Light Absorption Characteristics of Brown Carbon and Black Carbon from Biomass Burning and Fossil Fuel Combustion in Dhaka

Abstract

Dhaka is the most polluted city in Bangladesh. The number of transport (vehicles), burning sources, and industries are increasing day by day both in the city and rural areas. The most important pollutants are brown carbon (BrC) and black carbon (BC) which absorb light at shorter wavelengths and longer wavelengths respectively. As an important type of light-absorbing aerosol, BrC greatly impacts the Earth's radiative balance and is also thought to be a climate-forcing agent. This study investigates the optical properties, heating rate of BrC and BC, chemical functional groups, and total carbon content in the particulate matter released from biomass burning and fossil fuel combustion. Light absorption properties like mass absorption efficiency (MAE₃₆₅), absorption angstrom exponent (AAE), and refractive index (k_abs-BrC) were determined by using UV-visible spectrophotometer (only for BrC) and Aethalometer (for both BrC & BC). This study found that light absorption properties of biomass burning > fossil fuel. A greater light absorption coefficient at 365 nm or 370 nm indicated the greater contribution of the chromophore. In FTIR analysis showed that biomass and fossil fuel samples showed a strong peak at ~850, ~1360, 1370, 1306, and near 1640 cm^-1. This indicated major component of BrC was organic nitrate which is responsible for the light absorption. BrC was expected to be significantly attributed to total light absorption, demonstrating the potential climatic impact of biomass burning and fossil fuel combustion events in rural Southeast Asia.