The Brown-Black Continuum of Light-Absorbing Combustion Aerosols

Brown carbon (BrC) exhibits highly variable light-absorption properties, with the imaginary part of the refractive indices (k) varying over several orders of magnitude. This poorly understood variability poses a challenge to accurately determining the BrC climate effect. Here, we present a framework to explain the variability in BrC k. We hypothesize that a fraction of BrC is composed of black carbon (BC) precursors whose transformation to BC is not complete, and that there is a continuum of light-absorption properties along which BC and BrC lie. To test this hypothesis, we performed controlled-combustion experiments using benzene and toluene. By systematically varying the combustion conditions, we isolated BrC components along the brown black continuum progressing from light (k = 0.004 at 550 nm) to dark (k = 0.25 at 550 nm). Using laser-desorption-ionization mass spectrometry and thermodenuder measurements, we show that the BrC progression from light to dark is associated with an increase in molecular size and decrease in volatility. The darkest BrC has molecular sizes of several 1000 Da, is refractory, and is optically more similar to BC than the lighter BrC, blurring the lines between the optical properties of BrC and BC.