| 摘要: | 本文於2021年12月在中央研究院臺中都市空氣污染研究站（都市測站）及2022年2月至3月在鹿林山空氣品質背景測站（鹿林山測站）量測氣膠水溶性無機離子（Water-Soluble Inorganic Ions, WSIIs）實時動態變化，探討PM2.5質量濃度上升時段WSIIs變化趨勢及來源，並連結氣膠粒徑分布及光學特性進行討論。
2021年冬季都市測站發生兩次濃度上升事件，PM2.5小時濃度峰值分別為38 μg m-3及61 μg m-3，主要WSIIs（SO42-、NO3-、NH4+）濃度都有明顯上升，日間NH4NO3形成與夜間N2O5水解是NO3-的主要來源，SO42-與NH4+相關性較低，SO42-可能是經由SO2與OH·氧化反應形成。都市測站NO2-濃度上升，受到相對濕度(RH)影響，NOx的液相反應第一次比第二次事件有更顯著的影響，同時也發現兩次事件液相反應都是NO2-主要形成途徑。NO3-氣膠主導都市測站氣膠對太陽輻射的散光能力，都市測站NO3-和SO42-與綠光散光係數相關性高，搭配前驅氣體與CO濃度的變化趨勢，顯示受到化石燃料燃燒和移動污染源排放的影響。本文篩選不同RH，驗證氣膠散光係數隨著RH增加而上升。
2022年春季鹿林山測站偵測到五次生質燃燒（Biomass Burning, BB）氣團傳輸，每次BB事件都能觀察到PM2.5濃度、WSIIs濃度、綠光吸、散光係數的上升。值得注意的是，BB事件期間14:00 – 18:00常觀察到谷風，WSIIs濃度上升伴隨大氣能見度明顯下降;相對濕度上升期間，氣膠粒徑吸濕增大，體積眾數粒徑集中在300 – 400 nm，NO3-與PM1綠光散光係數有顯著中等相關，說明谷風將山下污染氣團傳輸至測站，氣膠吸濕潮解，影響大氣能見度。BB事件期間，氣膠數目眾數粒徑集中在100 – 200 nm粒徑區間，谷風時段則集中在60 – 80 nm粒徑區間，顯示谷風較BB事件期間氣膠數目集中在更細粒徑。在鹿林山測站SO42-與PM1綠光散光係數相關係數較NO3-高，表示大氣背景觀測站受到區域化石燃料燃燒氣膠影響顯著。
總結來說，都市測站PM2.5來源以移動污染源為主，濃度上升期間可能還受到遠處固定污染源傳輸影響，氣膠以細粒徑散光成分為主，RH上升導致氣膠潮解使綠光散光係數明顯上升。高山測站在BB事件期間，PM2.5質量濃度上升，SO42-濃度與都市測站幾乎相同，谷風期間氣流的高RH影響氣膠化學成分及光學特性。
;This study investigates the real-time variations of aerosol Water-Soluble Inorganic Ions (WSIIs) measured at the Urban Air Pollution Research Station (urban station) of the Academia Sinica in December 2021 and at Lulin Atmospheric Background Station (LABS; 2,862 m above mean sea level) from February to March 2022. The study aims to explore the WSIIs variations and sources during PM2.5 concentration rising periods and to discuss their relationships with aerosol size distributions and optical properties.
During the winter of 2021, the urban station experienced two events of PM2.5 concentration rise, with peak hourly concentrations of 38 μg m-3 and 61 μg m-3, respectively. The concentrations of major WSIIs (SO42-, NO3-, and NH4+) showed a significant increase. The formation of NH4NO3 in the daytime and hydrolysis of N2O5 during the nighttime were the two primary pathways of NO3-. There was a weak correlation between SO42- and NH4+, suggesting that SO42- may have been generated through the oxidation reaction of SO2 and OH·. The concentration rise of NO2- at the urban station was influenced by relative humidity (RH) with a greater impact during the first than the second event for the liquid phase reaction of NOx. Moreover, NO2- formation for both events was primarily through liquid phase reactions. At the urban station, aerosol NO3- dominated solar radiation scattering. The correlation between NO3- and SO42- at the station was high for the green light-scattering coefficient, suggesting the influences of both fossil fuel combustion and mobile source emissions. Aerosol deliquescence increased its light scattering coefficient, verified by varying RH levels in the study.
During the spring of 2022, the LABS detected five events of air mass transportation resulting from Biomass Burning (BB). Each BB event was associated with increases in PM2.5 concentration, WSIIs concentration, green light-absorption, and -scattering coefficient. Note that valley wind was often observed from 14:00 to 18:00 during the BB event. This leads to an increase in WSIIs concentration and a significant reduction in atmospheric visibility. As the RH rises, the aerosol size increases due to hygroscopic growth to result in the modal diameter of aerosol volume concentration distributed at 300 – 400 nm. There is a significant moderate correlation between NO3- and the green light-scattering coefficient of PM1, indicating that valley wind transport polluted air masses from below the mountain to the measuring station. As the aerosol absorbs moisture and deliquesces, it affects atmospheric visibility. During the BB event, the modal diameter of the aerosol number concentration was distributed in the 100 – 200 nm size range, whereas during the period of valley wind, it was accumulated in the 60 – 80 nm size range. The results indicate that valley winds result in aerosol number concentration being accumulated in smaller particle sizes than during the BB event. At LABS, the correlation coefficient between SO42- and the green light-scattering coefficient of PM1 is higher than that of NO3-, indicating that the LABS is significantly affected by regional fossil fuel combustion aerosols.
In summary, PM2.5 at the urban station is mainly derived from mobile emission sources, and during periods of concentration increase, it may also be influenced by distant transport of stationary sources. The aerosols are mainly composed of fine scattering components, and aerosol deliquescence caused by the RH increase leads to a significant enhancement in the green light-scattering coefficient. In contrast, the PM2.5 mass concentration at LABS increases, and the SO42- concentration is almost the same as at the urban station during the BB periods. The high RH of the valley wind affects the chemical composition and optical properties of the aerosols. |
