33. 已知正方体$ABCD-A_{1}B_{1}C_{1}D_{1}$的棱长为2，点$P$为线段$AA_{1}$的中点，若点$P\in$平面$\alpha$，且$AC_{1}\bot$平面$\alpha$，则平面$\alpha$截正方体$ABCD-A_{1}B_{1}C_{1}D_{1}$所得截面的周长为$($　　$)$