Structural Collapse and Coating Composition Changes of Soot Particles During Long-Range Transport

Soot particles play an important role in warming the atmosphere, but their optical absorption is highly uncertain due to their variable morphology and mixing states. Compared to short-range transport, soot particles during long-range transport normally undergo complicated aging processes. Here, we investigated the changes in microphysical properties and mixing states of soot particles during their long-range transport in Eastern China. The dominant mixing state of soot particles transformed from partly coated at 60% by number to embedded status at 67% when they were transported to a downwind region 1,000 km away under cold fronts. The fractal dimension (Df) increased from 1.79 & PLUSMN; 0.05 for partly coated soot and 1.86 & PLUSMN; 0.07 for embedded soot to 1.83 & PLUSMN; 0.06 and 1.93 & PLUSMN; 0.05 following their transportation, respectively. Our study shows that aging processes of soot particles with chain-like morphology caused their structural collapse. Moreover, we found that coating materials of aged soot particles changed from secondary inorganic-dominated to organic-dominated species during their long-range transport, which suggests the aqueous formation of secondary organic aerosols on soot-containing particles. The thick organic coating formation in some particles further induced soot redistribution from the particle center into the coating. We highlight that the Df at 1.83-1.93 is appropriate for assessing radiative absorption of long-range transported soot particles in Eastern China and propose that soot redistribution may offset & SIM;13% optical absorption enhancement for long-range transported soot particles. The microscopic changes in aged soot particles should be considered to precisely evaluate their optical absorption in the large-scale haze layer. In recent years, East Asian winter monsoon has been suggested to be strengthened due to climate change and continues to cause long-range transport of haze pollutants in Eastern China. Soot particles (i.e., black carbon) are typical primary aerosols with strong light absorption, and their absorption magnitudes mainly depend on their morphology and mixing states. Soot particles can undergo complicated aging processes during transport, which change their morphology and mixing states. However, there is no detailed information on the aging of soot particles in long-range transported haze masses with high PM2.5 concentrations. Here, we raise several questions and try to answer them: how are morphology and mixing states of soot particles changed during long-range transport? What are aging processes and mechanisms of long-range transported soot particles? How can the optical absorption of soot particles change due to the variation in their morphology and mixing states? These answers are important for understanding aging processes of soot particles and improving simulations for their radiative effects. Therefore, we conducted field campaigns and analyzed morphology and mixing states of soot particles during their long-range transport. This study not only provides critical information about long-range transported soot particles but also evaluates changes in their radiative absorption. Aging processes of soot particles during long-range transport affect their morphology and induce their structural collapseCoating compositions of long-range transported soot particles are changed from inorganic-dominated to organic-dominated componentsRedistribution of long-range transported soot particles in individual particles reduces their optical absorption enhancement