The ornamental design for air treatment device, as shown and described.
FIG. 1 is a perspective view of the air treatment device of the new design;
FIG. 2 is a left side elevational view thereof, the right side elevational view being identical thereto;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
FIGS. 1-6 show features of the air treatment device in phantom lines. These features are shown for environmental purposes and do not form any part of the claimed design. Further, the phantom lines immediately adjacent the solid-line portions of the design form the boundary of the design, with the phantom lines forming no part of the claimed design.