Environmental controls and nonlinear size dependence of the Ångström exponent over the Tengger Desert

The Ångström exponent (α) is a dimensionless parameter describing the wavelength dependence of aerosol extinction and is commonly used as a proxy for particle size. Conventional parameterizations often assume a monotonic α–size relationship, the nonlinear response of α to particle size variations in arid dust sources remains poorly understood. This study quantifies the nonlinear relationship and its environmental controls using 20 days of high-temporal-resolution, in situ measurements collected directly on the Tengger Desert sand dunes. The results show that α ranged between 1.0 and 1.5, reflecting a persistent fine-particle contribution, but declined sharply to 0.1–0.5 during local dust outbreaks dominated by coarse particles. A polynomial model captured three distinct scattering regimes associated with dune-emitted dust, corresponding to Rayleigh, Mie-transition, and non-selective scattering. Rate analysis further revealed that abrupt coarse-particle enhancements and α reductions were characteristic of dust storm and humid conditions, whereas under arid, low-wind regimes α co-varied positively with coarse-particle fluctuations. Overall, these findings demonstrate that particle-size structure and its climatic modulation over active dunes exert a first-order control on the optical response of α. The study provides a quantitative characterization of the nonlinear regulation of α by particle-size distributions, offering a new framework for linking dust microphysical properties with optical behavior in arid source regions.