FIG. 1 is a perspective view of an environmental monitoring device showing our new design. FIG. 2 is front view thereof. FIG. 3 is a back view thereof. FIG. 4 is a first side view thereof. FIG. 5 is a second side view thereof. FIG. 6 is a top view thereof. FIG. 7 is a bottom view thereof; and, FIG. 8 is another side view thereof. The broken lines in all views are for illustrative purposes only and form no part of the claimed design.