<p><strong>Abstract.</strong> Black carbon (BC) is an important climate forcer in the atmosphere. Amplification of light absorption can occur by coatings on BC aerosols, an effect that remains one of the major sources of uncertainties for accessing the radiative forcing of BC. In this study, the absorption enhancement factor (E<sub>abs</sub>) was quantified by the minimum R squared (MRS) method using elemental carbon (EC) as the tracer. Two field campaigns were conducted in urban Guangzhou at the Jinan university super site during both wet season (July 31–September 10, 2017) and dry season (November 15, 2017–January 15, 2018) to explore the temporal dynamics of BC optical properties. The average concentration of EC was 1.94 ± 0.93 and 2.81 ± 2.01 μgC m<sup>−3</sup> in the wet and dry seasons, respectively. Mass absorption efficiency at 520 nm by primary aerosols (MAE<sub>p520</sub>) determined by MRS exhibit a strong seasonality (8.6 m<sup>2</sup>g<sup>−1</sup> in the wet season and 16.8 m<sup>2</sup>g<sup>−1</sup> in the dry season). E<sub>abs520</sub> was higher in the wet season (1.51 ± 0.50) and lower in the dry season (1.29 ± 0.28). Absorption Ångström exponent (AAE<sub>470-660</sub>) in the dry season (1.46 ± 0.12) were higher than that in the wet season (1.37 ± 0.10). Collective evidence showed that the active biomass burning (BB) in dry season effectively altered optical properties of BC, leading to elevated MAE, MAE<sub>p</sub> and AAE in dry season comparing to those in wet season. Diurnal E<sub>abs520</sub> was positively correlated with AAE<sub>470-660</sub> (<i>R</i><sup>2</sup> = 0.71) and negatively correlated with the AE33 aerosol loading compensation parameter (<i>k</i>) (<i>R</i><sup>2</sup> = 0.74) in the wet season, but these correlations were significantly weaker in the dry season, which may be related to the impact of BB. This result suggests that lensing effect was dominating the AAE diurnal variability during the wet season. The effect of secondary processing on Eabs diurnal dynamic were also investigated. The E<sub>abs520</sub> exhibit a clear dependency on secondary organic carbon to organic carbon ratio (SOC/OC). E<sub>abs520</sub> correlated well with nitrate, implying that gas-particle partitioning of semi-volatile compounds may potentially play an important role in steering the diurnal fluctuation of E<sub>abs520</sub>. In dry season, the diurnal variability of E<sub>abs520</sub> was associated with photochemical aging as evidenced by the good correlation (<i>R</i><sup>2</sup> = 0.69) between oxidant concentrations (O<sub>x</sub>=O<sub>3</sub>+NO<sub>2</sub>) and E<sub>abs520</sub>.</p>

This supplement contains 3 tables and 16 figures.