Optical characteristics of brown carbon in the atmospheric particulate matter of Dhaka, Bangladesh: Analysis of solvent effects and chromophore identification

The prevalence of brown carbon (BrC) in the atmosphere has experienced a notable upsurge owing to human activities of anthropogenic origin. This study aims to examine the optical characteristics of BrC in both deionized (DI) water and organic solvents (OS), alongside the identification of BrC chromophores within the ambient atmosphere of Dhaka, Bangladesh. Particulate matter (PM) samples were collected on quartz filters using a low-volume sampler from December 2021 to May 2022 at Mukarram Hussain Khundker Bhaban, University of Dhaka. The concentration of BrC was measured using soot analyzer, optical properties of BrC were determined using UV-Vis spectrometer, and BrC chromophores were identified with GC-MS. Average concentration of BrC was 19.13 ± 5.71 μgm^-3. The average of absorption coefficient (b_{abs_365}), mass absorption efficiency (MAE), absorption angstrom exponent (AAE), and refractive index (k_{abs_365}) of BrC_DI have been observed to be 38.75 ± 21.90 Mm^-1, 2.16 ± 1.42 m² g^-1, 1.51 ± 0.08, 0.06 ± 0.04, respectively. The absorption coefficient and MAE of BrC_OS are 1.3 and 1.36 times, respectively higher than that of BrC_DI. Thirty chromophores of BrC have been identified, predominantly consisting of oxygenated compounds. Derivatives of Bisphenol A (C₂₇H₄₄O₂Si₂) were detected in all samples of oxygenated compounds, primarily originating from the combustion of plastic and the incineration of e-waste. Additionally, compounds containing nitrogen and sulfur, such as C₁₄H₂₆N₂O, C₃₁H₅₅N, and C₃₁H₄₉NO₃S, were identified, largely attributed to biomass combustion and traffic emissions. These chromophores play a significant role in the absorption of solar radiation, thus influencing atmospheric photochemistry.