Optical properties of brown carbon aerosols in Dhaka’s atmosphere: evaluating climate forcing potential

Brown carbon (BrC) has a substantial impact on the earth’s radiative stability and is considered a climate-forcing agent. The concentration and optical characteristics of BrC were analyzed in ambient particulate matter (PM) of Dhaka, Bangladesh, during November–December 2019. PM samples were collected on quartz filters using a low-volume air sampler during day and night. BrC was extracted from the filters utilizing two distinct solvents: deionized water and methanol. Mass concentration and density of BrC was calculated using an aethalometer, whereas optical properties were investigated by UV–Visible spectroscopy. At night, the average concentration of BrC was 71 ± 17 µg/m³, 1.6 times higher than the daytime concentration of 44 ± 12 µg/m³. Absorbance of methanol-soluble BrC (MeS-BrC) was higher than the water-soluble BrC (WS-BrC), since BrC was extracted more efficiently in the organic solvent. This resulted in greater values for optical parameters of MeS-BrC, compared to WS-BrC. Absorption coefficient, b_abs of BrC extracted in methanol, was 1.19 to 1.51 times higher than BrC extracted in water. MeS-BrC had more scattering capacity than WS-BrC, evident from the mass absorption efficiency (MAE) values. Absorption Angstrom exponent (AAE) of BrC in both the solvents was > 1, which indicated the presence of UV absorbing BrC in aerosols, that may be emitted from biomass burning. Higher absorbance was noticed at a greater pH and shorter wavelength for WS-BrC, indicating the deprotonation of phenolic -OH group in BrC.