| dc.description.abstract | The purpose of this study is to study the vertical optical properties of aerosols observed at Mt. Lulin Atmospheric Background Station (2862 m; 23.47°N, 120.87°E) from April 2006 to December 2008 with simultaneous measurements with a multi-filter rotating shadowband radiometer (MFRSR) and a Cimel’s sunphotometer (CIMELs). Solar direct flux data of MFRSR measurements were retrieved to obtain aerosol optical depth (AOD500nm), based on Krotkov et al. (2005a). The AOD500nm, CO and PM10 were relatively high in spring (March-May), due to the impact of biomass burning from Southeast Asia. The variation of monthly mean AOD500nm and PM10 was similar, with the maximum values occurring in March. Monthly mean Ångström exponent values were lower (MFRSR:0.35-0.76, CIMELs:0.73-0.91) between May and August and higher between December and March (MFRSR:1.15-2.14, CIMELs:1.63-2.24). However, relative humidity and columnar water vapor showed an opposite trend. Besides, the AOD500nm increased in the afternoon, as well as the PM10. The Ångström exponent significantly decreased in the afternoon, but relative humidity gradually increased.
AOD500nm and PM10 had a better correlation, relative high-pressure weather conditions under particularly in winter (R:0.82-0.86), while a poor correlation in the summer and autumn. The HYSPLIT trajectory analysis helped classify air mass sources. The minimum AOD500nm was associated with the air mass from the ocean, while maximum value was associated with the air mass from Southeast Asia. The minimum and maximun Ångström exponent were associated with the air mass from the ocean and from the high-level, respectively. The AOD500nm evidently increased during the dust and biomass burning events, and was about 2.7 times and 18 times that in maritime air mass. Based on particle size distribution and Ångström exponent, the dust aerosol had a large mode, while biomass burning aerosol had a fine mode. The ratio of direct flux to diffuse flux in the dust event, biomass burning, and background were 7.42, 1.41, and 15.32, respectively, indicating that the largest vertical column as aerosol loading appeared in the biomass burning event, while the smallest occurred in the clean background air.
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