Aggregation Induced Enhancements in Aerosol Absorption and Scattering across the Black-Brown Continuum
JOSHIN KUMAR, Payton Beeler, Benjamin Sumlin, Rajan K. Chakrabarty,
Washington University in St. Louis Abstract Number: 510
Working Group: Carbonaceous Aerosol
AbstractThe global radiative forcing of carbonaceous aerosols is one of the largest sources of uncertainty in current climate models. BrC is an amorphous organic aerosol formed in biomass burning events, such as wildfires, and its radiative impact remains poorly understood. Aggregates of BrC primary spheres (monomers) have been recently observed in abundance in wildfire smoke. Aggregation could alter their optical properties and direct radiative forcing, yet very little is known about this phenomenon. This study could improve the representation of BrC aerosols in climate models by enhancing the current understanding of their optical properties.
In this study, we utilize the framework presented by Saleh et al. (2020) for the optical classification of particles in the black-brown continuum but focus on two sub-classes of BrC: dark brown carbon (d-BrC) and weakly-absorbing brown carbon (w-BrC). We calculate and compare the mass absorption coefficient (MAC), mass scattering coefficient (MSC), single scattering albedo (SSA), asymmetry parameter of dark brown carbon (d-BrC) and weakly-absorbing brown carbon (w-BrC), and determine their absorption and scattering enhancements due to aggregation. We use polydisperse diffusion limited cluster-cluster aggregation simulations to generate 90 aggregates of varying monomer diameters, and discrete dipole approximation to calculate optical properties of aggregates at different refractive indices, monomer diameters, and incident wavelengths.
BrC aggregate's optical properties are more sensitive to the number and mean diameter of monomers than their polydispersity. d-BrC has almost twice the MAC of w-BrC at 350 nm, but w-BrC has MSC values almost double that of d-BrC. Both MAC and MSC decrease with wavelength. Aggregation enhances optical properties, with smaller size parameters and lower imaginary part of the complex refractive index of aggregates resulting in stronger absorption and scattering.
Reference: Saleh, R.: From measurements to models: toward accurate representation of brown carbon in climate calculations, Current Pollution Reports, 6, 90-104, 2020.