The challenge of improving visibility in Beijing

Atmospheric Chemistry and Physics Discussions This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract The " Blue Sky Project " was proposed in 1998 to investigate by how much emissions should be reduced to increase blue sky frequency in Beijing, which hosted the Summer Olympics in 2008. This paper focuses on the temporal variation of visibility and its dependence on meteorological conditions and suspended particles at Beijing using 5 the hourly observed visibility data in Beijing Capital International Airport (BCIA) from 1999 to 2007. It has been found that about 47.8% (24.2%) of the hours in Beijing are " bad " (" good ") hours with visibility below 10 km (equal or higher than 20 km) between 1999 and 2007. Due to the high Relative Humidity (RH), summer is the season with the lowest mean visibility in a year. Although PM 10 index was reported in a decreasing 10 trend, the increase of RH has resulted in a decreasing trend of visibility over BCIA in the summer from 1999 to 2007. To ensure blue sky (" good " visibility) for Olympics 2008, daily mean PM 10 index should be reduced to 44 from 86.5. This requires that not only vehicle emissions, but also other emissions should be limited. Observations during Olympics 2008 verify that blue-sky-hour rate has been increased significantly 15 after mean PM 10 index was reduced to 56, however, the visibility during the same period of 2009 has returned to the mean standard before 2008.