Abstract
Abstract. Biomass burning aerosol is a major source of PM2.5, and significantly affects Earth's radiative budget. The magnitude of its radiative effect is poorly quantified due to uncertainty in the optical properties of aerosol formed from biomass burning. Using a broadband cavity-enhanced spectrometer with a recently increased spectral range (360–720 nm) coupled to a size-selecting aerosol inlet, we retrieve complex refractive indices of aerosol throughout the near-ultraviolet and visible spectral region. We demonstrate refractive index retrievals for two standard aerosol samples: polystyrene latex spheres and ammonium sulfate. We then retrieve refractive indices for biomass burning aerosol from 13 controlled fires during the 2016 Missoula Fire Science Laboratory Study. We demonstrate that the technique is highly sensitive to the accuracy of the aerosol size distribution method and find that while we can constrain the optical properties of brown carbon aerosol for many fires, fresh smoke dominated by fractal-like black carbon aerosol presents unique challenges and is not well-represented by Mie theory. For the 13 fires, we show that the accuracy of Mie theory retrievals decreases as the fraction of black carbon mass increases. At 475 nm, the average refractive index is 1.635 (±0.056) +0.06 (±0.12)i, and at 365 nm, the average refractive index is 1.605 (±0.041) +0.038 (±0.074)i.
Highlights
Biomass burning is one of the largest global contributors to accumulation-mode aerosol mass, with estimated emissions of 15–57 Tg yr−1 (Pan et al, 2020)
refractive index (RI) were retrieved for 13 fires assuming Mie theory, and the quality of the fits is related to the black carbon content of the smoke, demonstrating that smoke with higher BC generally cannot be fit well with Mie theory
This paper describes the development of a new broadband cavity-enhanced spectrometer that derives the RI of biomass burning aerosol with low BC content over a very broadband wavelength region in the ultraviolet and visible spectral regions
Summary
Biomass burning is one of the largest global contributors to accumulation-mode aerosol mass, with estimated emissions of 15–57 Tg yr−1 (Pan et al, 2020). Many global models and satellite retrieval algorithms assume atmospheric aerosol particles are predominately spherical and calculate total aerosol extinction using Mie theory with a small set of constant RIs for different aerosol types (Liao et al, 2003; Levy et al, 2007; Omar et al, 2009; Lamarque et al, 2012). Complex RIs have been reported for standard aerosol samples, such as nigrosin and Suwannee River Fulvic Acid (Washenfelder et al, 2013; Zhao et al, 2017), and for aged organic aerosol (Flores et al, 2014a, b; He et al, 2018; Li et al, 2020) These measurements have all been conducted with spherical, homogeneous particles generated in laboratory or chamber experiments. We analyze smoke from 13 fires at the Fire Sciences Laboratory, present detailed examples where the retrieval algorithm can and cannot be used to accurately characterize the complex refractive index, and discuss the implications for remote-sensing retrievals
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