Aerosols

Aerosols

Context

A groundbreaking study led by IIT Madras and published in Science Advances revealed that aerosol pollution is a primary driver of the intensifying winter fog across North India. The research highlights a "vicious cycle" where aerosols not only trigger fog but also make it significantly thicker and longer-lasting, disrupting aviation and public health in the Indo-Gangetic Plain.

About Aerosols

What are they?

Aerosols are tiny solid or liquid particles suspended in the atmosphere. They range in size from a few nanometers to several micrometers and can remain airborne for days or weeks.

Origin and Types:

• Natural Sources: Mineral dust from deserts, sea spray (salt), volcanic ash, and smoke from wildfires.
• Human Sources: Vehicle exhausts, industrial smoke, biomass burning (crop residue), and coal combustion.
• Formation:
  • Primary Aerosols: Emitted directly as particles (e.g., soot/black carbon).
  • Secondary Aerosols: Formed in the air through chemical reactions of gases like sulfur dioxide and nitrogen oxides .

Key Findings: The IIT Madras Study (2026)

The study introduced a new metric called AODFOG (Aerosol Optical Depth above Fog) to quantify how pollution above the fog layer affects its behavior.

• Fog Invigoration: High aerosol concentrations act as "seeds" (Cloud Condensation Nuclei), allowing more water vapor to condense.
• Thickening Mechanism: On highly polluted days, fog layers in North India were found to be 15–20% thicker (reaching heights of 400–600 meters).
• Vertical Mixing: Aerosols at the top of the fog layer enhance radiative cooling and latent heat release, which creates buoyancy and "stirs" the fog, preventing it from dispersing.
• Nighttime Intensity: This "invigoration" is most pronounced during the night, leading to the zero-visibility conditions frequently seen at Delhi's IGI Airport.

Implications of Aerosols

1. Environmental and Weather

• Cloud Formation: Aerosols are essential for clouds; without them, water vapor wouldn't have a surface to condense upon.
• Albedo Effect: Reflective aerosols (like sulfates) bounce sunlight back into space, causing global dimming and surface cooling.
• Warming Effect: Absorptive aerosols (like black carbon or soot) soak up solar energy, warming the upper atmosphere while cooling the surface—this stabilizes the air and traps pollutants near the ground.

2. Public Health

• Respiratory Impact: Fine particles ($PM_{2.5}$) penetrate deep into the lungs and enter the bloodstream, worsening asthma and bronchitis.
• Cardiovascular Issues: Long-term exposure is linked to heart disease and stroke.

Challenges and Way Forward

• The Feedback Loop: Fog traps aerosols near the ground, which in turn makes the fog denser, leading to "severe hazy episodes" where pollution levels spike by 30–40%.
• Regional Transport: Research shows aerosols from North India can travel as far as the southeast coast (Chennai), worsening air quality thousands of kilometers away.
• Policy Needs: Tackling winter fog requires a shift from just "managing visibility" to aggressively reducing aerosol emissions from biomass burning and vehicles.

Conclusion

Aerosols are no longer seen as just a byproduct of pollution but as active "weather makers." The ability of these tiny particles to thicken fog and alter local temperatures makes them a critical factor in India's climate and public safety strategies.