  Recently, the superstring was covariantly quantized using the BRST-like
operator $Q = \oint \lambda^\alpha d_\alpha$ where $\lambda^\alpha$ is a pure
spinor and $d_\alpha$ are the fermionic Green-Schwarz constraints. By
performing a field redefinition and a similarity transformation, this BRST-like
operator is mapped to the sum of the Ramond-Neveu-Schwarz BRST operator and
$\eta_0$ ghost. This map is then used to relate physical vertex operators and
tree amplitudes in the two formalisms. Furthermore, the map implies the
existence of a $b$ ghost in the pure spinor formalism which might be useful for
loop amplitude computations.
