Why in news?
A global study has classified atmospheric aerosols into seven distinct types. It used observations from 171 monitoring stations across six continents. Dust mixed with pollution was especially common over South Asia. Strongly absorbing aerosols produced the study’s greatest atmospheric heating.
Background
An aerosol is a tiny solid particle or liquid droplet suspended within the surrounding air.
Aerosols range from a few nanometres to tens of micrometres, with each micrometre equalling one-millionth of a metre.
Some aerosols remain airborne for hours or days, depending on their size, altitude and surrounding weather.
Primary and secondary aerosols
| Type | How it forms | Common examples |
|---|---|---|
| Primary aerosol | It enters the atmosphere directly as a particle or droplet. | Mineral dust, sea salt, smoke and volcanic ash are examples. |
| Secondary aerosol | Atmospheric reactions convert emitted gases into new particles. | Sulphates, nitrates and some organic particles are examples. |
Natural sources include deserts and volcanoes, while human sources include vehicles, factories, farming and biomass burning.
How do aerosols affect climate?
Aerosols alter solar energy moving through the atmosphere, producing a change called aerosol radiative forcing.
- Scattering: Bright particles reflect sunlight towards space, usually cooling Earth’s surface.
- Absorption: Dark particles, especially black carbon, absorb sunlight and warm the surrounding air.
- Cloud effects: Aerosols support droplet formation and can change cloud brightness, lifetime and rainfall.
- Snow effects: Dark particles settle on snow, reduce its reflectivity and speed melting.
What did the new study examine?
Researchers used the Aerosol Robotic Network (AERONET), a global network of ground-based instruments called sun photometers.
The instruments measure how particles scatter, absorb and polarise sunlight across more than thirty years of observations.
Researchers classified aerosols using two measurements that indicated particle shape and the strength of light absorption.
The seven aerosol categories
- Pure dust mainly contains mineral particles from dry land.
- Dust-dominated mixture contains mostly dust with some pollution.
- Pollution-dominated mixture contains mostly pollution with some dust.
- Very weakly absorbing aerosol reflects much more light than it absorbs.
- Weakly absorbing aerosol absorbs slightly more incoming light.
- Moderately absorbing aerosol has a stronger light-absorption effect.
- Strongly absorbing aerosol absorbs substantial light and strongly heats air.
Major regional findings
- Pure mineral dust dominated many monitoring sites across northern Africa.
- Dust-dominated and dust-pollution mixtures were common across South Asia.
- Less-absorbing urban aerosols were common in Europe and North America.
- Strongly absorbing aerosols dominated several tropical biomass-burning regions.
What do the numerical results mean?
Strongly absorbing aerosols produced atmospheric forcing of 30.14 ± 8.04 watts per square metre. Heating reached 0.85 kelvin per day.
Very weakly absorbing aerosols produced atmospheric forcing of 7.83 ± 4.12 watts per square metre. Heating reached only 0.22 kelvin per day.
These category averages are not daily local forecasts, while the plus-minus values show variation around each average.
Why is South Asia especially important?
South Asia receives desert dust alongside major pollution emissions, which often combine within one moving air mass.
Black carbon can coat or mix with mineral dust, increasing sunlight absorption and atmospheric heating.
Heating at different heights can alter atmospheric stability, cloud formation, monsoon circulation and regional rainfall.
Health and policy relevance
Many aerosols form particulate air pollution, whose finest particles can enter deep regions of human lungs.
Climate models must represent each aerosol type correctly, because uniform treatment can distort temperature and rainfall estimates.
Better classification can improve satellite retrievals, pollution monitoring and action against the most harmful emission sources.
Conclusion
Aerosols have no uniform climate effect because their composition determines sunlight reflection and atmospheric heating.