Light absorption property and potential source of particulate brown carbon in the Pearl River Delta region of China

Brown carbon (BrC) is a type of light-absorbing component of organic aerosol (OA), covering from near-ultraviolet (UV) to visible wavelength ranges, and thus may cause additional aerosol radiative effect in the atmosphere. While high concentrations of OA have been observed in the Pearl River Delta (PRD) region of China, optical properties and the corresponding radiative forcing of BrC in PRD are still not well understood. In this work, we conducted a set of comprehensive measurements of atmospheric particulate matters from 29 November 2014 to 5 January 2015 to investigate aerosol composition, optical properties, source origins and radiative forcing effects at a suburban station of Guangzhou. Particle absorption Ångström exponent (AAE) was deduced and utilized to differentiate light absorption by BrC from black carbon (BC). The results showed that the average absorption contributions of BrC were 25.9 ± 9.0 % at 370 nm, 19.7 ± 7.9 % at 470 nm, 14.1 ± 6.9 % at 520 nm, 11.6 ± 5.6 % at 590 nm and 7.7 ± 4.4 % at 660 nm, respectively. A sensitivity analysis of the evaluation of absorption Ångström exponent of BC (AAEBC) was conducted based on the Mie theory calculation, assuming that the BC-containing aerosol was internally mixed, with a core-shell configuration. The corresponding uncertainty of BrC absorption contribution was acquired. We found that variations in the imaginary refractive index (RI) of BC core can significantly affect the estimation of BrC absorption contribution. However, BrC absorption contribution was relatively less sensitive to the real part of RI of BC core and was least sensitive to the real part of RI of non-light absorbing shell. BrC absorption was closely related to aerosol potassium cation content (K+), a common tracer of biomass burning emission, which was most likely associated with straw burning in the rural area of western PRD. Diurnal variation of BrC absorption revealed that primary organic aerosol had a larger BrC absorption capacity than secondary organic aerosol (SOA) had. Radiative transfer simulations showed that BrC absorption may cause 2.2 ± 2.3 W m−2 radiative forcing at the top of atmosphere (TOA) and contribute 14.2 ± 6.2 % of the aerosol warming effect. A chart was constructed to conveniently assess the BrC radiative forcing efficiency in the studied area with reference to a certain aerosol single-scattering albedo (SSA) and BrC absorption contribution at various wavelengths. Evidently, BrC radiative forcing efficiency was higher in shorter wavelength.