Effects of coatings on laser-induced incandescence measurements of black carbon

Description

Refractory black carbon particles are believed to have a large influence on climate through direct radiative forcing and by reduction of surface albedo of snow and ice in the cryosphere. The optical properties of atmospheric particles containing black carbon are uncertain, and the specific sources of these particles found in polar regions are also not well known. Laser-induced incandescence (LII) has been employed to measure atmospheric black carbon, but the interpretation of LII data is often complicated by the presence of coatings on these particles. Refractory black carbon particles found in the atmosphere are often coated with unburned fuel, sulfuric acid, water, ash, and other combustion by-products and atmospheric constituents. Coatings can alter the optical and physical properties of the particles and therefore change the response of optical diagnostics. A fuller understanding of how coatings affect LII is needed before the technique can be applied reliably to a wide range of particles. We have investigated the effects of coatings on combustion-generated black carbon particles using time-resolved LII measurements. Particles were generated in a coflow diffusion flame, extracted, cooled, and coated with oleic acid. The diffusion flame produces highly dendritic soot aggregates with similar properties to those produced in diesel engines. A thermodenuder was used to remove the coating. A scanning mobility particle sizer (SMPS) was used to monitor aggregate sizes, a centrifugal particle mass analyzer (CPMA) was used to measure coating mass fractions, and transmission electron microscopy (TEM) was used to characterize particle morphologies. The results demonstrate striking differences in LII temporal evolution and dependence on laser fluence between coated and uncoated particles. The LII signal appears to be sensitive to coating-induced particle morphology and optical changes. These results can be understood in the context of energy and mass balance during laser heating and conductive and evaporative cooling and are consistent with predictions based on an LII model that includes a heavy organic coating.