Fixing the Reference Frame for PPMXL Proper Motions Using Extragalactic
  Sources
  We quantify and correct systematic errors in PPMXL proper motions using
extragalactic sources from the first two LAMOST data releases and the
Veron-Cetty & Veron Catalog of Quasars. Although the majority of the sources
are from the Veron catalog, LAMOST makes important contributions in regions
that are not well-sampled by previous catalogs, particularly at low Galactic
latitudes and in the south Galactic cap. We show that quasars in PPMXL have
measureable and significant proper motions, which reflect the systematic
zero-point offsets present in the catalog. We confirm the global proper motion
shifts seen by Wu, Ma, & Zhou (2011), and additionally find smaller-scale
fluctuations of the QSO-derived corrections to an absolute frame. We average
the proper motions of 158,106 extragalactic objects in bins of 3x3 degrees and
present a table of proper motion corrections.
