An Improved Absorption Ångström Exponent (AAE)-Based Method for Evaluating the Contribution of Light Absorption from Brown Carbon with a High-Time Resolution

Abstract

While brown carbon (BrC) might play a substantially important role in radiative forcing, an estimation of its light absorption contribution with high-time resolution is still challenging. In this study, a multi-wavelength (370-950 nm) Aethalometer was applied to obtain the wavelength dependent light absorption coefficient (σ abs ) of aerosols both before and after being heated to 250°C. An improved absorption Ångström exponent (AAE)-based method was developed to evaluate the contribution of BrC to light absorption at a wavelength of 370 nm (σ abs,BrC /σ abs,370nm ). The σ abs,BC at 370 nm was determined from the field measured AAE values for the wavelengths from 880 to 950 nm with a one-hour resolution. The simultaneous measurements of heated aerosols help confirm the negligible influence of BrC on the σ abs values across the range of 880-950 nm. Meanwhile, σ abs,BrC /σ abs,370nm was also estimated with previously reported methods by assuming that the AAE was equal to 1 (Method I) as well as a new approach based on the light absorption enhancement (Method II). While the estimated σ abs,BrC /σ abs,370nm based on our developed method and Method I is highly correlated (r 2 = 0.78), the difference could be as large as > 20% on average. The obtained mean σ abs,BrC /σ abs,370nm was negative with Method II, indicating the net production of BrC when the aerosols were heated. The difference between the values for σ abs,BrC /σ abs,370nm obtained by our developed method and by Method II was ~40% on average and much higher (> 50%) during the noon hour, when secondary organic aerosols and sulfate were abundant. We propose that it is more suitable to use an AAE around 0.7 for "pure" BC to evaluate the contribution of BrC to light absorption in the PRD region. The developed method thus helps improve our understanding of the light absorption and climate forcing of BrC.

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 …”

“…It has been shown that this assumption may produce a considerable uncertainty in produced results (Lack & Landridge, ). Others proposed the AAE of BC to be <1 (e.g., Zhang et al, ). The larger BC particles (or aggregates of small particles) may have indeed the AAE values <1 depending on their diameter and refractive index (Figure b).…”

Estimating Absorption Ångström Exponent of Black Carbon Aerosol by Coupling Multiwavelength Absorption with Chemical Composition

“…Although experimental investigations and numerical calculations are the methods most commonly used to characterize the AAE BC , several studies also have attempted to use atmospheric observations or improved measurements to obtain the site-specific AAE BC values. ,− For instance, Yuan et al constrained AAE BC based on a linear relationship found between the measured AAEs and the ratios of organic/BC in the atmosphere . However, the general applicability of this method is debatable.…”

“…Taking the correlation between the AAE and OC/BC ratio in this study as an example (Figure S8), the intercept of the equation is 1.19 (AAE BC ), and AAE correlates weakly with the OC/BC ratios ( r = 0.52), which implies large uncertainties would result if the extrapolated intercept is used as a representative value for AAE BC . Several studies have retrieved AAE BC through measurements of thermal-denuded absorption in the atmosphere, , but this approach is constrained by the heating temperature and can be affected by variations caused by coating-composition-dependent absorption efficiency.…”

Aerosol optical properties and brown carbon in Mexico City

“…The choice of AAE BC is critical for the correct attribution of b abs,BC,370 . 67,69 An AAE BC value of 1.0 is oen adopted, despite studies have found it might vary according to the BC core, particle size, morphology, mixing state and coating thickness of the particles. 70,71 The impact of these variables can be approximated by the Mie theory using the aerosol size distribution data.…”