Field-Constrained Diurnal Viscosity Variability of Urban PM2.5 and Implications for N2O5 Uptake across Global Megacities

Abstract Aerosol phase state governs condensed-phase diffusion and heterogeneous reactivity, yet viscosity constraints for ambient urban PM2.5 remain scarce. Here, we quantified relative humidity (RH)-dependent viscosities of urban PM2.5 collected in Ansan, South Korea, during summer 2024 using a poke-and-flow technique coupled with fluid-dynamics simulations, yielding 6.2 × 104 to 1.3 × 107 Pa s at RH ∼20–40% and exceeding ∼108 Pa s at RH < ∼10%. Integrating the Ansan data set with previously reported PM2.5 viscosities from Seoul and Beijing, a unified temperature–RH parametrization was derived using the Vogel–Tammann–Fulcher framework and applied via a machine-learning surrogate to hourly meteorological data from 14 global megacities over September 2023–August 2024. Predicted PM2.5 viscosity was systematically lower at night than during the day across all cities, driven by RH-induced plasticization that outweighed nighttime cooling, although these predictions were based on 24 h PM2.5 composition and therefore did not include time-resolved changes in aerosol chemical composition. Incorporating viscosity-dependent N2O5 diffusivity into a resistor-model framework yielded mean nighttime N2O5 uptake coefficients of approximately 0.01–0.07, up to approximately 1 order of magnitude below the conventional liquid-particle assumption of 0.1, though substantial uncertainty remains from propagated viscosity measurement errors and parametrization assumptions. These results suggest that viscosity measurements from field-collected PM2.5 can provide improved estimates of diurnal aerosol-phase conditions relevant to nocturnal N2O5 reactivity and nitrate formation in urban atmospheres.