UV-Vis-IR Spectral Complex Refractive Indices and Optical Properties of Brown Carbon Aerosol from Biomass Burning

TL;DR

This study measures the spectral optical properties and complex refractive indices of brown carbon aerosols from biomass burning, revealing wavelength and fuel parameter dependencies crucial for climate modeling.

Contribution

It provides in situ spectral measurements and refractive index retrievals of BrC aerosols from peatland wildfires, highlighting the influence of fuel parameters on optical properties.

Findings

Refractive index real part n constrained between 1.5 and 1.7.

Imaginary part κ decreases from 0.014 to 0.003 as wavelength increases from 375 to 532 nm.

κ is sensitive to fuel packing density, suggesting mechanisms affecting aerosol absorption.

Abstract

Constraining the complex refractive indices, optical properties and size of brown carbon (BrC) aerosols is a vital endeavor for improving climate models and satellite retrieval algorithms. Smoldering wildfires are the largest source of primary BrC, and fuel parameters such as moisture content, source depth, geographic origin, and fuel packing density could influence the properties of the emitted aerosol. We measured in situ spectral (375-1047 nm) optical properties of BrC aerosols emitted from smoldering combustion of Boreal and Indonesian peatlands across a range of these fuel parameters. Inverse Lorenz-Mie algorithms used these optical measurements along with simultaneously measured particle size distributions to retrieve the aerosol complex refractive indices (m=n+i{\kappa}). Our results show that the real part n is constrained between 1.5 and 1.7 with no obvious functionality in wavelength ({\lambda}), moisture content, source depth, or geographic origin. With increasing {\lambda} from 375 to 532 nm, {\kappa} decreased from 0.014 to 0.003, with corresponding increase in single scattering albedo from 0.93 to 0.99. For {\lambda} = 532 nm, both {\kappa} and single scattering albedo showed no spectral dependency. We discuss differences between this study and previous work. The imaginary part {\kappa} was sensitive to changes in FPD, and we hypothesize mechanisms that might help explain this observation.