Discrepancies between brown carbon light-absorption properties retrieved from online and offline measurements

We compared the imaginary part of the refractive indices (k) of brown carbon (BrC) retrieved from online and solvent-extraction offline light-absorption measurements. BrC with variable light-absorption properties was generated from the controlled combustion of three structurally different fuels: toluene (aromatic), isooctane (branched alkane), and cyclohexane (cyclic alkane). The online retrieval method involved combining real-time measurements of aerosol absorption coefficients (at 422, 532, and 781 nm) and size distributions with Mie calculations. The offline method involved extracting BrC samples with two organic solvents, methanol and dichloromethane, followed by light-absorption measurements of the extracts using a UV-vis spectrophotometer. For the least absorbing BrC, k values of the extracts in both solvents were similar to those of the aerosol. However, for darker BrC, k values of the BrC extracts in dichloromethane were smaller than those of the BrC aerosol, and k values of the BrC extracts in methanol were the smallest, with the discrepancy among the three increasing with increasing BrC darkness. These results indicate that BrC produced in this study was more soluble in dichloromethane than methanol, and the BrC solubility in both solvents decreased with increasing BrC darkness. Finally, k of BrC aerosol and extracts followed the same trend of decreasing wavelength dependence with increasing k as previous data. This further supports the brown-black continuum of light-absorption properties and that the differences observed between k of the BrC aerosol and extracts are due to the inefficiency of solvent extraction and not due to intrinsic differences between the online and offline methods. Copyright © 2020 American Association for Aerosol Research