Long-Term Variation Study of Fine-Mode Particle Size and Regional Characteristics Using AERONET Data

To identify the long-term trend of particle size variation, we analyzed aerosol optical depth (AOD, tau) separated as dust (tau(D)) and coarse-(tau(PC)) and fine-pollution particles (tau(PF)) depending on emission sources and size. Angstrom exponent values are also identified separately as total and fine-mode particles (alpha(T) and alpha(PF)). We checked these trends in various ways; (1) first-order linear regression analysis of the annual average values, (2) percent variation using the slope of linear regression method, and (3) a reliability analysis using the Mann-Kendall (MK) test. We selected 17 AERONET sun/sky radiometer sites classified into six regions, i.e., Europe, North Africa, the Middle East, India, Southeast Asia, and Northeast Asia. Although there were regional differences, tau decreased in Europe and Asian regions and increased in the Middle East, India, and North Africa. Values of tau(PC) and tau(PF), show that aerosol loading caused by non-dust aerosols decreased in Europe and Asia and increased in India. In particular, tau(PF) considerably decreased in Europe and Northeast Asia (95% confidential levels in MK-test), and tau(PC) decreased in Northeast Asia (Z-values for Seoul and Osaka are 2.955 and 2.306, respectively, statistically significant if vertical bar z vertical bar >= 1.96). The decrease in tau(PC) seems to be because of the reduction of primary and anthropogeznic emissions from regulation by air quality policies. The meaningful result in this paper is that the particle size became smaller, as seen by values of alpha(T) that decreased by -3.30 to -30.47% in Europe, North Africa, and the Middle East because alpha(T) provides information on the particle size. Particle size on average became smaller over India and Asian regions considered in our study due to the decrease in coarse particles. In particular, an increase of alpha(PF) in most areas shows the probability that the average particle size of fine-mode aerosols became smaller in recent years. We presumed the cause of the increase in alpha(T) is because relatively large-sized fine-mode particles were eliminated due to air quality policies.