Strong Haze-Black Carbon-Climate Connections Observed Across Northern and Eastern China

Radiative absorption of black carbon (BC) is of great importance in air quality and climate change. Simultaneous mitigation of haze pollution and climate warming requires an in-depth understanding of haze-BC-climate connections based on direct observations. We simultaneously performed measurements in the North China Plain (NCP) and the Yangtze River Delta (YRD) during the winter of 2018-2019, when cross-regional haze transport frequently occurred. We found that BC particles exhibited a thick coating thickness (100-120 nm) under haze pollution conditions in the NCP and were then transported to the YRD with regional haze development. BC particles in the downwind region showed a bimodal distribution of core-size-resolved aging degree, which can be used to separate and quantify local and regional BC. The results showed that the regional haze transport from the NCP brought 1-3 mu g m(-3) BC into the YRD. These regional BC particles exhibited similar to 1.6 times stronger absorption efficiency than locally emitted BC. The direct radiative effects of BC in the downwind region were estimated to be enhanced by similar to 2 times when regional haze pollution was transported from the NCP to the YRD. The amplification of BC's climate effect during regional haze pollution was attributed to the increases in both the mass concentrations and light absorption efficiency of BC, which were responsible for 40%-50% and 50%-60%, respectively. Our work identified strong haze-BC-climate connections across the NCP and YRD and highlighted the importance of BC in the effects of haze pollution on regional climate. Plain Language Summary Black carbon (BC) aerosols strongly absorb solar radiation and have a large contribution to both haze pollution and climate warming. The model estimates of BC radiative effects during their lifecycle have considerable uncertainties due to the complex physical and chemical processes of BC aerosols from local emissions to long-range transport in the atmosphere. Improving estimates of BC radiative effects requires some direct measurements of BC absorption properties to constrain model predictions. However, it challenges field measurements to obtain this information at the regional scale. Our simultaneous observations in the North China Plain and the Yangtze River Delta using a single-particle technique helped to distinguish the regional origins of BC. Based on field measurements, we quantified the optical properties of BC at local and regional scales. The results showed that the radiative effects of BC at the regional scale were significantly amplified by haze formation and transport, indicating the important role of BC in haze-climate interactions. The work provides a clue for the simultaneous mitigation of haze pollution and climate warming.