Temperature–RH Dependent Viscosity of Organic Aerosols from 273 to 303 K: Implications for Global N₂O₅ 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, spanned ~9 orders of magnitude. A Vogel–Fulcher–Tammann fit with experimentally derived fragility (D_f = 13) extended the parameterization to 230–310 K and 0–100 % RH. When coupled with zonal-mean tropospheric T–RH fields (2020–2024), the parameterization yielded global distributions of viscosity and organic-phase mixing time (τ_mix,org) for 200-nm particles: liquid states prevailed below ~2 km, semisolid regimes occupied ~2–9 km (latitude dependent), and near-glassy conditions emerged above ~9 km; τ_mix,org was <1 h in the boundary layer but frequently exceeded 1 h aloft. 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 highlight the need for temperature-sensitive viscosity in next-generation air-quality and climate models.