The Purple Mountain Observatory has almost lost its ability to perform as a professional observation post and may change into a museum because of heavy air pollution in Nanjing, Jiangsu province, astronomers said.

According to the provincial meteorological center, the intensity of sunlight has decreased by 14 percent in the past 50 years. Compared with 10 years earlier, the number of days on which astronomical observations can effectively be practiced in a year has decreased by 50.

Also, because of the high concentration of PM2.5 (particles with less than 2.5 micrometers in diameter), the observatory needs to spend large amounts of money to clean the professional equipment.

According to Liu Yu, an expert from the National Astronomical Observatories, it costs more than 100,000 yuan ($16,000) to clean a large-diameter reflecting telescope each time. The lenses of telescopes may need to be cleaned every week to guarantee normal use.