Abstract
Abstract. The strong spectral dependence of light absorption of brown carbon (BrC) aerosol is regarded to influence aerosol's radiative forcing significantly. The Absorption Angstrom Exponent (AAE) method has been widely used in previous studies to attribute light absorption of BrC at shorter wavelengths for ambient aerosols, with a theoretical assumption that the AAE of "pure" black carbon (BC) aerosol equals to 1.0. In this study, the AAE method was applied to both urban and rural environments in the Pearl River Delta (PRD) region of China, with an improvement of constraining the realistic AAE of "pure" BC through statistical analysis of on-line measurement data. A three-wavelength photo-acoustic soot spectrometer (PASS-3) and aerosol mass spectrometers (AMS) were used to explore the relationship between the measured AAE and the relative abundance of organic aerosol to BC. The regression and extrapolation analysis revealed that more realistic AAE values for "pure" BC aerosol (AAEBC) were 0.86, 0.82, and 1.02 between 405 and 781 nm, and 0.70, 0.71, and 0.86 between 532 and 781 nm, in the campaigns of urbanwinter, urbanfall, and ruralfall, respectively. Roadway tunnel experiments were conducted and the results further confirmed the representativeness of the obtained AAEBC values for the urban environment. Finally, the average light absorption contributions of BrC (± relative uncertainties) at 405 nm were quantified to be 11.7 % (±5 %), 6.3 % (±4 %), and 12.1 % (±7 %) in the campaigns of urbanwinter, urbanfall, and ruralfall, respectively, and those at 532 nm were 10.0 % (±2 %), 4.1 % (±3 %), and 5.5 % (±5 %), respectively. The relatively higher BrC absorption contribution at 405 nm in the ruralfall campaign could be reasonably attributed to the biomass burning events nearby, which was then directly supported by the biomass burning simulation experiments performed in this study. This paper indicates that the BrC contribution to total aerosol light absorption at shorter wavelengths is not negligible in the highly urbanized and industrialized PRD region.
Highlights
Light absorbing carbonaceous aerosols including black carbon (BC) and brown carbon (BrC) are the primary matters absorbing light in the atmosphere
There were two main methods to identify the absorption of BrC in total aerosol absorption at shorter wavelengths in previous studies: one was to use theoretical Mie models to calculate the light absorption of BrC with input of ambient chemical, physical, and optical measurements of bulk aerosol (Lack et al, 2012b); the other was based on optical measurement followed by absorption Angstrom exponent (AAE) calculation, which was simpler and widely used with a criterion of Absorption Angstrom Exponent (AAE) for “pure” BC aerosol (AAEBC) (Clarke et al, 2007; Favez et al, 2009; Yang et al, 2009; Bahadur et al, 2012; Chung et al, 2012a)
The results indicated that no matter at the urban site or rural site in Pearl River Delta (PRD), BC still played a dominant role in total aerosol light absorption at 405 and 532 nm, but the contribuwww.atmos-chem-phys.net/16/1433/2016/
Summary
Light absorbing carbonaceous aerosols including black carbon (BC) and brown carbon (BrC) are the primary matters absorbing light in the atmosphere. There were two main methods to identify the absorption of BrC in total aerosol absorption at shorter wavelengths in previous studies: one was to use theoretical Mie models to calculate the light absorption of BrC with input of ambient chemical, physical, and optical measurements of bulk aerosol (Lack et al, 2012b); the other was based on optical measurement followed by absorption Angstrom exponent (AAE) calculation, which was simpler and widely used with a criterion of AAE for “pure” BC aerosol (AAEBC) (Clarke et al, 2007; Favez et al, 2009; Yang et al, 2009; Bahadur et al, 2012; Chung et al, 2012a). The focus of this paper was to reasonably quantify the light absorption of BrC aerosol in PRD with effective uncertainty evaluation using the AAE method
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