| Summary: | 碩士 === 國立中央大學 === 大氣科學學系 === 107 === Aerosol chemical components (i.e., black carbon (BC), brown carbon (BrC)) regulate the optical properties of aerosol, which are highly sensitive to atmospheric radiative forcing estimation. Previously, through an intensive international field experiment, i.e. The 7 SEAS project, the complex aerosol environment of Indochina was investigated. However, the role of absorbing aerosols during biomass burning season on the environment is still unclear. In this paper, the long-term cimel sunphotometer AERONET (Aerosol Robotic Network) measurements and its inversion products from 6 sites in Indochina have been studied to understand the seasonal and spatial characteristic differences in aerosol optical properties and the mixture of BC with other aerosol components in smoke haze.
The long-term data analysis revealed that the monthly mean aerosol optical depth (AOD) was higher in the months of March and April than the rest of months observed by all AERONET sites in the region, inferring the influence of regional biomass-burning activities. From the aerosol size distribution, the number of small particles was even larger than large particles during dry period. Due to the spatial differences in geographic and land use patterns, the AERONET sites in northern Indochina show a higher AOD compared to that of in southern Indochina. In comparison to the yearly-mean aerosol optical properties, a lower single-scattering albedo (SSA) and higher fine-mode fraction (FMF) values in February and March, suggested the domination of smaller and stronger absorbing particles during this period.
Two methods (i.e., aerosol type cluster and aerosol component retrieval) had been applied to determine the aerosol type and chemical components during the biomass-burning season. The correlation coefficient between component retrieval and chemical sampling was 0.6 at DAK site. For Chiang Mai site (northern Indochina), the cluster method revealed biomass-burning aerosol type in February and March, when a large fraction of absorbing aerosols (BC 5% and BrC 40%) were also observed according to the result from aerosol component retrieval. It is worth to mention that the peak of biomass burning month (i.e. March) in northern Indochina may not necessary with high BC fraction (i.e., lowest SSA) once dust particles are mixed. For southern Indochina (Bac Lieu site), a mixed aerosol type with low absorbance was determined.
Furthermore, we categorized the days into event days and non-event days based on AOD value during biomass-burning months (March in 2014 and 2015). As a result, the tendency of aerosol optical properties between DAK and CM site showed similar pattern, which suggested that aerosol spatial distribution was more homogeneous during non-event days. As contrast, AOD and AE tendency were different at DAK and CM site during event days. We suggested that the peak AOD value of event day at CM site was due to aerosol accumulation from previous day. For DAK site, the peak AOD value of event day happened in the afternoon which may due to nearby source region. The difference of geographic location and altitude between two sites cause the inhomogeneity of aerosol optical properties over the region. For the BC and BrC fractions during even day, BC fraction showed higher value while BrC fraction showed lower for both CM and DAK sites. This results of this study will help us to understand the seasonal variability of aerosol optical properties, the aerosol composition, and the event day and non-event day of biomass-burning month over Indochina peninsula. The results will also be useful in future field experiment planning, model/satellite retrievals evaluations, and aerosol radiative forcing estimation.
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