| dc.description.abstract | To investigate the effect of PM mass concentration and composition characteristics on visibility at different time scales and particle sizes. PM mass concentration, chemical composition, and meteorological conditions were measured in Tunghai University from October 2017 to August 2019. Tapered element oscillating microbalance (TEOM) and β-attenuation monitor (BAM) were used to measure the after heating and ambient cases of particulate matter in real-time. The carbonaceous species in PM2.5 were measured using a aethalometer (AE33) and the semi-continuous OC-EC field analyzer. Water-soluble inorganic ion components were measured with an online gas and aerosol component monitor.
The results revealed that the average mass concentrations of PM1 and PM2.5 in after heating cases are 13.4 µg/m3 and 24.2 µg/m3, respectively. The average mass concentrations of ambient PM1 and PM2.5 are 18.9 µg/m3 and 28.1 µg/m3, respectively. The mass concentration of PM2.5 has significantly decreased. However, PM1 has a slower decreasing tendency than PM2.5 during the observation period. A strong correlation (R2=0.88) is found between the measured and reconstructed PM2.5 mass concentration with a slope of 0.89, indicating that the total mass of major chemical components in PM2.5 could closely represent the measured PM2.5. Water-soluble inorganic ions (NO3-, SO42-, NH4+) and carbonaceous species (OC, EC) are the main components of PM2.5. The mass concentrations of OC, NO3-, and NH4+ in PM2.5 are higher in winter and spring, and lower in summer. Compared with the sources of traffic pollution during the summer, there may be more secondary organic matter formation, and more likely from the southerly or northwesterly winds. The sources of southerly wind probably come from the industrial zone. Pollutants in industrial zone may contain volatile organic substances, which can be used as precursors to form SOC. When visibility is degrading, the water-soluble inorganic ions account for about 40 to 50% of PM2.5 and carbonaceous species account for about 10 to 20% of PM2.5. Nitrate may increase significantly in low visibility, and ozone and relative humidity may affect the formation of nitrate.
According to the above results, simultaneous changes in chemical composition and mass concentration affect visibility, but the changes in particle size may offset the benefit of reduction. Thus, it is still necessary to consider the chemically resolved aerosol size distributions. In terms of reduction and improvement strategies, it can not only focus on the reduction of sulfate, but still need to strengthen the reduction of nitrate.
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