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. Brown carbon (BrC) is a special type of organic aerosol (OA), capable ofabsorbing solar radiation from near-ultraviolet (UV) to visible wavelengths,which may lead to an increased aerosol radiative effect in the atmosphere.While high concentrations of OAs have been observed in the Pearl River Delta(PRD) region of China, the optical properties and corresponding radiativeforcing of BrC in the PRD are still not well understood. In this work, weconducted a set of comprehensive measurements of atmospheric particulatematter from 29 November 2014 to 2 January 2015 to investigate aerosolcompositions, optical properties, source origins, and radiative forcingeffects at a suburban station in Guangzhou. The particle absorptionAngstrom exponent (AAE) was deduced and utilized to distinguishlight absorption by BrC from that by black carbon (BC). The results showedthat the average absorption contributions of BrC were 34.1±8.0  % at370 nm, 23.7±7.3  % at 470 nm, 16.0±6.7  % at 520 nm, 13.0±5.4  % at 590 nm, and 8.7±4.3  % at 660 nm. Asensitivity analysis of the evaluation of the absorption Angstromexponent of BC (AAE BC) was conducted based on the Mie theorycalculation assuming that the BC-containing aerosol was mixed with thecore–shell and external configurations. The corresponding uncertainty inAAE BC was acquired. We found that variations in the imaginaryrefractive index (RI) of the BC core can significantly affect the estimationof AAE BC . However, AAE BC was relatively less sensitive to the realpart of the RI of the BC core and was least sensitive to the real part ofthe RI of the non-light-absorbing shell. BrC absorption was closely relatedto aerosol potassium cation content ( K+ ), a common tracer of biomassburning emissions, which was most likely associated with straw burning inthe rural area of the western PRD. Diurnal variation in BrC absorptionrevealed that primary organic aerosols had a larger BrC absorption capacitythan secondary organic aerosols (SOAs). Radiative transfer simulationsshowed that BrC absorption may cause 2.3±1.8  W m −2 radiativeforcing at the top of the atmosphere (TOA) and contribute to 15.8±4.4  % of the aerosol warming effect. A chart was constructed toconveniently assess the BrC radiative forcing efficiency in the studied areawith reference to certain aerosol single-scattering albedo (SSA) and BrCabsorption contributions at various wavelengths. Evidently, the BrCradiative forcing efficiency was higher at shorter wavelengths.

Light absorption properties and potential sources of particulate brown carbon in the Pearl River Delta region of China