Quantification of Brown Carbon Mass Absorption Cross Section from Sources through the Application of Physical and Mathematical Segregation of Black Carbon

Abstract:

Quantification of the black carbon (BC) and brown carbon (BrC) components of source emissions is critical to understanding the impact combustion aerosols have on atmospheric light absorption. Multiple-wavelength absorption of particulate matter emissions was measured from combustion of wood, agricultural biomass, coals, leaf litter, and petroleum distillates in controlled combustion settings. Aethalometer corrected BC absorption was segregated mathematically from the total light extinction to estimate the BrC absorption from individual sources. Results were compared to elemental carbon (EC)/organic carbon (OC) concentrations to determine composition’s impact on light absorption. The bulk carbonaceous aerosol and BrC Mass absorption cross section (MAC) were variable across source types and light wavelengths. Sources such as incense and peat emissions showed ultraviolet wavelength (370nm) BrC absorption over 175 and 80 times (respectively) the BC absorption but only 21 and 11 times (respectively) at 520nm wavelength. The bulk EC MAC_{EC, λ} (average at 520nm=9.0±3.7 m² g^-1; with OC fraction <0.85 = ~7.5 m² g^-1) and the BrC OC mass absorption cross sections (MAC_BrC,OC,λ) were calculated; at 370 nm ultraviolet wavelengths; the MAC_BrC,OC,λ ranged from 0.8 m² g^-1 to 2.29 m² g^-1 (lowest peat, highest kerosene), while at 520nm wavelength MAC_BrC,OC,λ ranged from 0.07 m² g^-1 to 0.37 m² g^-1 (lowest peat, highest kerosene/incense mixture).

Samples from the same combustions sources were water and organic solvent extracted, filtered to physically remove BC, and the extracts were re-aerosolized in a controlled suspension chamber. The MAC_BrC,OC,λ derived from the re-suspended OC were compared to the mathematically derived MAC_BrC,OC,λ and were shown to have similar absorption spectra, however variability between the methods were observed, likely due to variations in particle size distributions, particle mixing state, and uncertainty associated with the OC quantification. The results show that BrC can be an important contributor to aerosol light absorption when OC is present in significant quantities (>0.9 OC/TC). However, this absorption is only becomes significant near and/or below 520 nm wavelength.