Measurement report: Mesoscale heterogeneity in the intensive optical properties of long-range-transported Shinmoedake-related volcanic aerosol observed with dual-site polarization Raman–Mie lidars

Abstract. Long-range transport can substantially modify volcanic aerosol, but it remains unclear whether aerosol from a common eruptive source retains a consistent optical state across neighboring remote receptors. We investigated elevated aerosol observed over Anqing and Shouxian in eastern China during the July 2025 Shinmoedake activity using eruption records, Himawari-9 imagery, Sentinel-5P sulfur dioxide observations, backward trajectories, and same-design multi-wavelength polarization Raman–Mie lidars. Independent source and transport evidence indicates that both receptor layers were associated with the same regional Shinmoedake-related transport episode after approximately 3–4 days. Under strictly matched sampling, the two sites showed similar median 355-nm backscatter but markedly different polarization and spectral properties. Median particle depolarization ratios at 355 nanometres were 0.019 at Anqing and 0.101 at Shouxian, while the corresponding backscatter Ångström exponents were +2.10 and -0.35. The wavelength dependence of depolarization also showed opposite behavior at the two sites. These contrasts cannot be explained by a simple difference in aerosol amount, and available meteorological and regional aerosol constraints do not support straightforward explanations based on local humidity, boundary-layer mixing, or sulfate abundance. The observations are consistent with source- and transport-history-dependent differences in particle optical state, potentially involving size-selective removal, aging, and mixing. Our results demonstrate that aged volcanic aerosol within a common regional transport event can exhibit substantial mesoscale optical heterogeneity, with implications for aerosol identification, spatial representativeness, and model evaluation.