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

Guohua Zhang,Lei Li,Mei Li,Xiufeng Lian,Xinming Wang,Guoying Sheng,Xinhui Bi,Qinhao Lin,Duohong Chen,Long Peng

doi:10.4209/aaqr.2017.12.0566

Abstract

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 (σabs) of aerosols both before and after being heated to 250°C. An improved absorption Angstrom exponent (AAE)-based method was developed to evaluate the contribution of BrC to light absorption at a wavelength of 370 nm (σabs,BrC/σabs,370nm). The σabs,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 σabs values across the range of 880–950 nm. Meanwhile, σabs,BrC/σabs,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 σabs,BrC/σabs,370nm based on our developed method and Method I is highly correlated (r2 = 0.78), the difference could be as large as > 20% on average. The obtained mean σabs,BrC/σabs,370nm was negative with Method II, indicating the net production of BrC when the aerosols were heated. The difference between the values for σabs,BrC/σabs,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.

Highlights

Most climate models regard black carbon (BC) as the sole light-absorbing particulate material
This paper provides an improved An improved absorption Ångström exponent (AAE)-based method of quantitatively estimating the light absorption by brown carbon (BrC) in real-time based on multi-wavelength light absorption measurements
The analysis shows that the wavelength dependence of BC light absorption can be better approximated for higher (880–950 nm) wavelengths

Summary

Introduction

Most climate models regard black carbon (BC) as the sole light-absorbing particulate material. Some organic species (i.e., brown carbon/BrC) could absorb solar radiation, and play an important role in radiative forcing (Liu et al, 2014; Liu et al, 2015; Shamjad et al, 2015). High contribution of BrC to light absorption is reported worldwide from satellite and ground observations, and modeling studies (Bahadur et al, 2012; Feng et al, 2013; Laskin et al, 2015, and references therein). The variable absorptivity of light-absorbing carbonaceous aerosols is poorly known, making it challenging to account for their roles in radiative forcing (Andreae and Gelencser, 2006; Laskin et al, 2015). An accurate estimation of BrC light absorption is essential both for constraining large uncertainties in current aerosol forcing estimates and for control policy development (Bergstrom et al, 2007; Zhang et al, 2013b; Saleh et al, 2014)

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Conclusion