台灣中部之季節性粒徑分布與新微粒生成

Abstract

大氣中的氣膠由各種粒徑範圍的微粒組成，而不同粒徑範圍的氣膠具有不同的影響。微粒粒徑分布 (Particle Size Distribution, PSD) 描述了不同粒徑範圍內微粒濃度的變化。新微粒生成 (New Particle Formation, NPF) 為氣體前驅物轉變為微粒的過程，是氣膠二次生成的重要過程之一，會導致成核模 (< 25 nm) 微粒數目濃度大幅增加，從而劇烈改變PSD，這一現象對全球氣候及人體健康產生重大影響。由於NPF在不同季節和地區的發生頻率和強度不同，本研究於臺灣臺中地區架設了IMPACT監測站，利用電移動度粒徑分佈掃描儀 (SMPS) 及氣動粒徑分布儀 (APS) 對從11.8奈米至20微米的粒徑分布進行為期一年半的監測，以探討不同季節間PSD的特徵及變化，同時利用Ranking Analysis的方法針對臺灣本土的NPF現象進行全面性的討論，以解析其發生條件、頻率及影響。 結果顯示，四季的PSD受氣象條件影響，在春季及冬季觀察到較高的表面積濃度及體積濃度，而這兩種濃度皆由積累模 (100-1000 nm) 粒子主導。春季的低風速及冬季的低邊界層高度不利於大氣擴散，使污染物容易累積，影響空氣品質並導致能見度劣化。此外，春冬兩季午間因光化學作用的加強，微粒發生二次生成，導致更高的PM1.0質量濃度。除了本地污染外，冬季強勁的東北季風還帶來了傳輸污染物的影響，使冬季近六成的時間段被定義為PM1.0污染事件 (PM1.0 > 17.96 μg/m³)。當PM1.0污染事件發生時，觀察到了硝酸鹽的二次生成現象。 夏季是質量濃度相對低的季節，但卻發現了頻繁的NPF現象。將NPF分為十個不同強度以進行條件比較，並將強度前20% 的觀測日定義為Strong-NPF，其中夏季有高達27~50%的時間被定義為Strong-NPF。比較結果顯示，夏季較高的太陽輻射和溫度皆為影響NPF的重要氣象因素，同時，夏季較低的質量濃度也有利於NPF的發生，使夏季成為臺灣NPF的好發季節。NPF多於早晨6:00開始，隨著太陽輻射及溫度的上升，成核模數目濃度大幅增加且微粒持續增長至艾肯模 (25-100 nm)。此時凝結速率 (Condensation Sink, CS) 與PM1.0質量濃度皆持續增加，NPF的現象會持續至中午12:00後，並隨NPF強度增加而延長。當CS12-25小於1.71×10-4 1/s 時，容易發生高強度的NPF。 由於NPF多發生於較乾淨的環境中，PM1.0的些微上升對現階段空氣品質的影響有限，但NPF期間大幅上升的成核模數目濃度會增加氣膠對健康的威脅。最後，透過NPF的案例分析揭示了二氧化硫 (SO2) 及臭氧 (O3) 間反應產生硫酸 (H2SO4) 可能為NPF的主要化學反應機制。

Atmospheric aerosols consist of particles across a wide range of sizes, each having distinct impacts. Particle Size Distribution (PSD) describes the variation in particle concentration across different size ranges. New Particle Formation (NPF) is the process by which gaseous precursors convert into particles, representing a crucial mechanism in secondary aerosol formation. This process significantly increases the number concentration of nucleation mode particles (< 25 nm), dramatically altering the PSD, which in turn has significant implications for global climate and human health. Since the frequency and intensity of NPF varies with season and region, this study involved the IMPACT monitoring station in Taichung, Taiwan, which used a Scanning Mobility Particle Sizer (SMPS) and an Aerodynamic Particle Sizer (APS) to conduct an 18-month monitoring campaign of particle size distributions ranging from 11.8 nanometers to 20 micrometers. The study aims to explore the seasonal characteristics and variations of PSD and to comprehensively discuss the local NPF phenomena in Taiwan using ranking analysis to elucidate their conditions of occurrence, frequency, and impacts. The results indicate that seasonal PSD is influenced by meteorological conditions, with higher surface area and volume concentrations observed in spring and winter, primarily driven by accumulation mode (100-1000 nm) particles. The low wind speeds in spring and low boundary layer heights in winter hinder atmospheric dispersion, leading to the accumulation of pollutants, which severely affects air quality and reduces visibility. Additionally, intensified photochemical reactions during the midday in spring and winter led to secondary particle formation, resulting in higher PM1.0 mass concentrations. Besides local pollution, the strong northeast monsoon in winter also contributed to pollution transport, resulting in almost 60% of the winter period being defined as PM1.0 pollution events (PM1.0 > 17.96 μg/m3). During these PM1.0 pollution events, secondary nitrate growth was also observed. Summer had relatively low mass concentrations but frequent NPF events. NPF was classified into ten intensity levels for conditional comparison, with the top 20% of observation days defined as Strong-NPF. In summer, 27-50% of the time was classified as Strong-NPF. The comparison indicates that higher solar radiation and temperatures in summer are significant meteorological factors influencing NPF. Additionally, the lower mass concentrations in summer are conducive to NPF, making summer the peak season for NPF in Taiwan. NPF typically starts around 6:00 A.M., with nucleation mode number concentrations increasing sharply with increasing solar radiation and temperature and particles growing into the Aitken mode (25-100 nm). During this time, condensation sink (CS) and PM1.0 mass concentrations increased, with NPF phenomena continuing past noon and extending with higher NPF intensities. High-intensity NPF was more likely to occur when CS12-25 was below 1.71×10⁻4 1/s. Since NPF frequently occurs in cleaner environments, the slight rise in PM1.0 has a limited impact on current air quality. However, the significant rise in nucleation mode particle number concentration during NPF increases the health threat posed by aerosols. Finally, case studies on NPF have shown that the reaction between SO2 and O3 to form H2SO4 may be a primary mechanism driving NPF.