Aerosols emitted in north India not only increase the pollution levels in that region, but due to anticyclonic wind circulation the emitted aerosols get transported towards the southeast coast of India and Bay of Bengal.
The regional transport of aerosols from north India to southeast coastal India occurs for two to four days at a stretch twice or thrice a month during the period December to March. The transported aerosols by themselves and by altering the thermal structure of the atmospheric column from the surface to a height of four-five km, vastly increases the PM2.5 load along the southeast coast.
Based on satellite data, ground-based lidar sensor, radiosonde measurements, and back trajectory analysis, researchers from IIT Madras and SRM Institute of Science and Technology (SRM IST) located in the outskirts of Chennai were able to investigate and characterise the regional aerosol transport over the eastern coast of India during the months of December to March during the period 2015 to 2024.
While the Lidar sensor at SRM Institute helps in providing aerosol profile, radiosonde measurements help in providing temperature and moisture profile, and back trajectory analysis helps in knowing from where the air mass is coming from. In addition, PM25 measurements routinely made at the U.S. Embassy and Consulate in Chennai on an hourly basis were used in the study.
The researchers also quantified the associated changes in the air temperature profiles leading to increased stability of lower-troposphere, and reduction in the height of the atmospheric boundary layer (ABLH) and measured the increase in the surface PM2.5 over Chennai.
“The transported aerosols are present between 1 km and 3 km altitude and are primarily absorbing types — black carbon and aged organic carbon. The absorbing aerosols have significant radiative effects that alter the boundary layer,” says Dr. Chandan Sarangi from the Department of Civil Engineering at IIT Madras and the corresponding author of a paper published recently in the journal Atmospheric Chemistry and Physics.
Aerosol concentration is higher during the days when aerosol transported from north India reaches the southeast coast and is discernible up to 5 km altitude, while during other days, aerosols are confined to altitudes less than 1.5 km.
Stabilisation of lower atmosphere
By absorption of incoming solar radiation, black carbon and aged organic carbon aerosols induce a drop in temperature at surface while increasing the temperature around and above the elevations they are found.
While aerosols present in the atmosphere even during clear days results in the cooling at the surface, they found that on days when aerosols transported from north India reaches the eastern coast, the surface cooling gets enhanced by 20-40 Watts per sq. metre.
“The temperature should be decreasing with an increase in altitude. But regional transport of aerosols changes the dynamics such that the atmosphere gets relatively warmer at higher altitudes and cools at the surface,” says Dr. Sarangi.
When temperature increases at higher elevation and decreases at the surface, the temperature gradient with increasing altitude reduces leading to stabilisation of the lower atmosphere. This results in the weakening of the heat exchange between Earth’s surface and the atmosphere, which disturbs the convective process.
“This results in the reduction of the atmospheric boundary layer height — where pollutants mix with the atmosphere — by up to 200-400 metres, during the regional transport events,” he says. “The reduced height of the mixing layer results in further concentration of aerosols and PM2.5 particles at the surface leading to increased severe hazy episodes. Data shows up to 30-40% increase in PM2.5 concentration due to aerosol-induced reduction in the mixing layer,” he added.
In all, the disturbance in the convective process and the reduced mixing layer results in increased concentration of absorbing aerosol at near-surface leading to lower atmosphere warming and surface cooling, says Dr. Sanjay Mehta from Department of Physics at SRM IST and a coauthor of the paper.
Upper air radiosonde measurements collected by India Meteorological Department (IMD) over Kolkata, Bhubaneswar, Vizagapatam, Chennai and Karaikal were used in the study; lidar measurements were made only at the SRM IST off Chennai. The impact of transported aerosols on air quality in megacities such as Kolkata, Bhubaneswar, and Chennai is greater than smaller cities such as Vizagapatam and Karaikal, suggesting a role of local emissions. “In the case of Chennai, there is over 50% increase in PM2.5 concentration during the days when aerosols are transported from north India,” Dr. Mehta says.
Reduced mixing layer height
The researchers identified a positive feedback loop wherein transported aerosols led to reduced atmospheric boundary layer height (ABLH). This led to enhanced local pollution as the pollutants were less likely to be dispersed due to reduced ABLH causing further aerosol accumulation,” Dr. Sarangi explains.
In the case of Kolkata, the relative enhancement in the temperature due to regional transport of aerosols from north India starts at 300 metres height and extends up to 2 km, while in Bhubaneswar and Vizagapatam, it starts at 500 metres height and extends up to 2.5 km. In the case of Chennai and Karaikal (which is further south of Chennai), it starts at 500 metres height and extends up to 3 km. The increase in the altitude of atmosphere warming due to the transported aerosols from Kolkata to Chennai and Karaikal is consistent with the fact that the long-range-transported aerosol plumes get elevated at downwind locations.
In comparison, on days when aerosols are not transported from north India, the height of the mixing layer varies between 1.5 km and 2 km in the case of Kolkata, Bhubaneswar and Vizagapatam, and is between 2 km and 2.5 km in Chennai and Karaikal, which then decreases to 300-400 m for Kolkata, Bhubaneswar and Vizagapatam, and 500 m to 1 km for Chennai and Karaikal.
In the case of Chennai, the researchers observed that warming occurs throughout the column to 3 km over both Chennai and Karaikal. “This suggests that aerosol-induced warming not only enhances the temperature at the altitude where aerosols occur but can also affect the air temperature near the surface,” Dr. Sarangi says.
Compared with clear days (January 23, 2018, and January 28-29, 2018), on January 24-27, 2018, when aerosols were transported from north India to Chennai, the researchers found that the number of transported aerosols was 50-60% higher between 1 km and 2.5 km height. And the height of the mixing layer decreased from 1.4 km to just 300 metres (78% reduction) between January 24 and January 25, 2018.
What the study means
“Our study demonstrates that air quality management in Chennai must consider transboundary aerosol transport. It is essential to put in place strategies to reduce regional emissions, especially during seasons with strong north-south flow,” Dr. Sarangi says. “The study highlights the need for integrated modelling frameworks that include aerosol-radiation interactions for accurate air quality forecasting.”
Published – August 18, 2025 07:56 pm IST
