Solvent effects on chemical composition and optical properties of extracted secondary brown carbon constituents

Brown carbon (BrC) aerosol is an important contributor to positive radiative forcing on Earth, but its influence on the global radiative budget remains uncertain due to the limitations of current analytical methods used to characterize its optical and chemical properties. While online optical instruments provide in situ characterization of BrC light-absorption properties, the wavelengths are often limited due to the light source of optical instruments. Offline measurements can cover the entire UV-Vis wavelength range, but the use of solvents may introduce artifacts. In this study, we compared the mass absorption coefficients at 375 nm (MAC(375)) measured by online and offline instruments for aerosol samples generated from the nighttime oxidation of heterocyclic volatile organic precursors. Offline samples were extracted with both acetonitrile (ACN) and methanol (MeOH) to investigate the potential solvent effects on the angstrom ngstrom absorption exponent and BrC chromophores. Our results indicated considerable variations between MeOH-extracted samples and ACN-extracted samples, with pyrrole SOA being especially susceptible to solvent effects. Multi-instrumental analysis confirmed that MeOH could induce solvent artifacts by reacting with conjugated carbonyl chromophores, such as phthalic anhydride, maleic anhydride and maleimide, and subsequently modify the light absorption properties of BrC constituents. Our findings highlight the importance of methanolysis in the formation of artifacts, whereas an inert solvent such as ACN would be ideal for proper characterization of BrC chromophores. Consequently, the results from offline analysis using MeOH as the solvent should be interpreted with caution. Copyright (c) 2022 American Association for Aerosol Research

Automated summary

This study compared the mass absorption coefficients of brown carbon aerosol samples measured by online and offline instruments and found that the choice of solvent had an impact on the measurement results. Methanol-extracted samples showed significant solvent effects, while using an inert solvent such as acetonitrile allowed for more accurate characterization of brown carbon properties.