Aerosols enhance winter fog lifetime over North India

Anthropogenic aerosols affect cloud properties and change theirlifetime, a phenomenon dubbed as aerosol-cloud interactions (ACi). Radiation fog is asurface-level cloud formed due to night-time radiative cooling of the land surface. Each year,north India experiences several prolonged fog events. These multi-day fog episodes canaffect surface visibility and air quality, affecting human health and the transportation sector.From long-term observations, we find that the fog duration and foggy days are enhancedduring high aerosol loading periods over north India.However, the mechanistic role of heavy aerosol pollution on the evolution, lifetime,and frequency of North Indian fog episodes is still poorly understood. Using chemistry-coupled regional model simulations, we find ACi leads to fog lifetime enhancement byproducing smaller droplets and reducing the droplet deposition rate. During the daytime, theenhanced activation of aerosols into droplets prevents evaporation from the surface.Interestingly, during this period, the aerosol radiative effect also helps produce conducivesurface conditions to delay fog dissipation. The unequivocal role of aerosol effects on the foglifetime over North India suggests the urgent need to regulate particulate pollution to reducelong periods of fog.