Characterizing efflorescence regimes in organic–inorganic aerosols using thermodynamically modeled viscosity

Atmospheric aerosols, especially internally mixed organic–inorganic aerosols, exhibit complex phase behaviors that affect their size evolution, optical properties, and chemical reactivity, ultimately impacting climate and human health. Although parameterizations for secondary organic aerosol phase state exist, quantitative descriptions of efflorescence behavior in organic–inorganic aerosols remain underdeveloped. Herein, we evaluated chemical parameters, including O:C ratio, organic mass fractions, glass transition temperature (Tg), and viscosity (η), and identified aerosol viscosity as the primary variable associated with efflorescence relative humidity (ERH) in organic–inorganic aerosols. We developed a viscosity–ERH framework, which defines boundary conditions that separate viscosity-humidity space into three empirical regions. Above the fitted line, aerosols remain in an aqueous state, whereas below the line, efflorescence occurs. Additionally, efflorescence is unlikely to occur when η exceeds a threshold value (>4.76×102Pas). The predicted boundary line was validated using an independent dataset, and a multivariate regression model incorporating η and Tg improved the statistical performance of the boundary line but was limited by Tg parameterization for complex organic–inorganic mixtures. Our findings highlight the association between aerosol viscosity and efflorescence and emphasize the need to develop improved aerosol viscosity measurement techniques to better constrain aerosol phase behavior in atmospheric models.