The Contribution of Black Carbon-Containing Particles to PM_2.5: Variability, Drivers, and Impacts

Atmospheric black carbon (BC) particles play an important role in air pollution, climate change, and human health. Evaluating BC's impacts by model simulation requires an understanding of the distribution of other aerosol species between those containing BC and those free of BC particles during the atmospheric aging process. Previous studies observed a large variability in the mass fraction of BC-containing particles in PM_2.5 (<i>F</i>_{BC-containing}), complicating the determination of their impacts. In this work, we conducted field observations in various polluted environments across China for process-level understanding of <i>F</i>_{BC-containing}. We find that the variability in <i>F</i>_{BC-containing}, ranging from 10 to 90%, can be elucidated by the concept of atmospheric oxidation capacity (AOC). The contrast between observations of <i>F</i>_{BC-containing} during haze events is determined by whether the initial aging of freshly emitted BC is limited by daytime AOC levels. We quantify and parametrize <i>F</i>_{BC-containing} by resolving BC aging under different AOC conditions, indicating efficient formation of secondary aerosol on BC-containing particles when daytime-average concentrations of O_<i>x</i> (i.e., O₃ + NO₂) exceed 80 μg m^-3. Our study provides valuable insights into BC mixing state representations under different environments, facilitating accurate evaluation of BC's impacts in atmospheric models.