Chemical composition and light absorption of brown carbon emitted from residential coal combustion in China

Brown carbon (BrC) is a type of light-absorbing organic compounds with a high capacity to absorb light in the low-wavelength visible and near-ultraviolet regions, which is ubiquitous in atmospheric aerosols, rainwater, and cloudwater samples. BrC can not only alter the light absorption and radiative forcing of aerosols but can also influence the formation of cloud condensation nuclei; therefore, it has a potential impact on atmospheric chemistry and climate change. Numerous studies have demonstrated that combustion processes are significant sources of atmospheric BrC, however most of these studies were focused on the emissions of biomass burning. Knowledge of primary BrC from coal combustion is still limited. In the study, smoke particles emitted from the combustion of residential coals with different geological maturity were collected in a combustion system. Then BrC fractions, including water soluble organic carbon (WSOC), water soluble humic-like substances (HULIS_w), alkaline soluble organic carbon (ASOC) and methanol soluble organic carbon (MSOC) were extracted and characterized for their abundances, chemical, and light absorption properties.

 

Our results showed that the abundance and light absorption of the coal combustion-derived BrC fractions were strongly dependent on the extraction methods used and the coal maturity. The abundances of MSOC fraction was significantly higher than WSOC and ASOC fractions and even higher than the sum of WSOC and ASOC, indicating that most organic compounds in smoke particles were soluble in pure methanol. The WSOC and MSOC fractions from the combustion of low maturity coal had relatively low SUVA₂₅₄ and MAE₃₆₅ values, indicated that they had relatively low levels of aromatic structures and light absorption.

 

The WSOC and MSOC fractions were characterized by ultrahigh-resolution mass spectrometry. The results showed that S-containing compounds (CHOS and CHONS) are found to be the dominant components of the WSOC, whereas CHO and CHON compounds make a great contribution to the MSOC samples. Noted that a greater abundance of S-containing compounds was found in the smoke produced from coal combustion compared to biomass burning and atmospheric samples, indicated that coal combustion could be an important source of atmospheric S-containing compounds in certain areas. The findings also suggest that organic molecules with a high aromaticity index and low polarity showed stronger light absorption. In summary, our study indicated that coal combustion is a potential source of atmospheric BrC and their abundance, chemical, and light absorption were strongly dependent on the extraction methods used and the coal maturity.