| 摘要: | 大氣氣膠在空氣中無所不在，其可透過影響太陽輻射而改變全球氣候，或導致區域能見度的劣化，同時也可能會造成人體健康的危害，如心血管疾病、肺功能的下降、呼吸系統的刺激等。近年來，隨著社會大眾對於空氣品質的重視，PM2.5為眾人所熟知，且對於細懸浮微粒的管制是以質量濃度為主要管制標準計量單位。然而，PM (particulate matters, 粒狀污染物)並非純物質，其為複雜的化學成分及不同粒徑之氣膠微粒混合物的統稱，因此總合之質量濃度未必能完全反映其在氣膠光學、太陽輻射或健康危害風險等影響。尤其對於大氣能見度，環境氣膠微粒的粒徑分佈更可劇烈改變其影響效應。
雖然現今已有許多國家針對氣膠的粒徑分佈進行測量，但在台灣的環境氣膠粒徑分佈則較為缺乏，再加上近期台中地區的能見度劣化議題為社會所關注。因此本研究在台中市地區建置一移動測站IMPACT (Integrated Measurements of Pollution and Aerosol Composition & Transformation)，設置光學係數、質量濃度、粒徑分佈、微量氣體、重金屬元素、水溶性無機鹽離子與氣象資料等儀器同時測量。本研究主要利用掃描式電移動度分析儀(Scanning Mobility Particle Sizer, SMPS)測量粒徑範圍約10 − 1000 nm的次微米氣膠微粒粒徑分佈，及氣動粒徑分析儀(Aerodynamic Particle Sizer, APS)量測粒徑範圍約0.5 − 20 µm的細懸浮微粒粒徑分佈，進一步了解氣膠粒徑分佈與其他參數在台中地區的物化特性，並分析其在能見度劣化過程中的特徵。
研究結果顯示在總觀測期間，粒徑分布的次微米幾何平均粒徑為44.6 ± 16.9 nm，微粒平均數目濃度約為27,340 ± 17,848 (#/cm3)。微米級幾何平均粒徑約為0.79 ± 0.29 µm，微粒平均數目濃度約為103.8 ± 101.9 (#/cm3)。第二年度相較於第一年度平均粒徑偏小而數目濃度增加，總數目濃度主要受到nucleation mode的影響最劇烈。由風花圖的結果發現當南風風向且風速介於2 − 3 m/s時幾何平均粒徑較大，總數目濃度以西北風向上濃度最高。在能見度劣化時，影響能見度最主要的粒徑落在約100 nm與400 nm左右，初步推估小粒徑(100 nm)的成份主要由有機物所主導而較大粒徑(300 nm)的成份主要由硫酸鹽或硝酸鹽所主導。氣象條件的變化在能見度劣化的情況下以凌晨及早上時段相對濕度較高，約為85 − 95%。相對地，GMD在事件期的變化受到夜間NOx的異相反應及相對濕度的影響導致微粒增長。而中午發生較低風速且高溫的條件下，使得微粒數目濃度稀釋而減少。這些氣象條件或可視為提供能見度劣化的適當環境條件，並可作為預警的指標。;Aerosol particles are ubiquitous in atmosphere and can influence global climate and regional visibility by scattering and absorbing solar radiation. Inhalation exposure to aerosol particles could lead to adverse health effects, including respiratory irritation, changes in pulmonary function, etc. In recent years, PM2.5 is well known with the public’s emphasis on air quality. Current air quality standards of particulate matters (such as PM2.5 and PM10 standards) are based on particle mass concentrations, however, PM is a collective name for a complex chemical composition and a mixture of aerosol particles with different particle sizes. Therefore, the total mass concentration may not fully reflect the impacts on solar radiation or health risks. Especially for atmospheric visibility, particle size distribution of ambient aerosol particles can play a vital role.
Monitoring particle size distributions (PSDs) of fine and ultrafine particles in ambient environment is a challenging task but becoming regular for air quality monitoring stations worldwide. Nevertheless, the related monitoring data in Taiwan is scarce. In addition, visibility impairment in middle Taiwan (Taichung area) is receiving lots of attention over the past years. Therefore, a mobile station “IMPACT” was established in Taichung area for this study. The optical coefficient, mass concentration, particle size distribution, trace gases, heavy metal elements, water – soluble inorganic ions and meteorological data were measured simultaneously from 2017 Sep. to 2019 Aug. Scanning Mobility Particle Sizer (SMPS) and Aerodynamic Particle Sizer (APS) are employed to measure ambient aerosol PSDs ranging from 10 nm – 1000 nm and 0.5 µm – 20 µm, respectively. The results show that during the observation period, the average geometric mean diameter (GMD) of the submicron PSD is 44.6 ± 16.9 nm, and the average particle number concentration is about 27,340 ± 17,848 (#/cm3). The average GMD and the average particle number concentration of fine particle is 0.79 ± 0.29 µm and 103.8 ± 101.9 (#/cm3), respectively. Compared with the first year, the average GMD in the second year was smaller but the number concentration increased. The increasing number concentration mainly resides in the nucleation mode. Based on the wind-roses analysis, the larger aerosol particle size was generally observed when southern wind at about 2 − 3 m/s, while the highest total number concentration was accompanied with northwest winds. When visibility is degrading, the number of aerosol particles with size between 100 to 400 nm are always demonstrating a boosting growth and the growth ratio distribution behave twin-mode with peaks around 100 nm and 300 nm. It is suspected that the smaller peak growth (at around 100 nm) is caused by secondary organics generation and the larger peak growth (at around 300 nm) is due to secondary sulfate or nitrate coating. |
