In Conversation with our laureates: S K Satheesh breaks down his work and its relevance today

You won the Infosys Prize for your study of soot, a black carbon aerosol particle. What is an aerosol? And how is soot different from other aerosols?

Atmospheric aerosols are solid or liquid particles suspended in the atmosphere.

Aerosols have the ability to alter climate by scattering and absorption of solar and terrestrial radiation. Aerosols are of natural and human-made (also known as anthropogenic) origin. While sea-salt aerosols (originating from oceans) and mineral dust aerosols (originating from deserts and arid/semi-arid regions) constitute major natural aerosols, sulphates, and black carbon are the major anthropogenic aerosol species.

Black carbon, or soot, is produced due to incomplete combustion of fossil fuels, biofuel, and biomass. Black carbon aerosols are highly absorbing in nature and hence can lead to atmospheric warming. Therefore, black carbon is considered as one of the most important aerosol species as far as climate impacts are concerned.

How are scattering and absorbing aerosols different? How does each influence the climate?

Radiative effects of different aerosol species vary depending on their composition.

While sulphates are scattering type, black carbon aerosols are highly absorbing type. Scattering aerosols are cooling agents since they scatter radiation back to space. Absorbing aerosols are warming agents because they have the ability to absorb radiation. The question of whether aerosols cool or warm the planet depends on the relative strengths of various species which make up the aerosol.

Atmospheric aerosols, in general, affect the climate significantly. Of late, several reports focus on the impact of carbon aerosols on the monsoon, despite contradictions and processes that are yet to be fully understood.

On a high level, what is the role of soot in atmospheric phenomena like monsoons and global temperature change?

Our field experiments revealed the presence of “elevated aerosol layers” at high atmospheric levels.

Since air is thin at this altitude, even the presence of a minor amount of black carbon and consequent absorption can lead to intense warming. Model simulations indicate that elevated aerosols can affect the atmospheric circulation pattern and, subsequently, the regional rainfall distribution over India.

In the case of surface aerosol sources, phenomena like clouds and rain remove aerosol particles from the atmosphere. But when black carbon particles are directly injected at an altitude of ten to twelve kilometers above the surface by airplanes, there are no removal processes except the action of gravity. It’s possible for these particles to reach stratospheric heights. If that happens, the aerosols can even affect our ozone layer!

Aerosols also have an interesting impact on Free Space Optical Communication (FSOC). Could you explain this?

Aerosols can influence FSOC systems through scattering and absorption.

While aerosols can decrease the surface temperature by scattering and/or absorption of solar radiation, absorbing aerosols can increase the atmospheric temperature. Unlike the atmospheric gases like nitrogen, oxygen, carbon dioxide etc, which are spatially homogeneous and follow a well-defined decrease in concentration with increase in altitude, aerosols have large spatio-temporal and vertical variations. These heterogeneities lead to large uncertainties in their contribution to atmospheric warming and cause fluctuations in the refractive indices of the atmosphere, which will cause the FSOC system performance to deviate from the expected estimates.

Now, let’s bring this a little closer to home. How are aerosols in the Indian atmosphere different to those in other regions? Are soot particles unique to the Indian subcontinent? And how do they affect the people of India?

Black carbon sources vary by region. Aerosols from fossil fuel burning sources have larger amounts of black carbon. Black carbon aerosols in the Indian atmosphere are not very different from those in other regions. Over India, however, there is a decreasing trend in black carbon concentration.

Inhalation of aerosols can cause health issues. Since the black carbon aerosols fall within the submicron size range, they are small enough to penetrate even the deepest parts of the lungs, causing respiratory illness.

In view of the ongoing climate crisis, how can we manage the release of harmful aerosols like soot? Is there a way to rid our atmosphere of these particles?

Owing to its absorption properties, black carbon has a warming effect on the atmosphere.

In the short term, we cannot do much about global climate change, because India contributes only about 7% to the global carbon dioxide emissions. However, the sources of aerosols are local. Therefore, it is possible to take up actions to mitigate the impact of aerosols on health. Any action which reduces emissions of greenhouse gases and aerosols (specifically the absorbing type) can contribute to reducing the effects of global warming. Such actions could include transitioning from motor to electric vehicles, and from conventional energy sources to renewable energy sources. Individuals, too, can contribute by switching off electric applicances when not in use, using solar heaters, walking short distances or using bicycles instead of using motor vehicles, planting trees, and so on.

Since we are already facing the consequences of 1°C global warming, it is also important to develop adaptation strategies. More attention must be diverted to the prediction of extreme weather events, floods, heat waves, and similar phenomena.