Contrasting nighttime heterogeneous and daytime photochemical aging drive the optical evolution of black carbon

Abstract. Black carbon (BC) particles 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) mass concentration during the sampling period 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 associated with enhanced nitrate signals and coating growth, potentially influenced by gas–particle partitioning, condensation, and coagulation. Daytime photochemical aging was characterized by rapid nitrate accumulation followed by increases in sulfate and oxidized organic species. Despite their distinct mechanisms, both aging pathways significantly elevated the MR and produced similar net enhancements in the mass absorption cross section (MAC) at 532 nm 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 primarily due to the offsetting effect of intensive fresh emissions during the day. Specifically, the apparent rates of change in the MAC of core–shell-like BC driven by nighttime heterogeneous reactions and daytime photochemical aging were determined to be 0.36±0.05 and 0.51±0.11 m2 g−1 h−1, respectively. This study provides observationally constrained insights into the contrasting diurnal evolution of BC mixing state and optical properties and quantifies the campaign-specific apparent MAC enhancement rates under urban atmospheric conditions.