  We present measurements of the optical broadband atmospheric extinction
coefficients and the night sky brightness at the Xuyi Observational Station of
Purple Mountain Observatory (PMO). The measurements are based on CCD imaging
data taken in the Sloan Digital Sky Survey g, r and i bands with the Xuyi
1.04/1.20m Schmidt Telescope for the Xuyi Schmidt Telescope Photometric Survey
of the Galactic Anti-center (XSTPS-GAC), the photometric part of the Digital
Sky Survey of the Galactic Anti-center (DSS-GAC). The data were collected in
more than 130 winter nights from 2009 to 2011. We find that the atmospheric
extinction coefficients for the g, r and i bands are 0.70, 0.55 and 0.38
mag/airmass, respectively, based on observations taken in several photometric
nights. The night sky brightness determined from images of good quality has
median val- ues of 21.7, 20.8 and 20.0 mag/arcsec2 and reaches 22.1, 21.2 and
20.4 mag/arcsec2 under the best observing conditions for the g, r and i bands,
respectively. The relatively large extinction coefficients compared with other
good astronomical observing sites are mainly due to the relatively low
elevation (i.e. 180 m) and high humidity of the Station.
