Last year, a study found 75% of aerosol absorption over the Indo-Gangetic Plain and the Himalayas comes from black carbon, or soot. It was alarming that pollutants from the plains were reaching the mountains. Also last year, the Arctic sea ice shrunk to its lowest in eight years and nearly half of what it was just four decades ago. Every decade, it is losing 13% of its ice. Scientists have now found what links the two — an 8,500-km expressway that takes soot straight from the heart of north India over central Asia to the icy expanse of the Arctic in just seven days.
A new study by researchers from the Finnish Meteorological Institute, the University of Washington and the National Oceanic and Atmospheric Administration has found that Asian — specifically, Indo-Gangetic — emissions explain much of the black carbon “events” over the Arctic.
“It is a combination of high pollution levels at the source and a pathway that does not dilute or remove the atmospheric pollutants before entering the Arctic,” lead author Dr John Backman from the Finnish Meteorological Institute told TOI. “For example, a more direct and drier route would transport more particulate air pollution from A to B compared to a longer route with more precipitation from that very same A to B.”
What black carbon does is it sticks to the surface of ice and turns it darker. The reflective surface, which would under normal circumstances bounce off sunlight, then starts absorbing more light. That sunlight turns into heat, its surface temperature starts increasing and the ice starts melting. “The Arctic is particularly sensitive to changes in surface temperature, and increases in surface temperature are coupled to sea ice extent,” the paper said. “Because black carbon is so destructive for the Arctic climate, it is essential to understand (its) properties, transport to, and concentrations in the Arctic.”
So, data from six stations in the Arctic — Alert, Barrow, Pallas, Summit, Tiksi and Zeppelin — were collected over a period of three years. “A model was used to calculate a path the air had travelled before arriving at the station. This path was calculated backwards in time using meteorological data from another model,” Backman said. “By combining a path the air had travelled with measured concentrations of pollutants we could construct a map showing which levels of pollutants had come from where.” They did this for all stations, put together individual “pollution maps” for each station and then came up with one large map.
Because black carbon does not linger for too long, they used a seven-day period as a benchmark. “It was a compromise,” Backman said. “The further back in time one calculates, the more uncertain the geographic location becomes. Not to have too much uncertainty, we chose seven days.”
After mapping, the densely populated areas of Europe and North America, closer to the Arctic, did not stand out. Nor did eastern China, otherwise identified as a global black carbon hotspot. Instead, “a significant source of black carbon aerosols seems to come from the northern parts of South Asia.” It measures well above the station average at almost all Arctic stations, and is the lowest during summer. The paper added, “The pathway identified does not contribute to isolated events of pollution for the individual sites but shows up in multiple measurement stations at the same time, indicating that the pollution intrusions spread out over wide areas in the Arctic.