Our paper in Environmental Research Letters focuses on atmospheric pollution over Kanpur, an industrial city lying in the centre of the IGP. Ramesh Singh formerly with the Indian Institute of Technology in Kanpur, and now at the School of Earth and Environmental Sciences at Chapman University, US, employed a sunphotometer belonging to NASA's global AERONET programme. The instrument measures the aerosol concentration in the atmosphere and derives various aerosol properties from the data.

The idea of continuously measuring the amount of aerosol particles above Kanpur over a long period of time came about when Singh analysed his earlier satellite data, which showed extensive pollution over the IGP. While living in Kanpur himself, Singh says he frequently had to put up with dense haze, fog and smog during the winter, which made it very difficult to drive because of the reduced visibility (less than 10 metres). The weather conditions also affected flights and trains, and generally made life difficult for people living in the region.

The new ten-year-long data series has been used extensively by both the Indian and international community in numerous investigations. What's more, several papers have been published in an effort to understand how atmospheric aerosols affect the climate and monsoon in this part of the world.

The present study shows that aerosol concentrations have increased over Kanpur in the last 10 years, particularly during the extended dry winter season, which is usually when air quality and visibility are poorest. Aerosol concentrations during winter are mainly made up of fine particulates, emitted by fossil-fuel burning in industry, coal-based thermal power plants and vehicles, and by biomass combustion during crude methods of cooking and heating. In the Indo-Gangetic plains, the density of coal-based power plants is very high. Such fine particulates can potentially cause serious respiratory problems.

"Interestingly, we found a contrasting weakly negative trend [over the last ten years] in aerosol concentrations during the spring and early summer, periods that are dominated by long-range transport of desert dust aerosols," says Singh. "Our data show a weak spring-time trend, which points to large inter-annual fluctuations in dust emissions from the Thar Desert in India and from as far away as the Arabian Peninsula. Overall, our observations of increasing amounts of aerosols, especially during winter, suggest increasing air pollution over Kanpur in particular and over northern India in general. We believe the increasing air pollution is linked to growing energy demands and emissions in the region."

The Kanpur AERONET is the only station of its kind to provide such long-term information about aerosol concentrations in southern Asia.