| 摘要: | 每年3至4月中南半島北部山區大規模生質燃燒(Biomass Burning, BB)，燃燒煙團傳輸到東亞影響範圍甚廣。本文於2020年9~10月以及2021年3~4月在鹿林山大氣背景觀測站(2,862 m a.s.l.)觀測大氣氣膠化學成分，並結合觀測站相關監測資料，分析氣膠的光學特性。
2019年底新冠肺(COVID-19)爆發，許多國家實施鎖國政策，人為活動減少。2020年秋季背景(Background, BK)期間，PM2.5及PM10的質量濃度分別為2 ± 1及3 ± 2 μg m-3，與2019年秋季相比下降了76%及70%。2021春季，PM2.5及PM10的質量濃度分別為20 ± 9及29 ± 13 μg m-3，PM2.5占PM10 68%；PM10是近五年濃度最高的一年，不受疫情影響。2021年春季三種氣流軌跡類型PM2.5碳成分受到BB、化石燃料燃燒、烹飪排放不等的影響；採樣期間發生阿里山森林大火，OC/ EC及Char-EC/ Soot-EC比值證實受到BB影響。當大氣氣膠在NH4+/SO42-莫耳比1.5時，NO3-與NH4+大多結合成NH4NO3。
以Revised IMPROVE公式計算大氣消光係數，2020年秋季約為17.0 Mm-1，2021年春季為53.6至104.6 Mm-1；空氣分子是秋季主要影響大氣消光因子，春季則是氣膠有機物及硫酸銨。計算2019~2021年的大氣消光係數與氣膠光學厚度判定係數R2 = 0.41 (n = 36, p < 0.01)，表示大氣層氣膠成分平均組成和鹿林山測站量測結果有相關但不全然相近。2016 ~ 2021年春季原生排放有機碳(POC)推估濃度有逐漸增加的趨勢，代表境外傳輸BB煙團排放碳成分比例增高，二次形成有機碳(SOC)濃度降低。近六年春季氣膠碳成分ECR(=SOC/(POC+EC)) 吸光較散光效應強；然而，ECR值逐漸升高，顯示PM2.5碳成分對太陽輻射的吸光效應還是有逐漸減弱的趨勢。
總結來說，東亞高山地區大氣氣膠以PM2.5為主，2021年春季氣膠濃度較國際新冠肺炎疫情前高。近六年(2016-2021)春季PM2.5碳成分對太陽輻射的吸光較散光效應強，但有逐漸減弱的趨勢。
;Each year, from March to April, large-scale biomass burning (BB) occurs in the northern mountainous regions of the Indochina Peninsula, and the resulting smoke plumes can impact a wide area in East Asia. This study conducted atmospheric aerosol chemical composition measurements at the Lulin Atmospheric Background Observation Station (2,862 m a.s.l.) from September to October 2020 and March to April 2021. The study also analyzed the optical properties of the aerosols in conjunction with relevant monitoring data from the station.
At the end of 2019, the outbreak of the COVID-19 pandemic led to many countries implementing lockdown policies, resulting in reduced human activities. During the background (BK) period in autumn 2020, the mass concentrations of PM2.5 and PM10 were 2 ± 1 μg m-3 and 3 ± 2 μg m-3, respectively, representing a decrease of 76% and 70% compared to autumn 2019. In spring 2021, the mass concentrations of PM2.5 and PM10 were 20 ± 9 μg m-3 and 29 ± 13 μg m-3, with PM2.5 accounting for 68% of PM10. Despite the pandemic, PM10 concentrations in spring 2021 reached their highest level in the past five years, unaffected by the COVID-19 impact. During spring 2021, PM2.5 carbonaceous components were influenced by BB, fossil fuel combustion, and cooking emissions. The occurrence of the Alishan forest fire during the sampling period was confirmed to have affected the OC/EC and Char-EC/Soot-EC ratios, indicating the influence of BB.
When the molar ratio of NH4+/SO42- in atmospheric aerosols was approximately 1.5, most of the NO3- combined with NH4+ to form NH4NO3. The atmospheric extinction coefficients were calculated using the Revised IMPROVE formula and were approximately 17.0 Mm-1 in autumn 2020 and ranged from 53.6 to 104.6 Mm-1 in spring 2021. In autumn, air molecules were the primary factors affecting the atmospheric extinction, while in spring, organic aerosols and ammonium sulfate played significant roles. The correlation coefficient (R2) between the calculated atmospheric extinction coefficients and aerosol optical thickness from 2019 to 2021 was 0.41 (n = 36, p < 0.01), indicating a significant but not entirely consistent relationship between the average aerosol composition and measurement results at the Lulin station.
From 2016 to 2021, there was a gradual increase in the estimated concentration of primary organic carbon (POC) during spring, indicating an increasing proportion of carbonaceous components from transported BB smoke plumes, while the concentration of secondary organic carbon (SOC) decreased. Over the past six years, the ECR (= SOC/(POC+EC)) of aerosol carbon components in spring showed a stronger light-absorbing effect than scattering, but there was a trend of gradual weakening.
In summary, PM2.5 dominates the atmospheric aerosols in the East Asian high mountain regions, and the aerosol concentration in spring 2021 was higher than pre-COVID-19 pandemic levels. Over the past six years (2016-2021), the light-absorbing effect of PM2.5 carbonaceous components on solar radiation has shown a gradually weakening trend, despite the stronger light absorption compared to scattering effects initially. |
