The ornamental design for a displayscreen for an air purifier, as shown and described.
FIGS. 1-9 are top perspective views of a display screen for an air purifier as an inner circular element changes its shape;
FIGS. 10-18 are left perspective views of FIGS. 1-9;
FIGS. 19-27 are front views of FIGS. 1-9;
FIGS. 28-36 are right perspective views of FIGS. 1-9; and,
FIGS. 37-45 are bottom perspective views of FIGS. 1-9.
The broken lines showing various elements of the 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.