The role of non-sphericity of externally mixed organic carbon in altering the backscattering linear depolarization ratio of smoke aerosols from biomass burning

The backscattering linear depolarization ratio (LDR) is a key parameter to identify particle types. Previous studies on smoke LDR have shown significant differences in their measurements, with the magnitudes varying widely under different study scenarios. Single-particle models involving internally mixed black carbon (BC) are applied to assess the LDR of smoke aerosols. However, handicaps have been found to apply such models to describe the bulk optical properties of aerosols, because of their overlook of the contribution of externally mixed organic carbon (OC) to the LDR. Smoke aerosols typically consist of a low proportion of BC particle population and a high proportion of externally mixed OC particle population. If the spherical assumption is applied to the calculation of smoke LDRs, the LDRs turned to be extremely low even approach zero. This leads to difficulties in explaining the observed variability and higher levels of smoke LDR. We conducted a prescribed burning experiment in Xichang, Sichuan Province, China, and did onsite measurement on the LDR of smoke at a wavelength (λ) of 532 nm using atmospheric laser lidar. Field smoke particles were collected using a single-particle sampler and the morphology of particles was then characterized by the transmission electron microscope (TEM). The results indicated that the LDR of local smoke varied between 0 and 20.1%, with rapid fluctuations. The TEM images confirmed the coexistence of both internally mixed BC and externally mixed OC in the smoke aerosols, with OC displaying an ellipsoidal morphology even on copper grids. Using the discrete dipole approximation, we subsequently evaluated the LDR of individual BC and OC. Based on light scattering theory, we further quantified the bulk LDRs of the aerosol aerosols. The results shown that the smoke LDR ranged from 0.0% to 28.2% in λ = 532 nm while accounting for the effect of externally mixed OC. The LDR is slightly influenced by BC and is significantly affected by the externally mixed OC. Furthermore, the LDR is primarily governed by the morphology and particle size distribution of the externally mixed OC. It is concluded that the high levels and rapid variations in the LDRs of smoke can be largely attributed by the non-sphericity and particle size distribution of externally mixed OC. This study advances the methodologies for LDR measurements and evaluations of smoke aerosols from biomass burning.