Contrasting Nighttime Heterogeneous and Daytime Photochemical Aging Drive the Optical Evolution of Black Carbon

Abstract. Black carbon (BC) play a critical role in the climate system, yet their atmospheric aging processes and consequent optical impacts in real-world atmospheres remain insufficiently understood. In this study, we present integrated single-particle measurements using a single particle soot photometer (SP2) and a single-particle aerosol mass spectrometer (SPAMS) during a field campaign in urban Shenzhen, China. The mean refractory BC (rBC) concentration was 1.2 μg m⁻³, with core mass median diameters (MMD) of 155–170 nm. The diurnal variation in the coating-to-core mass ratio (MR) indicated that BC underwent continuous aging. Nighttime aging was primarily driven by heterogeneous nitrate uptake, whereas daytime photochemical aging was characterized by condensation of nitrate, sulfate, and oxidized organics. Despite their distinct mechanisms, both aging pathways elevated the MR and produced similar net enhancements in the mass absorption cross section (MAC) with an overnight increase of ~0.8 m² g⁻¹ and a daytime increase of ~1.0 m² g⁻¹. These comparable net increments were due to the offsetting effect of intensive fresh emissions during the day. Specifically, the MAC enhancement rates driven by nighttime heterogeneous reactions and daytime photochemical aging were determined to be 0.34 and 0.59 m² g⁻¹ h⁻¹, respectively. This study provides direct observational evidence of diurnally contrasting BC aging pathways and quantitatively constrains the optical enhancement rates under real urban conditions.