Numerical Computation of Optical Properties of Internally Mixed Soot in Biomass Burning Constrained by Field and Laboratory Observations

Abstract:

Carbonaceous aerosol emitted from biomass burning (BB) contributes significantly to atmospheric aerosol loadings regionally and globally. Direct radiative forcing of BB aerosol is highly uncertain due to its complex composition, morphology and mixing state. Soot particles are the strongest light absorbing aerosols in BB smoke. In BB smoke, soot particles are normally internally mixed with other material and the mixing state can affect their optical properties. In this study we investigated morphology and mixing state of soot particles emitted from BB smoke from field and laboratory measurements. Smoke particles were collected 1) during the Las Conchas wildfire in New Mexico (June, 2011) and 2) at the U.S. Forest Service’s Fire Science Laboratory in 2012, during the fourth Fire Laboratory at Missoula Experiment (FLAME-4).

Single particles were analyzed with electron microscopy, and were categorized and characterized by their morphology, and mixing state. We found that soot particles were mostly heavily coated. Based on the characterization on field and laboratory samples, synthetic soot particles with various morphologies and mixing states were generated and their optical properties were numerically calculated using the discrete dipole approximation. We used organic material as a coating agent and investigated the spectral dependency of scattering and absorption for internally mixed soot particles. We found enhancement in scattering and absorption when most of the soot particle was embedded within the organic coating. The aim of this study is to improve our understanding of the effect of morphology and mixing on light scattering and absorption by soot particles and ultimately their effects on the direct radiative forcing.