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Using a sample of 117053 quasars identified in the PPMXL catalog, thesystematic errors of the proper motions in the PPMXL catalog are estimatedindependently as -2.0 and -2.1 mas yr$^{-1}$ in $\mu_{\alpha}\cos\delta$ and$\mu_{\delta}$, respectively. For objects with SDSS $r$ magnitude between 16.0and 21.0 mag and SDSS $g-r$ color between -0.4 and 1.2, there are no obviousmagnitude and color dependence in the systematic errors of the proper motionsin the PPMXL catalog. The random errors of the proper motions in the PPMXLcatalog change from $\sim4.0$ to $\sim8.0$ mas yr$^{-1}$ along with theincrease of the magnitude of the objects. No color dependence in the randomerrors of the proper motions in the PPMXL catalog is found. There are obviousright ascension $\alpha$ dependence of the systematic errors in both componentsof the proper motions in the PPMXL catalog. For quasars with declination$\delta>0$, the systematic errors of $\mu_{\delta}$ in the PPMXL catalogincrease linearly with $\delta$ with a slope of 0.07 mas yr$^{-1}$ deg$^{-1}$.  For comparison, using a subsample of quasars identified both in the PPMXL andSDSS DR7 catalogs, the systematic errors of the USNO-SDSS proper motions in theSDSS DR7 catalog are derived as 0.9 and 0.5 mas yr$^{-1}$ in$\mu_{\alpha}\cos\delta$ and $\mu_{\delta}$, respectively. There are no obviousmagnitude, color, and $\alpha$ dependence in the systematic errors of theUSNO-SDSS proper motions. For quasars with declination $\delta>0$, thesystematic errors of $\mu_{\delta}$ in the SDSS DR7 catalog increase linearlywith $\delta$ with a slope of 0.03. The random errors of the USNO-SDSS propermotions change from $\sim2.5$ to $\sim6.0$ mas yr$^{-1}$ along with the changeof the magnitude and color of the objects.

Independent determination of the systematic and random errors of the proper motions in the PPMXL catalog using quasars