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

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.