| dc.description.abstract | 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. | en_US |