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

Category: Optical/Radiative Properties and Remote Sensing

Article In Press
DOI: 10.4209/aaqr.2017.12.0566
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Guohua Zhang 1, Long Peng1,2, Xiufeng Lian1,2, Qinhao Lin1, Xinhui Bi1, Duohong Chen3, Mei Li4, Lei Li4, Xinming Wang1, Guoying Sheng1

  • 1 State Key Laboratory of Organic Geochemistry and Guangdong Key Laboratory of Environmental Resources Utilization and Protection, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
  • 2 University of Chinese Academy of Sciences, Beijing 100039, China
  • 3 State Environmental Protection Key Laboratory of Regional Air Quality Monitoring, Guangdong Environmental Monitoring Center, Guangzhou 510308, China
  • 4 Institute of Mass Spectrometer and Atmospheric Environment, Jinan University, Guangzhou 510632, China

Highlights

Light absorption coefficient (σ) of ambient/heated aerosols were measured.
The AAE-based method was improved to evaluate the σ of brown carbon (σabs,BrC).
The estimated σabs,BrC is > 20% larger than that estimated by assuming AAE = 1.
AAE around 0.7 for “pure” BC is suggested to evaluate σabs,BrC in the PRD region.


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 Ångström 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.

Keywords

Brown carbon Black carbon Light absorption Absorption Ångström exponent Aethalometer PRD region


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