Temperature–RH dependent viscosity of organic aerosols from 273 to 303 K: implications for global N 2 O 5 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-H 2 O 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 N 2 O 5 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, N 2 O 5 uptake coefficient leveled near ∼ 10 −3.5 aloft, and without it can drop to 10 −5 –10 −6 at viscosity 10 9 Pa s. These results provide evidence for the need for T - and RH-sensitive viscosity in next-generation air-quality and climate models.