| dc.description.abstract | 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.
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