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

Abstract

AbstractSoot 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 ± 0.05 for partly coated soot and 1.86 ± 0.07 for embedded soot to 1.83 ± 0.06 and 1.93 ± 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 ∼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.