Size‐Resolved Particle Effective Density in the Free Troposphere at Mt. Hua, China: Influence of Chemical Aging and Sources

Abstract Effective density ( ρ eff ) is a key physical property of atmospheric particles, yet it has rarely been investigated in free tropospheric environments. It is largely determined by chemical compositions and particle morphology, providing valuable insights into atmospheric evolution and air‐mass aging. Here, size‐resolved ρ eff was measured at Mount Hua (2060 m a.s.l.) in the Fenwei Plain, China, during autumn 2021, using a coupled differential mobility analyzer‐centrifugal particle mass analyzer‐condensation particle counter (DMA‐CPMA‐CPC) system. We found that the ρ eff increased from 1.40 g cm −3 at 50 nm to 1.54 g cm −3 at 350 nm, generally higher than those observed for similar size ranges at ground‐level sites (∼1.2–1.4 g cm −3 ), indicating a reduced contribution from low‐density particles and the predominance of aged aerosol components in the free troposphere. Black carbon (BC) exerted the strongest influence on ρ eff , and a 5% increase in BC mass fraction was associated with a decrease of approximately 0.06 g cm −3 in ρ eff . The retrieved BC effective density ranged from 0.85 to 1.20 g cm −3 , indicating that BC particles may remained non‐spherical shapes even within the free troposphere. During the observation, a vertical transport event led to ρ eff decline of ∼0.1 g cm −3 as near‐surface pollutants were entrained to the summit. Additionally, long‐range transport from northwestern China systematically lowered ρ eff due to distinct changes in chemical composition. These findings highlight the significant role of aerosol source and transport in shaping the physical and chemical properties of free tropospheric aerosols.