Black carbon is found worldwide but its presence and impact are particularly strong in Asia. Image: NASA, December 2009
New Delhi: Scientists have found that soot emitted in ‘hotspots’ in the Indo-Gangetic can travel all the way to the Arctic in as little as a week.
The findings, reported in a paper in the journal Geophysical Research Letters, add to a growing body of research on the problem of soot from various sources around the world that are deposited in the Arctic, accelerating the rate at which ice melts.
Scientists from the Finnish Meteorological Institute, Helsinki, and the University of Washington, Seattle, traced the path that soot, or ‘black carbon’, takes to the Arctic from different points of origin in the Indo-Gangetic Plain.
The scientists measured surface soot concentrations at six Arctic sites, standardised the measurements into a single data set, and analysed the quantities according to various source regions.
Using computer models, they estimated ‘back trajectories’ – the most likely central path over geographical areas – by following a parcel of air backward in hourly steps for a specific duration, in this case seven days.
Tracing trajectories this way helps pinpoint important sources of light-absorbing particles and aerosols, or suspensions of tiny solid particles or liquid droplets in the air, over the Arctic.
The team found that there was a higher concentration of polluting particles over Central Asia than over other regions.
Older studies have already been able to identify two clearly defined pollution ‘hotspots’ in Asia – the Indo-Gangetic Plain and Eastern China.
The new study reported that “pollution from the Indo-Gangetic Plain is shown to be far more likely to end up in the Arctic than has previously been shown. … This source area shows up in all low-altitude Arctic monitoring stations included in this study.”
That is, the pathways that the researchers identified don’t contribute to isolated ‘pollution events’ in the Arctic but show up in multiple measurement stations on the icy continent. And at at least half of these stations, the soot influx often exceeded the higher winter-time levels of light-absorbing aerosols.
Snow and sea ice reflect the bulk of sunlight back to the sky. But polluting particles or other impurities in the ice can absorb some of the light, warm up and speed up ice melt.
Scientists have previously reported that the Arctic is particularly sensitive to changes in surface temperature, which are in turn linked to the extent of sea ice. So the potential of soot to disturb surface temperatures in the Arctic makes its measurements “particularly important”, the paper notes.
John Backman, a scientist at the Finnish Meteorological Institute and one of the paper’s authors, told The Wire Science that his team’s study throws light on emissions that originate from the northern parts of South Asia.
“As pollution levels are quite high on the Indo-Gangetic plain, we identified those regions as a significant source of black carbon aerosols in the Arctic,” he said. This was the case, he added, for five of the six Arctic measurement stations included in the study.
“Our study did not actually try to quantify the amount of black carbon that is deposited onto snow and ice in the Arctic,” Backman explained. “What we found was which areas are important for the levels of black carbon in the Arctic, and the Indo-Gangetic Plain stood out.”
“Black carbon ends up in snow and ice by sticking to the surface or by sticking to snow and rain when it falls to the ground,” Backman said. “No black carbon in the atmosphere – then no black carbon in snow either, so they are related.”
It’s a known fact that winds can transport aerosols over long distances, but measurements of these distances are often limited and uncertain, according to Tirthankar Banerjee, an assistant professor at the Institute of Environment and Sustainable Development, Banaras Hindu University.
He and his colleagues studied the haze that hangs over South and Southeast Asia between September and February using satellite data from 2010 to 2020.
After analysing the source, types and vertical profiles of aerosols, their study paper, published on March 6, concluded: “No evidence of intercontinental transport of aerosols is, however, noted.”
So, Banerjee added, the new study represents a significant development. “Although there is speculation that these pollutants are mainly emitted from India and China, to reasonably [determine] the source location is impressive work.”
Previous reports of long-distance soot transport have included emissions from East Asia finding their way to the North Pacific, and South American, African and Australian emissions dominating soot loads over the South Pacific. “But the lifetime of black-carbon aerosols varies and depends on existing meteorological conditions,” Banerjee said.
So on the flip side, the method used in the new study to trace the transport of aerosols can’t track sudden fluctuations in local or regional meteorology, he pointed out.
Yutaka Kondo, a professor at the National Institute of Polar Research (NIPR) in Japan, also said the new study is “interesting” for its use of trajectories that connect measurement sites in the Arctic to Asian sources.
“Some high black-carbon events seem to be due to transport from South Asia, mainly India. This is a demonstration of pathways from South Asia to the Arctic,” he said. The findings thus add to older studies that were based on three-dimensional models and showed that some portion of the soot in the Arctic likely originated from South Asia.
However, he also said that the trajectory analysis is still an approximation. “Air parcels undergo mixing and removal by precipitation during transport from India to the Arctic. It is not possible to include these processes in the trajectory model used in this paper.”
In the past, Kondo’s team has measured soot mass at four Arctic sites accurately in order to use it to test the models. “It will be interesting to quantify the contributions of black-carbon emissions in India to black carbon observed in the Arctic by using our models in the near future,” Kondo added.
For the time being, the trajectory study suffices to say that India may be a more important source than its East Asian counterparts as far as soot in the Arctic is concerned. And “this is an important message,” according to Kondo. “There is a need to investigate this point further.”
This in turn means scientists will need accurate information about soot emissions out of India. Kondo and his colleagues at NIPR team are currently working with Indian scientists to measure soot in New Delhi, he finished.
T.V. Padma is a freelance science journalist.