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. Black carbon (BC) aerosols have been widely recognized asa vital climate forcer in the atmosphere. Amplification of light absorptioncan occur due to coatings on BC during atmospheric aging, an effect thatremains uncertain in accessing the radiative forcing of BC. Existing studieson the absorption enhancement factor (Eabs) have poor coverage on bothseasonal and diurnal scales. In this study, we applied a recently developedminimum R squared (MRS) method, which can cover both seasonal and diurnalscales, for Eabs quantification. Using field measurement data inGuangzhou, the aims of this study are to explore (1) the temporal dynamics ofBC optical properties at seasonal (wet season, 31 July–10 September; dryseason, 15 November 2017–15 January 2018) and diel scales (1 h timeresolution) in the typical urban environment and (2) the influencing factors onEabs temporal variability. Mass absorption efficiency at 520 nm byprimary aerosols (MAEp520) determined by the MRS method exhibited a strongseasonality (8.6 m2 g−1 in the wet season and 16.8 m2 g−1in the dry season). Eabs520 was higher in the wet season (1.51±0.50) and lower in the dry season (1.29±0.28). AbsorptionÅngström exponent (AAE470–660) in the dry season (1.46±0.12) was higher than that in the wet season (1.37±0.10). Collectiveevidence showed that the active biomass burning (BB) in the dry seasoneffectively altered the optical properties of BC, leading to elevated MAE,MAEp and AAE in the dry season compared to those in the wet season.Diurnal Eabs520 was positively correlated with AAE470–660 (R2=0.71) and negatively correlated with the AE33 aerosol loadingcompensation parameter (k) (R2=0.74) in the wet season, but thesecorrelations were significantly weaker in the dry season, which may berelated to the impact of BB. This result suggests that during the wetseason, the lensing effect was more likely dominating the AAE diurnalvariability rather than the contribution from brown carbon (BrC). Secondaryprocessing can affect Eabs diurnal dynamics. The Eabs520 exhibiteda clear dependency on the ratio of secondary organic carbon to organic carbon (SOC∕OC), confirming the contribution of secondary organic aerosols toEabs; Eabs520 correlated well with nitrate and showed a cleardependence on temperature. This new finding implies that gas–particlepartitioning of semivolatile compounds may potentially play an importantrole in steering the diurnal fluctuation of Eabs520. In the dry season,the diurnal variability in Eabs520 was associated with photochemicalaging as evidenced by the good correlation (R2=0.69) between oxidantconcentrations (Ox=O3+NO2) and Eabs520.

Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou