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Updated: Aug 20, 2019 04:34 IST

Indian Space Research Organisation (ISRO) and National Aeronautics and Space Administration (NASA) scientists have found the first observational evidence of pollution in a layer of atmosphere where it has never been seen before. This may have an impact on global and regional weather systems and climate, the scientists said.

A team of international scientists has found an interaction between aerosols and clouds near the tropopause – boundary layer between the upper troposphere and the lower stratosphere – that can warm or cool the atmosphere, thereby having an impact on climate.

A plastic balloon flight with scientific payloads floating at an altitude of 26km on July 16 found that ice crystals or cirrus clouds (wispy clouds that form at high altitudes) were formed at 18-19km owing to a high concentration of water vapour and aerosols. The observation was made during experiments conducted at the Tata Institute of Fundamental Research-Balloon Facility (TIFR-BF), Hyderabad, between July 3 and August 1.

Aerosols are very fine solid or liquid micro particles that are released from man-made and natural processes such as vehicle exhaust, waste-burning, wind-blown dust and volcanic eruptions. Clouds are formed when water vapour condenses upon aerosols, which serve as cloud condensation nuclei. Aerosols can reflect and absorb sunlight, and can be important for atmospheric chemistry and cloud formation.

“We were not expecting such high aerosol concentration at 18-19km. Though established theoretically, no one has reported aerosol-cloud interaction at that level,” said M Venkat Ratnam, senior scientist at the National Atmospheric Research Laboratory (NARL), which is under the Department of Space.

Jean-Paul Vernier, scientist, NASA, agreed. “This is the first observational evidence that aerosol-cloud interaction is also taking place at higher altitudes near the tropopause, and can warm or cool the atmosphere.”

For instance, cirrus clouds have the potential to warm or cool the atmosphere. This is because they absorb the earth’s thermal radiation or can reflect solar radiation, thereby cooling the atmosphere depending on their interaction with the type of aerosol.

Every monsoon season, thunderstorm clouds provide a vehicle for the transport of pollution into the stratosphere. In 2011, satellite observations by NASA detected a layer of pollutants – popularly called the Asian Tropopause Aerosol Layer (ATAL) – that puzzled the international scientific community.

“This layer can have serious implications on climate as it covers a large area from the Eastern Mediterranean sea to east China during the monsoons in mid-June to mid-September,” said Vernier.

Gwenael Berthet, scientist, CNRS-France added, “After the monsoon, ATAL spreads over the Northern Hemisphere with potential global impact.”

To therefore quantify the nature of the pollutants reaching the stratosphere, and explore how Asian pollution affects the monsoon over the Indian subcontinent, over the last four years ISRO and NASA carried out a series of balloon campaigns – Balloon Asian Tropopause Aerosol Layer (BATAL), from the TIFR-BF, Hyderabad.

Just as the discovery of ATAL was puzzling, another “unusual and unexpected” observation from the July 16 balloon launch has left climate scientists baffled. The team found that pollutants rose up to 5km before suddenly dropping – an indication that pollutants near the tropopause could be transported from anywhere in the Middle East to China – and will now analyse atmospheric conditions that caused pollutants to drop at that altitude, said Ratnam.

Conventionally, all pollutants should drop at boundary layer altitude of 1.5km to 2km. “A drop at 5km can have an impact on background clouds through aerosol-cloud interaction that takes place at that altitude. Properties of background clouds can also be modified, which means that it may not rain despite cloud formation,” said Ratnam. “These are complex interactions and their impacts can have a bearing on the monsoon.”

With a deficit of precipitation in most parts of India up to mid-monsoon, NARL director AK Patra said, “Data collected by scientists could help in understanding the relationship between pollution transport and its effect on precipitation in the Indian subcontinent.”

This year, scientists from NASA, NARL-ISRO, CNRS-France started experiments at TIFR-BF with advanced in-situ and remote sensing instruments between July 3 and August 1.

“TIFR Balloon Facility has developed sophisticated plastic balloons to float at the coldest part of atmosphere at the edge of the stratosphere (18km). Very few countries including India have this capability,” said professor Devendra Ojha, chairperson, TIFR-BF committee.

Suneel Kumar B, scientist-in-charge, TIFR-BF, added, “Most of the success lies in recovering the payload without damage, which is one of the main strengths of TIFR-BF.”