Investigation of the relationship between the fine mode fraction and Ångström exponent: Cases in Korea

Based on long-term AERONET observations in the Korean peninsula, we evaluate how well the Ångström exponent (AE) can describe the regional aerosol size well in comparison to the fine mode fraction (FMF). Despite close relationships between AE and FMF previously reported, we found that AE is not always linearly correlated to the FMF in the Korean peninsula, particularly during the summertime. Seasonal patterns of volume size distribution indicate that the volume median and peak radius of the fine mode are obviously enhanced in the summer. Since the ambient conditions in Korean summer are highly humid, cloudy, and stagnant, the size of regional fine mode aerosol can increase significantly due to hygroscopic growth and/or cloud processing in this highly polluted atmosphere. In contrast to the AE, the AE ratio (AE at longer wavelengths divided by AE at shorter wavelengths; AER) correlates better with the FMF variation in summer, but a little worse in the spring. This suggests that the combined consideration of AE and AER may provide a better characterization relative to FMF. Thus, we try to estimate the FMF based on the multiple linear regression method using both AE and AER. As expected, the estimated FMF shows remarkably high linear correlations with the AERONET FMF without any seasonal bias. These findings imply that the FMF in South Korea can be simply obtained if just the AOT measurement at multiple wavelengths (in the range from ultraviolet to near-infrared channels) are available.