| 05 Oct 2026
Synoptic-scale organization of multi-day black carbon variability in a regional-background environment over eastern China
Abstract. Black carbon (BC) variability reflects the combined effects of emissions, regional transport, atmospheric mixing, and removal processes, yet the temporal scales through which these processes organize receptor concentrations remain poorly constrained. Regional-background environments provide a useful perspective for identifying atmospheric controls because they are less dominated by immediate local emission fluctuations. Here, year-long hourly BC observations from a regional-background site in eastern China were combined with time-frequency analysis, backward trajectories, circulation-state transitions, and an independent 3–10 day time-domain evaluation to investigate the drivers of multi-day BC variability. BC exhibited enhanced variability at multi-day periods near one week, but the signal occurred intermittently rather than as a persistent weekly oscillation. Transport classification identified contrasting atmospheric states: Regional/slow flow was associated with higher BC (1.30 μg m-3), weaker winds (1.68 m s-1), and shallower planetary boundary-layer height (431 m) than northeastern/coastal (NE/coastal) flow (0.90 μg m-3, 2.22 m s-1, and 593 m, respectively). Transitions toward Regional/slow conditions were followed by BC increases, whereas transitions toward NE/coastal conditions produced decreases. The independently extracted 3–10 day BC component covaried with the persistence of these transport states (Spearman ρ = 0.345; segment-aware circular-shift p < 0.001). Together, these results show that the apparent near-weekly BC feature represents an emergent signature of recurring synoptic accumulation–ventilation states rather than a fixed weekly emission cycle. This study highlights the importance of incorporating atmospheric-state information when interpreting multi-day aerosol variability.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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