都市氣膠成為雲凝結核之特性探討

Abstract

氣膠的物理化學特性在大氣系統中扮演十分重要的角色，其特性會影響雲凝結核的形成，更進一步可改變地球的輻射收支平衡。在本研究中，為了研究都市氣膠活化為雲凝結核之影響，於2010年10月15日至11月01日期間在台北地區進行觀測。觀測期間，雲凝結核數量濃度主要是利用連續氣流式雲凝結核計數器在過飽度0.1至1%之範圍進行監測，凝結核的數量濃度則是以凝結核計數器量測。 由觀測資料顯示，凝結核數量濃度(NCN)與氣態汙染物有很好的相關性，表示台北的氣膠主要是人為來源所貢獻，像是廚房油煙以及機動車等排放。NCN範圍在 2000-50000 個/cm3，且在研究期間NCN有明顯的日夜變化，是因為白天受到本地汙染物的排放導致有較高值的發生。在過飽和度0.1至1%時，雲凝結核數量濃度(NCCN)為 500-7000 個/cm3，且NCCN會受到大氣中NCN變化之影響。 在本研究中，由氣膠粒徑分布以及量測的凝結核(CN)與雲凝結核(CCN)資料可推估氣膠成為雲凝結核之最小粒徑(Dss)，並進一步探討CCN的特性。在風速較大的條件下，平均的Dss及吸濕性參數(κ)分別為60 - 220 nm和0.05 - 0.15；風速較弱時，平均的Dss及κ分別為90 - 250 nm和0.01 - 0.1。結果說明，在風速強的條件下可稀釋本地的汙染物及傳送外來的老化氣膠，使得大氣氣膠具有較多的水溶性物質並較易形成雲凝結核。然而，κ值也顯示在風速較弱的情況時有較明顯的日夜循環，可能是因為人為活動影響晚上的氧化過程。因此，相較於本地所排放的初始氣膠，老化氣膠會傾向有較高的κ值，也就是較易成為雲凝結核。

The physical chemical properties of aerosol particles play a significant role in atmospheric system by affecting cloud condensation nuclei (CCN) activity and further changing radiative energy budgets on the Earth. In this study, how the urban ambient aerosols affecting the CCN activity was investigated over the period of 15 October to 01 November in Taipei, Taiwan. The CCN concentration were monitored using a continuous flow cloud condensation nuclei counter (CCNc, DMT) at supersaturation (SS) ranging from 0.1% to 1% and the condensation nuclei (CN) concentration were measured using a condensation particles counter (CPC, TSI). The in-situ observation shows that the strong correlation between the CN number concentration (NCN) vs. gas pollutants and suggested that the Taipei ambient aerosols are mainly contributed by anthropogenic sources such as cooking and vehicle emission. The NCN is ranged from 2000 to 50000 particles / cm3 and show a significant diurnal variation due to the local emission leads to higher NCN at daytime during this study. The CCN number concentration (NCCN) is ranged from 5000 to 7000 particles / cm3 at 0.1-1.0% supersaturation(SS) and is strongly dependent on NCN. In this study, with the particle size distribution and measured CN and CCN data, the required minimum particle diameter (Dss) acting as CCN was estimated and further investigated the CCN properties. For the strong wind condition, the average value of Dss is 60 to 220 nm (at 0.1-1.0% SS) and hygroscopicity parameter (κ) ~ 0.05 to 0.15. During the weak wind event, the average value of Dss is 90 to 250 nm (at 0.1-1.0% SS) and κ ~ 0.01 to 0.1, respectively. The results suggest that ambient aerosols have more hygroscopic species fraction and easily form to CCN possibly due to the dilution of freshly local emission by the inflow wind containing long range transported aged aerosols during the strong wind condition. However, the κ values also show a significant diurnal cycle as the wind speed was weak possibly due to the continued night oxidation process with less significantly interference caused by human activities. The aged particles tend to have higher κ values as compared to the freshly emitted local aerosols.