Widespread fog invigoration due to aerosols over Northern India.

Abstract

Thick fog events frequently occur in heavily polluted atmospheric boundary layers of Northern India. These events tend to persist longer, causing widespread socioeconomic disruption. Using long-term satellite observations across North India, we find that elevated aerosol concentrations are associated with thicker fog and larger droplets near the fog top. However, the underlying mechanism of this observed relationship remains unclear. High-resolution aerosol-coupled fog simulations suggest that increased aerosol loading enhances fog droplet number concentrations, leading to greater latent heat release. This, in turn, results in buoyancy-induced vertical mixing within the fog. Simultaneously, the increased water content intensifies longwave radiative cooling at the fog top, promoting cool-saturated conditions that further favor the formation of condensate and larger droplets. Together, the aerosol-induced vertical mixing-driven by thermodynamic uplift and radiative cooling-amplifies condensational processes, causing fog invigoration. We find that this phenomenon is especially pronounced during nighttime. Overall, our findings indicate a mechanistic pathway by which aerosols increase fog thickness over Northern India.