臺灣中部地區氣膠特性探討：以溪頭為例

Abstract

氣膠影響著人類生活環境的空氣品質，其物理化學特性更可能改變雲霧的生成，使局部地區天氣發生變化，進而改變大氣環流，而光學特性除了影響能見度外，亦會對地表的輻射強迫有所影響。本研究以臺灣大學溪頭實驗林為觀測地點(23.40°N, 120.47°E, 1,178 m above sea level (asl))，於2018年12月1日至24日針對氣象、氣態汙染物、氣膠的吸濕特性以及官能基組成進行觀測分析，並以同位素分析探討氣膠的可能來源及其化學反應過程。氣膠的吸濕係數(κ)是由雲凝結核計數器(CCNc)、凝結核計數器(CPC)、掃描式電移動度微粒分徑器(SMPS)，並經由κ-科勒公式推算而來。本實驗亦利用多階微孔沉降衝擊器(MOUDI)採集不同粒徑的氣膠粒子，將收集後的粒子進行紅外線光譜儀(FTIR-ATR)的分析，來測定硫酸根、硝酸根、銨根、以及黑碳的質量濃度，收集到的部分濾紙再以同位素質譜儀(IR-MS)進行氮15 (δ15N)與氧18 (δ18O) 同位素組成的分析。實驗結果發現，利用CCNc, CPC, SMPS僅能推得粒徑小於120微米的吸濕特性，其κ在觀測期間約介於0.05至0.5之間，在風速較高的期間得到的κ值較高，顯示κ與氣膠來源與生成具有一定程度的相關性。利用紅外線光譜分析測得的硫酸根、硝酸根、銨根、以及黑碳濃度在白天較晚上高，可能是因為白天谷風將都會區的氣膠粒子及前驅物傳送至溪頭地區造成，而在有霧的情況下，這些官能基的濃度也會較無霧時來得高，推測有可能是有霧的情況下，邊界層的高度較低且有較強的邊界層逆溫，使得汙染物不易藉由紊流(turbulence)向上傳送混合。硫酸根及銨根的粒徑分佈在有霧的時候有往較大粒徑偏移的趨勢，可能是因為在高相對濕度的環境之下，無機鹽類會吸濕成長；次微米的硝酸根離子在有霧的時候濃度顯著上升，顯示氣態硝酸在高濕度及有足夠銨離子的情況下，會被中和於氣膠之中。同位素分析的結果顯示，質量加權平均後的銨根δ15N日平均值介於+3.7至+16.3‰之間，硝酸根的δ15N則在+1.5至+5.2‰之間，推測氣膠粒子中的銨根與硝酸根主要來自於人為源排放；而硝酸根的δ18O則是在+70至+80‰間，顯示硝酸根主要是藉由臭氧氧化而來。此研究顯示了都會地區人為汙染物的排放可能經過大氣局部環流傳送至溪頭森林，且霧的形成可能會導致氣膠的化學組成發生改變。

A field campaign in Xitou Experimental Forest of National Taiwan University (23.40°N, 120.47°E, 1,178 m asl) from December 1st to 24th in 2018 was conducted to investigate the aerosol composition, interaction between local circulation and aerosol physical-chemical properties, and to identify the probable sources of aerosol in a rural site in central Taiwan. The single hygroscopicity parameter, κ, of aerosols was derived from the measurements using a cloud condensation nuclei counter (CCNc), an ultrafine condensation particle counter (UCPC) and a scanning mobility particle sizer (SMPS). The filter samples collected using a multi-orifice uniform deposit impactor (MOUDI) were quantified off-line using a Fourier transform infrared spectroscopy with an attenuated total reflection gadget (FTIR-ATR) for selected functional groups (NH4+, SO42-, NO3-, and black carbon, BC). The δ15N of particulate NH4+ (p-NH4+), particulate NO2- or NO3- (p-NOx-) and the δ18O of NO3- was analyzed by an isotopic ratio mass spectroscopy (IR-MS) to infer the source and chemical pathway of aerosols. From CCNc analysis, κ can only be derived for the particle diameter <120 nm and showed a strong time dependence likely due to the sources and the chemical processes. Mountain-valley circulation makes the concentration of aerosol functional groups higher in the daytime than that in the nighttime. Higher concentration of NH4+, SO42-, NO3-, and BC was observed in foggy period, which might be caused by the stronger boundary layer inversion and weaker upward turbulent mixing during the foggy period. Higher NO3- concentration at diameter of 0.56-1 µm during foggy periods was observed, which is mostly caused by the partitioning of gaseous HNO3 and neutralized by NH4+, forming relative stable particulate NH4NO3. The ambient size distribution for particles containing NH4+ and SO42- shifted to larger mode during the foggy period and was caused by the hygroscopic growth of aerosols under higher relative humidity (RH) condition. The daily mass-weighted average of p-NH4+ δ15N values ranged from +3.7‰ to +16.3‰ and δ15N of p-NOx- from +1.5‰ to +5.2‰ is likely caused by anthropogenic sources such as traffic emission; and the δ18O of p-NOx- is +70‰ to +80‰, indicating that the oxidation of NOx by O3 is the main chemical pathway to form p-NOx-. The results show that local circulation transporting urban pollutants plays an important role in Xitou forest, and fog formation might further change the chemical composition of the aerosol.