Insights Into the Evolution Behavior and Hygroscopicity of Black Carbon-Containing Particles Based on Observations of Their Chemical Compositions in Beijing's Autumn

Black carbon-containing particles (BCc) are key short-lived climate forcers, exerting strong light absorption that influences climate. Their atmospheric behavior evolves during aging, necessitating chemical characterization to elucidate their environmental and climatic impacts. Despite this, detailed studies of BCc compositions and evolution under current atmospheric conditions remain scarce. In this study, we deployed a soot particle aerosol mass spectrometer (SP-AMS) alongside advanced instrumentation during a month-long field campaign in Beijing in autumn 2022 to examine the chemical compositions and evolution of BCc. Our results reveal a strong link between BCc coatings and air pollution levels, with organic compounds and nitrate as the predominant coating materials. Nitrate displayed pronounced temperature sensitivity and uneven distribution between BCc and other sub-micron particles. During high-ozone pollution episodes, secondary organic aerosols dominated the organic fraction of BCc coatings, whereas in lower-ozone conditions, secondary contributions elevated with increasing air pollution, likely driven by aqueous-phase reactions. Hygroscopicity estimates for BCc and bulk PM₁, derived from the Zdanovskii−Stokes−Robinson mixing rule, consistently diverged, with differences diminishing as coatings thickened. These findings provide critical insights into the atmospheric fate of BCc, underscoring the pivotal role of chemical characterization in understanding their behavior and broader implications for climate and air quality.