Contrasting annual variation of organic and elemental carbon in the Pearl River Delta: Insights from three years of continuous observations and the unique conditions of the COVID-19 lockdown.

Carbonaceous aerosols-key constituents of fine particulate matter (PM2.5)-play a critical role in shaping air quality, human health, and climate change. In China, extensive air pollution control measures have substantially reshaped emission patterns in recent years; however, their impacts on the sources and formation of carbonaceous aerosols remain insufficiently understood. To address this knowledge gap, this study investigates the sources and formation of carbonaceous aerosols in the Pearl River Delta (PRD) region through three years of continuous atmospheric observations (2019-2021), encompassing the unique conditions of the COVID-19 lockdown. The results reveal contrasting temporal trends between organic carbon (OC) and elemental carbon (EC). OC concentrations showed a steady decline, primarily driven by reduced formation of secondary organic carbon (SOC). This suppression is closely associated with the PRD's NOx-rich atmosphere environment, a relationship that reversed during the lockdown due to a sharp reduction in NOx concentration. In contrast, EC levels remained relatively stable or slightly increased, underscoring the persistent impact of fossil fuel combustion, particularly from industrial activities and mobile emissions. Source apportionment using a Positive Matrix Factorization (PMF) model indicates that fossil-related emissions account for approximately 70% of carbonaceous aerosols. These contributions showed limited reduction over the study period and occasional increases during specific episodes. Collectively, these findings highlight the need to strengthen fossil fuel emission controls and to design PM2.5 mitigation strategies tailored to the atmospheric condition of the PRD region.