The ornamental design for a displayscreen for an air purifier, as shown and described.
FIGS. 1-7 are top perspective views of a display screen for an air purifier as an inner circular element changes its shape;
FIGS. 8-14 are left perspective views of FIGS. 1-7;
FIGS. 15-21 are front views of FIGS. 1-7;
FIGS. 22-28 are right perspective views of FIGS. 1-7; and,
FIGS. 29-35 are bottom perspective views of FIGS. 1-7.
The broken lines showing various elements of a display screen for an air purifier are included for the purpose of illustrating non-claimed subject matter and form no part of the claimed design.