Key Laboratory of Atmospheric Chemistry (LAC) Centre for Atmosphere Watch and Services (CAWAS) Chinese Academy of Meteorological Sciences Chinese Meteorological Administration Beijing 100081 China;
SKLLQG Institute of Earth Environment Chinese Academy of Sciences Xi'an 710075 China;
Graduate School of the Chinese Academy of Sciences Beijing 100039 China;
National Meteorological Information Center Chinese Meteorological Administration Beijing 100081 China;
visibility; aerosol; horizontal extinction coefficient; air pollution index; Xi'an;