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Visibility impairment has become an important environmental issue receiving great attention from both the scientific community and the public. Long-term meteorological data from Baoji, China, were collected to investigate the trend in visibility change from 1980 to 2012. The 33-year average visual range is 12.0 km. The best 20% of the visibility observations in a calendar year shows a general decreasing trend from 1994 onwards, while the worst 20% exhibits a slight increasing trend from 1997 onwards. These results suggest the progressive degradation of air quality in Baoji in recent years. Intensive PM_2.5 measurements were conducted from March 2012 to February 2013 to determine the causes of visibility impairment. An analysis based on IMPROVE equation reveals that PM_2.5 organic matter (OM) contributes to 34.2% of the light extinction coefficient (b_ext) on an annual basis, followed by (NH₄)₂SO₄ (30.0 …