Direct Measurement of the Absorption Cross-Section of Uncoated and Coated Soot by Photoacoustic Spectroscopy

Atmospheric aerosols such as soot play a fundamental role in Earth’s atmospheric chemistry and climate. As with most atmospheric components, soot’s influence on warming or cooling of the atmosphere by interacting with the solar radiation is receiving increased scrutiny. It has been proposed that coated soot might absorb radiation more efficiently than uncoated soot, thus warming the climate more than previously suspected. In the present work, these issues are addressed through direct laboratory measurements of the absolute absorption of both coated and uncoated soot particles. Soot is generated in a well-controlled ethylene/air diffusion flame. Dibutyl phthalate is employed as a surrogate for sulfuric acid due to the similarities in the real components of their indices of refraction and the absorption cross-section of mobility selected uncoated soot and dibutyl phthalate coated soot particles are measured using photoacoustic spectroscopy. Results show that a non-absorbing coating increases the absorption cross-section by 90% relative to the uncoated particles and supports the high end of IPCC estimates of warming from atmospheric aerosols. Effect of relative coating thickness on Cabs for soot with various core sizes, as selected by either a single-DMA or Tandem-DMA system. Absorption amplification of up to a factor of two are observed for coated soots.