在台灣台北都市地區之氣膠揮發性、吸濕性及有效密度之探討

Abstract

在老化的過程中，氣膠可能透過內混或外混之方式與水溶性物質或是揮發性有機物質混合。因此，揮發性、吸濕性及有效密度是相互關聯的，並且在大氣環境中會隨時間有著動態的變化。然而，這些氣膠特性尚未同時被檢視，尤其是在台灣。因此，本研究發展出即時且現地之量測技術，透過結合揮發性串聯式電移動度分徑儀、氣膠質量分析儀及雲凝結核計數器(VT-DMA-APM-CCNc)，能夠系統性且同步地監測在北台灣都市地區超細懸浮微粒(UFPs)之揮發性、吸濕性及有效密度。每日排放之氣體濃度與交通流量具有相似之行為，而這些氣體是二次無機和有機氣膠的潛在前驅物。環境條件[如：依強度區分之太陽光照度、風速及相對濕度 (降雨事件)]對於揮發性物質相分配及有效密度之影響將被檢視。此外，凝結於微粒上之高和低揮發性物質密度將會被估計。結果表明，光化學反應是太陽光照度的函數，其與氣膠老化具有密切相關。老化微粒之有效密度將會上升，而揮發性會下降。根據這些發現，低揮發性化合物被視為老化的物質且其和老化程度有關。此外，同步量測之參數包括AAE及Ks，闡明了微粒之光學性質及構型的改變。在中午，總是會發現最大值之AAE及最小值之Ks，這也暗示著微粒在中午是相對老化的，且被二次有機物質披覆並逐漸接近規則球形。此外，降雨事件之案例研究說明濕沉降造成之慣性衝擊是粗微粒(如：粒徑大小 > 1 μm)之重要去除機制之一，且PM1 和PM10之比值有顯著上升。高濕度將透過微粒形成之水膜而加速老化反應。綜合上述結果，在高強度光照、低風速及高相對濕度情況下，氣膠之揮發性、吸濕性及有效密度將於都市環境中有著快速的轉變。

During aging processes, aerosol could be internally or externally mixed with water-soluble compounds or volatile organic compounds (VOC). Thus, volatility, hygroscopicity, and effective density are intercorrelated and dynamically evolving with time in ambient environment. However, these aerosol’s properties have not been examined at the same time, especially in Taiwan. Therefore, in this study, a real-time and in-situ measurement technique, which is a hyphenated volatile tandem differential mobility analyzer, aerosol particle mass analyzer and cloud condensation nuclei counter (VT-DMA-APM-CCNc) system, is developed to systematically explore the relationship among volatility, CCN activity and effective density of ultrafine particles (UFPs) in the urban area of northern Taiwan. The daily traffic-emitted gases, the potential precursors for secondary inorganic and organic aerosols, demonstrated a similar diurnal pattern with traffic flux. The effects of ambient conditions, such as the solar irradiance classified by the intensity, wind speed and relative humidity (precipitation event), on the phase partitioning of volatile materials and effective density were investigated. In addition, the density of condensed high- and low-volatile materials would be estimated. Results showed photochemical reaction, which is a function of solar irradiance, is closely related to aerosol aging. Effective density is increased while volatility is decreased for aged particles. According to these findings, low-volatile compounds are considered to be aged material and related to the degree of aging. In addition, the synchronous measurements of AAE and Ks can also shed the lights on the changes of optical property and morphology after aging. The maximum AAE and minimum Ks were always found at noon. These imply that the particles were aged at noon and coating with secondary organic matters and closed to regular spheres. Furthermore, the case study of precipitation event shows that wet deposition is one of the most important particle removal mechanisms in the atmosphere for coarse particles (i.e. particle size > 1.0 μm) due to inertial impaction, and the ratio of PM1 to PM10 significantly increased. High RH would accelerate the aging reaction by forming water film. In summary, under high solar irradiance, low wind speed, and high RH, aerosol’s volatility, hygroscopicity and effective density would be quickly changed in urban environment.