I claim the ornamental design for a portable aroma diffuser, as shown and described.
FIG. 1 is a front view of a portable aroma diffuser showing my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a left-side view thereof;
FIG. 4 is a right-side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof;
FIG. 7 is a front perspective view thereof; and,
FIG. 8 is a rear perspective view thereof.
The shade lines shown in the drawings indicate the character and contour of surfaces represented and should not be confused with surface ornamentation.