Temperature–RH dependent viscosity of organic aerosols from 273 to 303 K: implications for global N ₂ O ₅ uptake

Abstract. 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 (Df= 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 (τmix,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 τmix,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.