實驗。兩種採樣器所採集到的前驅氣體NH3、HONO、HNO3及SO2之過原點的線性迴歸決定係數(R2)為0.76-0.83之間，斜率範圍為0.92-1.04；大氣中主要的水溶性無機鹽微粒為NH4+、NO3-及SO42-，R2範圍為0.89-0.96，斜率範圍為0.84-1.1；Na+、K+及Cl-相較於NH4+、NO3-及SO42-濃度較低，量測誤差較大，因此R2較低，其範圍為0.77-0.83，斜率範圍為0.75-0.95。 PPWD-PILS連續監測系統所偵測的PM2.5濃度值與降雨、風向等氣象因素極度相關。 PPWD-PILS在中山測站七月份中，NH3平均值濃度為6.26± 3.09 ppbv，為濃度最高的前驅氣體，SO2濃度隨著白天溫度上升濃度增高，而HONO白天因進行光化反應故濃度較低；PM2.5中NH4+、NO3-及SO42-有明顯的日夜變化且其平均值分別為1.55± 1.16、0.52± 0.5及4.56± 3.14 μg m-3。基於NO3-、SO42-在高濃度NH4+的環境下，(NH4)2SO4與NH4NO3為PM2.5中主要的無機鹽微粒。 本研究也利用ISORROPIA II 熱平衡濃度值與PPWD-PILS觀測值，計算大氣熱平衡的情況下前驅氣體與PM2.5微粒濃度，前驅氣體NH3觀測值濃度較ISORROPIA II熱平衡濃度值低，而氣體中HNO3與PM2.5中NH4+、SO42-觀測值濃度較熱平衡濃度值高。不論是自動監測系統PPWD-PILS與手動採樣器PDS或是自動監測系統PPWD-PILS與ISORROPIA II熱平衡模擬系統之間皆有良好的比對結果，也驗證自動化監測系統PPWD-PILS的可靠性並可取代傳統手動採樣方法。

An online monitoring system for simultaneous measurements of ambient PM2.5 inorganic ions (NH4+, NO3-, SO42- Na+, Cl- and K+) and associated precursor gases(NH3, HONO, HNO3 and SO2), PPWD-PILS, is developed. The accuracy of PPWD-PILS was validated by comparing it with PDS 24-hr denuder/filter-pack data and was performed from March 2014 to July 2014 in NCTU campus and Taipei City, Taiwan. Results showed good correlations between PPWD-PILS and PDS for precursor gases and PM2.5 ion. The linear regression slope and R2 ranged from 0.92-1.04 and 0.75-0.97 for precursor gases, from 0.76-0.83 and 0.77-0.94, for PM2.5 ions. Concentration trends of precursor gases and PM2.5 are related to the rain event and wind direction. NH3 was the most abundant precursor gas with mean concentration of 6.26 ppbv and to present a similar diurnal variation to SO2. A reverse trend for HONO indicated daytime photolysis and nocturnal heterogeneous reaction of it. The major PM2.5 ions, NH4+、NO3- and SO42-, showed similar diurnal patterns with the mean concentrations of 1.55、0.52 and 4.56 μg m-3, respectively. With NO3- and SO42- neutralized in an ammonia-rich atmosphere, (NH4)2SO4 and NH4NO3 were inferred to be the dominant inorganic salts in PM2.5. Thermodynamic modeling for measured aerosols and gases was carried out using ISORROPIA II. This large discrepancy between observation and prediction could be attributed in part to departure from equilibrium or measurement uncertainty.