Influence of Hydrogen Content on the Optical Properties of Carbonaceous Aerosols: Insights from Atomic Scale Simulations

JOSHIN KUMAR, Gwan-Yeong Jung, Rohan Mishra, Taveen Kapoor, Rajan K. Chakrabarty, Washington University in St. Louis

     Abstract Number: 246
     Working Group: Carbonaceous Aerosols

Abstract
The role of carbonaceous aerosols in climate models is critically influenced by their optical properties, contributing significantly to uncertainties in global radiative forcing predictions. Recent studies have utilized particle-scale modeling and experimental approaches to examine the optical characteristics within the black-brown continuum of these aerosols. However, there remains a substantial gap in understanding how atomic-scale structures of these carbonaceous particles and their electronic properties correlate with these optical behaviors. This study aims to bridge this gap by exploring the relationship between the atomic structure, hybridization, and bulk optical properties of carbonaceous aerosols through atomic-scale simulations using Density Functional Theory (DFT).

Utilizing molecular dynamics and DFT, we first constructed atomic models of pristine black carbon and subsequently doped these structures with varying concentrations of hydrogen, within experimentally validated composition ranges. For each configuration, we then computed the structural, electronic, and optical properties including pair correlation function, average hybridization of carbon atoms, density of states, and complex refractive indexes.

Starting with amorphous black carbon structures, increasing hydrogen concentration leads to a higher proportion of sp3 hybridization and a corresponding decrease in the imaginary (absorbing) refractive index. Increasing hydrogen concentration simulated progressively browner particles indicating shift from black to brown part of the absorption continuum that align with Saleh et al. (2020). Notably, an increase in hydrogen content also results in changes to the short-range pair correlation function, primarily due to enhanced C-H bonding, while maintaining the amorphous nature at longer ranges.

These findings provide a foundational understanding of how variations in hydrogen concentration impact the structural, electronic, and optical properties of carbonaceous aerosols at an atomic level. This study underscores the potential of first-principles simulations in predicting and tuning aerosol impacts on climate modelling.

Reference: Saleh, R.: From measurements to models: toward accurate representation of brown carbon in climate calculations, Current Pollution Reports, 6, 90-104, 2020.