Temperature–RH dependent viscosity of organic aerosols from 273 to 303 K: Implications for global N₂O₅ uptake

Organic aerosol (OA) viscosity and phase state govern multiphase diffusion and reactivity, yet systematic constraints across tropospheric temperature (T)–relative humidity (RH) space remain limited. We measured the viscosity of sucrose-H₂O droplets (OA surrogate) over 273–303 K and ∼ 20 %–90 % RH using bead-mobility and poke-and-flow methods, with viscosity spanning ∼ 8 orders of magnitude across the investigated RH range at each temperature. A Vogel–Fulcher–Tammann fit with experimentally derived fragility (D_f = 13 ± 1) extended the parameterization by ∼ 43 K beyond the measurement range, to 230–310 K and 0 %–100 % RH, with substantial uncertainty at low temperatures. When coupled with zonal-mean tropospheric T–RH fields (2020–2024), the parameterization was used to infer global distributions of viscosity and organic-phase mixing time (τ_mix,org) for 200 nm particles, suggesting predominantly liquid states below ∼ 2 km, semisolid regimes across ∼ 2–9 km (latitude dependent), and near-glassy conditions at higher altitudes; with τ_mix,org typically being < 1 h in the boundary layer but often exceeding 1 h aloft. The resulting global fields highlight the potential atmospheric implications of temperature- and RH-sensitive viscosity across the troposphere. Calculations indicated the N₂O₅ uptake coefficient was generally ≥ 10⁻² for liquid particles in the boundary layer, decreased by ∼ 1–2 orders above ∼ 2–4 km as bulk diffusion became rate-limiting; with surface hydrolysis, N₂O₅ uptake coefficient leveled near ∼ 10^−3.5 aloft, and without it can drop to 10⁻⁵–10⁻⁶ at viscosity 10⁹ Pa s. These results provide evidence for the need for T- and RH-sensitive viscosity in next-generation air-quality and climate models.