Optical Properties of Laboratory and Ambient Biomass Burning Aerosols: Elucidating Black, Brown, and Organic Carbon Components and Mixing Regimes

Abstract

Biomass burning (BB) is an important global source of aerosol and trace gases that degrade air quality, decrease visibility, and impact climate and human health. Refractory black carbon (rBC), brown carbon (BrC), and organic aerosol are major components of BB emissions. BB aerosol composition is highly variable at the source and depends on fuel composition and combustion phase. Atmospheric aging alters fresh BB aerosol through processes that are complex and dynamic. To better understand the variability in optical properties, we report fresh aerosol laboratory measurements from burning southwestern U.S. fuels and compare them to aged ambient BB aerosol from wildfires over a range of atmospheric time scales. Our BB aerosol analysis uses the relationship between the absorption Ångström exponent and single‐scattering albedo (SSA) to identify rBC, BrC, and organic aerosol‐dominated regimes that are defined using Mie theory. This model framework is used to interpret the large variability in optical properties measured in laboratory burns. In contrast, we find the observed absorption Ångström exponent‐SSA relationship for ambient BB aerosol to be less variable and more clustered together with increased atmospheric aging. This transition from fresh to aged behavior is attributed to the homogenization of the BB aerosol from mixing and aging over several hours. Finally, BB aerosol in ambient fire plumes that have aged for several hours exhibits larger SSAs than laboratory flaming burns. We conclude that BrC/OC mixtures play a larger role than rBC in the positive climate forcing of BB aerosol than what would be projected from laboratory results.

Mie Scattering Captures Observed Optical Properties of Ambient Biomass Burning Plumes Assuming Uniform Black, Brown, and Organic Carbon Mixtures

“…This remains true even if we change the real part of refractive index of BC to 1.7 (Figure c) or change the imaginary part to 0.3 (Figure d), or if we adopt the refractive index of 1.95–0.79i. Thus, in agreement with earlier publications (Lack & Cappa, ; Romonosky et al, ; Zhang et al, ), and rounding to the higher one decimal place number, we conclude that the AAE(405/781) of a mixture of BC and spectrally flat organic material is limited by …”

“…This provides an opportunity to estimate the imaginary part of the refractive index (assuming that the real part is known) of BC from the maximum SSA value (Figure b). For example, the samples used in laboratory measurements by Romonosky et al () and shown in their Figures and , suggest by comparison with our Figure , the imaginary part of the refractive index close to 0.5i for BC, and close to 0.2i for nigrosin.…”

“…We approximate the AAE between the two wavelengths λ 1 and λ 2 by where λ 1 and λ 2 are the two wavelengths at which the AAE is calculated and the b absλ1 and b absλ2 are the absorption coefficients at the selected wavelengths λ 1 and λ 2 . In this report, we will be working with the wavelengths at 405 and 781 nm determined by instruments we use (Gomez et al, ; Romonosky et al, ). Our phase space is defined by two variables: the AAE and a single‐scattering albedo (SSA) at the wavelength 405 nm given by where b scat and b ext are the scattering and extinction coefficients.…”

“…We use the presented Mie scattering model to deduce an imaginary part of refractive index of BrC. The list of southwestern fires (Romonosky et al, ) together with the two Atmospheric Radiation Monitoring (ARM) field observations (Green Ocean Amazon [GOAmazon] and Layered Atlantic Smoke Interactions with Clouds [LASIC]) are provided in Table .…”

“…The scattering characteristics are obtained using the Mie scattering formulation assuming a spherical geometry. A similar Mie scattering model was used, for example, by Feng et al (2013), Lu et al (2015), and Romonosky et al (2019).…”