NEW DELHI: Air pollution in north India doesn't only affect local residents. Emissions from the region's vehicles, farm fires and power plants may be lifted by the monsoon right up to the stratosphere, with possible implications for regional and global climate NASA scientists have detected a layer of tiny aerosol particles in the upper troposphere and lower stratosphere - just below the ozone layer - that forms over Asia during the summer monsoon. Satellite data shows that this layer of pollutants stretches vertically between 13-18km and horizontally from the eastern Mediterranean Sea to western China, said Jean-Paul Vernier, the senior scientist at NASA's Langley Research Centre who first wrote, along with his colleagues, about the phenomenon in 2011.The layer doesn't seem to have been present before the late 1990s, and has been linked to rising emissions in India and China over the past few decades, especially sulphur dioxide from coal-fired power plants. Until recently it was thought that only volcanoes had the power to project pollutants so far up.The existence of this Asian Tropopause Aerosol Layer (ATAL), as it has been named, has been confirmed by balloon experiments in China and India, including joint efforts by NASA, India's National Atmospheric Research Laboratory (NARL), Tata Institute of Fundamental Research and Banaras Hindu University.NARL director Achuthan Jayaraman described the layer as a large disc. "It's thicker over north India and thins out over south India," he said.One hypothesis, Jayaraman said, is that particulates from the "brown cloud" of pollutants over south Asia -especially the Indo-Gangetic plain - may be escaping into the atmosphere.Simulations by Vernier's group suggest the layer is dominated by sulphur and carbon-based aerosols, 70% of which originate from human-created emissions in India and China. Another study has put that proportion at 30%. More work is needed to resolve the issue, Vernier said.The layer could have significant effects on climate, depending on its composition. Sulphate aerosols cool the atmosphere, for instance, while soot has a warming impact. ATAL is not yet a threat to the ozone layer in the stratosphere, Vernier said, adding, "if it increases, it could become one".How could pollution in north India travel so far up into the atmosphere? The answer may lie in the timing of the layer's formation. Monsoon rains are thought to wash out all the pollutants that have accumulated in the summer over north India. But some may survive. These then get pushed upwards by the strong convection provided by the monsoon system."The Summer Asian Monsoon appears to provide an input of aerosols into the stratosphere equivalent to a small volcanic eruption," said Vernier.In winter, temperature inversion traps pollutants on the ground. "However, when pollution increases in winter, the chances are that pollution in summer will also be higher," said Vernier.Aerosols like dust and soot can also provide feedback that leads to enhanced upward transport, added William Lau, senior research scientist at the University of Maryland. Lau's latest work shows that desert dust, soot, carbon monoxide gas from biomass burning, as well as water vapor are transported to the ATAL, including from the Indo-Gangetic plain.A similar action seems to take place in western China. "Much of the lifting of pollutants into the upper atmosphere occurs over South and Southeast Asia and over the Tibetan Plateau," said Arnico Panday, senior atmosphere scientist at the International Centre for Integrated Mountain Development, Kathmandu.A similar but much smaller layer over North America is thought to be dominated by emissions from forest fires.