Black Carbon aerosol

Dr. Chakrabarty's group performs detailed experimental and modeling studies to understanding the complex fractal shape of Black Carbon (BC) aerosols. One of their first major contributions was the experimental validation and error quantification of three well-known non-spherical aerosol optical theories —Rayleigh-Debye-Gans (RDG) approximation, volume-equivalent Mie theory, and integral equation formulation for scattering (IEFS)–using photoacoustic spectroscopy and nephelometry. Their findings produced the most accurate model of BC physical and optical properties to date, which revealed a ≈3-times over-estimation of scattering by current schemes using core-shell models.

In another first, Dr. Chakrabarty's group discovered that large-scale open fires emit a form of soot particle – percolated aggregates (PAs), previously unrecognized in the ambient atmosphere – in quantities greater than 70% by number and mass concentration. The particle mobility diameters (D_m) and specific surface areas of soot PAs are ten and three times greater, respectively, than those of conventional sub-micron soot particles. These unusually large D_m values render these particles undetectable using conventional aerosol mobility sizing instruments. Alternatively, the aerodynamic diameters – used for estimating the probability of deposition within lungs – of these aerosols are similar to those of sub-micron (conventional) particles. These observations suggest that soot PAs may have deleterious effects on human health and the environment, and previously unaccounted for impacts on climate forcing. In follow-up findings, his research group calculated the radiative properties and direct forcing by these aerosols in the short visible wavelengths. A surprising finding was the enhanced mass absorption cross section (MAC) of BC aerosols in the thermal infrared solar spectrum. Compared to equivalent-size spheres, the MAC values of sub-micron aggregates and PAs showed enhancements of ≈150 and 400%, respectively.

Field observations carried out worldwide have shown that greater than 75% of Black Carbon (BC) in the atmosphere is internally mixed with non-refractory materials (e.g., organic carbon compounds, sulfates). Dr. Chakrabarty’s group performs detailed three-dimensional morphological characterization of BC aggregate aerosols, surface coated with organic materials in varying degrees, as found occurring in the atmosphere. Rigorous structure-factor and other advanced analysis of these aggregates showed that aggregate fractal dimension D_f remained invariant at 1.8 with increasing coating mass. They found organic coating to affect only the fractal pre-factor, an understudied parameter that controls the aggregate shape anisotropy and local packing fraction of monomers. Their findings upended the conventional view that D_f values range between 1.8 ≤ D_f ≤ 3.0 for internally mixed BC aerosols in the atmosphere.

Next, his group identified unique scaling relationships for MAC and enhancement in MAC (E_MAC) as a function of increasing internal mixing ratios. Their goal was to enable the integration of the established scaling relationships inexpensively into current climate models. Scaling is one of the most powerful concepts in statistical physics and many-body, non-equilibrium systems, such as aerosols. This concept provides mechanistic insight and reduces the multivariate state-space into simple power-law expressions between the most important parameters. The field of atmospheric science has yet to embrace this concept toward solving the BC light absorption problem. The MAC and E_MAC of BC aerosols followed a universal 1/3 scaling relationship as a function of increase in coating. This law was shown to be observed for global BC light absorption dataset collected globally over USA, UK, and Asia. His group also quantitatively estimated the correction factors that need to be applied to core-shell aerosol models used by current climate models. This research was highlighted as an editors’ suggestion in Physical Review Letters (November 2018)