Abstract Fogs formed by aerosol activation in supersaturated air often cause extremely poor visibility, yet accurate prediction remains challenging and models typically account for only fog droplet extinction. The role of fog interstitial aerosols has remained largely unexplored due to limited observational capabilities. Here, we developed an advanced optical instrument capable of online measurements of scattering and hygroscopic properties for both interstitial and activated aerosols, enabling direct determination of aerosol hygroscopicity and effective supersaturation in fogs and clouds. A new method is together proposed to quantify the light scattering enhancement of interstitial aerosols under supersaturation. Observations on the North China Plain reveal that hygroscopic growth of interstitial aerosols dominates fog visibility degradation, contributing on average >85% of total extinction, depending on fog water content. These results highlight the critical role of interstitial aerosols in fog visibility reduction and their potentially significant contribution to radiative effects in clouds with low water content.

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