Effects of Fog on AQI:

Fog a Natural Meteorological Phenomenon:

Fog is a natural weather phenomenon that forms when water vapor condenses into tiny droplets suspended near the Earth’s surface. While it significantly reduces visibility, fog itself is not a pollutant. Unlike smog—which is a combination of smoke and fog—natural fog consists only of atmospheric moisture.

How Fog Interacts with Air Quality:

Fog often develops under specific meteorological conditions such as low wind speeds, stable atmospheric layers, and temperature inversions. These conditions restrict the vertical and horizontal movement of air, preventing pollutants from dispersing. As a result, pollutants already present in the atmosphere accumulate near the surface, leading to a rise in the Air Quality Index (AQI).

Key contributing factors include:
Low Wind Speeds: Limit the dispersion of pollutants, allowing them to remain concentrated near ground level.
Temperature Inversions: Warmer air aloft traps cooler air below, effectively sealing pollutants in the lower atmosphere.
High Humidity: Enhances chemical and physical processes that increase fine particulate matter, particularly PM2.5.

Example Scenario:
Consider a situation where the AQI is already around 200 (Unhealthy category). Fog formation coincides with stagnant atmospheric conditions. Existing pollutants remain trapped close to the surface. Continuous emissions from vehicles, industries, and household activities further add to the pollutant load. With little or no wind, pollutant concentrations increase, keeping AQI elevated or pushing it higher. Air quality improves only when winds strengthen or atmospheric mixing increases, allowing pollutants to disperse and fog to dissipate.

Key Takeaways:
Fog does not generate pollution; it creates conditions that trap existing pollutants. Elevated AQI during foggy periods is mainly due to reduced dispersion and ongoing emissions. Improvement in air quality occurs once wind speeds increase and the fog clears. While fog is a natural phenomenon, its interaction with pollution underscores the need for close AQI monitoring during foggy conditions, particularly in urban and industrial regions where emissions are continuous. PM2.5 is defined by dry particle size. In high humidity, particles absorb moisture and appear larger, increasing measured concentrations, even though their original size remains within PM2.5.

AQI situation worsens during fog which is not under our control.

That’s how nature works !