Light absorption properties of brown carbon from biomass burning emissions.

A laboratory-scale experiment was conducted to determine the light absorption properties of brown carbon (BrC) produced from the incomplete combustion of 14 different biomasses. Particulate matters (PM) emitted from biomass burning were collected on the quartz fiber filters with a low volume sampler. BrC from filter samples was extracted with two different solvents (methanol and water), and absorption characteristics of BrC were determined using a UV-Vis spectrophotometer. The absorption coefficient (b), mass absorption efficiency (MAE), absorption angstrom exponent (AAE), and absorbing portion of refractive index (k) were calculated for each biomass from the absorbance of the extracted solution. Methanol-soluble BrC (MeS-BrC) showed higher absorbance than water-soluble BrC (WS-BrC) in all biomasses. MeS-BrC has higher b than WS-BrC, suggesting that the rate of light absorption on BrC extracted in methanol was higher. The absorption coefficients (b) were varied among biomasses-rain tree had the highest value of b, whereas jute stick had the lowest. The mass absorption efficiency of BrC (MAE) was evaluated in both water and methanol extracts, and it was found that the MAE for MeS-BrC in the biomasses was greater than that of WS-BrC. The highest MAE value (13.02 mg) was identified in the jackfruit tree, whereas the lowest MAE value (0.1 mg) was observed in the jute stick. The absorption angstrom exponent (AAE) of both WS-BrC and MeS-BrC was determined which represents the light absorption capacity of the aerosol particles. The highest AAE value was found in cow dung, and the lowest was found in rain tree. The increasing pH of the WS-BrC solution increased its optical absorption. However, this study revealed that the light absorption properties of brown carbon emitted from commonly used biomasses were varied significantly.