Effects of dicarboxylic acid coating on the optical properties of soot

Soot is a major component of atmospheric aerosols responsible for absorption of visible solar radiation. Internal mixing of soot with transparent materials can enhance its ability to absorb and scatter light, resulting in a larger role of soot in climate forcing. We have investigated the absorption and scattering of visible light (532 nm) by soot aerosol internally mixed with succinic and glutaric acids using a combination of a cavity ring-down spectrometer and an integrating nephelometer. The measurements were performed for flame-generated soot aerosol with well-characterized morphology and mixing state in the particle size range from 155 to 320 nm. Thin coatings of dicarboxylic acids on soot aggregates (with a mass fraction of 0.1-0.4) enhance significantly light scattering (up to 3.8 fold) and slightly light absorption (less than 1.2 fold). Cycling the coated soot aerosol through high relative humidity (humidified to 90% RH and then dried to 5% RH) promotes further increase in light absorption and scattering for soot internally mixed with glutaric acid, but not for soot mixed with succinic acid. The larger effect of glutaric acid on light absorption and scattering is caused by the irreversible restructuring of soot aggregates induced by the coating material. Our results indicate that the enhancement in the optical properties of soot by transparent coatings is strongly related to the ability of the coating materials to change the morphology of soot aggregates.