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 (sigma(abs)) of aerosols both before and after being heated to 250 degrees C. An improved absorption angstrom ngstrom exponent (AAE)-based method was developed to evaluate the contribution of BrC to light absorption at a wavelength of 370 nm (sigma(abs,BrC)/sigma(abs,370nm)). The sigma(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 sigma(abs) values across the range of 880-950 nm. Meanwhile, sigma(abs,BrC)/sigma(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 sigma(abs,BrC)/sigma(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 sigma(abs,BrC)/sigma(abs,370nm) was negative with Method II, a indicating the net production of BrC when the aerosols were heated. The difference between the values for sigma(abs,BrC)/sigma(abs,370nm) obtained by our developed method and by Method II was similar to 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.

Aerosols contribute the largest uncertainty to the total radiative forcing estimate, and black carbon (BC) that absorbs solar radiation plays an important role in the Earth's energy budget. This study analysed the aerosol optical properties from 22 February to 18 March 2014 at the China Meteorological Administration Atmospheric Watch Network (CAWNET) station in the Pearl River Delta (PRD), China. The representative values of dry-state particle scattering coefficient (sigma(sp)), hemispheric backscattering coefficient (sigma(hbsp)), absorption coefficient (sigma(absp)), extinction coefficient (sigma(ep)), hemispheric backscattering fraction (HBF), single scattering albedo (SSA), as well as scattering Angstrom exponent (alpha) were presented. A comparison between a polluted day and a clean day shows that the aerosol optical properties depend on particle number size distribution, weather conditions and evolution of the mixing layer. To investigate the mixing state of BC at the surface, an optical closure study of HBF between measurements and calculations based on a modified Mie model was employed for dry particles. The result shows that the mixing state of BC might be between the external mixture and the core-shell mixture. The average retrieved ratio of the externally mixed BC to the total BC mass concentration (r(ext-Bc)) was 0.58 +/- 0.12, and the diurnal pattern of r(ext-Bc) can be found. Furthermore, considering that non-light-absorbing particles measured by a Volatility-Tandem Differential Mobility Analyser (V-TDMA) exist independently with core-shell and homogenously internally mixed BC particles, the calculated optical properties were just slightly different from those based on the assumption that BC exist in each particle. This would help understand the influence of the BC mixing state on aerosol optical properties and radiation budget in the PRD. (C) 2016 Elsevier Ltd. All rights reserved.