India is making a big push for solar energy, with power capacity expected to double this year. But some of the gains, especially in north India, could be offset by a growing problem: air pollution A new study, the first of its kind in India and one of a handful globally, has found that dust and particulate matter (PM) may be reducing the energy yield of solar power systems in north India by 17-25 per cent annually.Half this reduction comes from dust and particles deposited on the surface of solar panels and which forms a physical barrier to light entry, said Duke University professor Mike Bergin, who led the study. Researchers allowed panels to accumulate dust for a month. Most importantly, half the decline in energy yield came from ambient pollution--haze that reduces the amount of sunlight reaching the ground, a phenomenon known as solar dimming. "This study thus shows that improving air quality can lead to a big improvement in solar energy yield," said Bergin. "Cleaning panels is not enough."Solar energy is the linchpin of the India's renewable energy mission with a target of 100GW of solar power capacity by 2022. The Indian government offers many concessions and incentives to the developers.Solar power plants depend on the availability of sunlight, or solar irradiance. Anything that obstructs sunlight from photovoltaic panels--whether cloudy skies or sand on the panels reduces potential energy generation. Some studies have looked at the impact of dust, especially in Middle Eastern countries.A 2016 study from Baghdad, for instance, found an 18.74% decline in efficiency for solar modules left uncleaned for a month. Another 2014 paper from Colorado, USA found that 4.1% light transmission was lost for every gm2 of dust accumulated on the photovoltaic plate.But air pollution has received less attention. In one rare study, researchers investigated the power output of ten photovoltaic sys tems in Singapore during a haze episode in 2013 due to forest fires in Indonesia. They found that poor air quality caused yield losses of 15­25% in a 10-­week period.Loss of irradiation in a single day peaked at nearly 50%, said Andre Nobre , lead author and head of operations at Cleantech Solar in Singapore. The study did not look at particulate deposits because frequent rain keeps solar panels clean in Singapore, he noted. "For a city like New Delhi, you have the added effect [of] soiling on the panels from the fact that it is a much drier and dirtier city."Bergin's study is the first to quantify the combined impact of ambient particles and deposited matter. Bergin and his colleagues analysed deposits on solar panels at the IIT campus in Gandhinagar , and tracked energy yield before and after cleaning. They then estimated change in solar panel transmittance per unit mass deposited, and developed a model.They found proportions of dust and pollution were roughly equal in north India. What fraction of dust was wind-blown and what was from human-activity was not analysed, noted Bergin, who previously looked at pollution's effect on the Taj Mahal The solution for one part of the pollution problem is simple enough: frequent cleaning. Power generation jumped by on average of 50% after each cleaning, the study found.However, companies cannot control cloud cover or smog, noted Ashish Khanna, the ED & CEO, Tata Power Solar , and instead must "anticipate plausible dips" in irradiation.Air pollution is now a factor in business decisions, according to Nobre, whose company has solar projects across Asia including India. "Developers like ourselves will be extra cautious when signing power purchase agreements with clients with facilities located in highly polluted zones," Nobre said. "Our returns are impacted by air pollution, which in turn end up increasing electricity tariffs we are able to offer. As the fleet of PV systems is drastically growing in India, there could be millions of dollars in revenue being lost."