The ornamental design for a microbialairsampler, as shown and described.
FIG. 1 is a right front perspective view of a microbial air sampler showing my new and ornamental design, a left front perspective view being a mirror image; and,
FIG. 2 is an elevational right side view thereof, a left side view being a mirror image.
The broken lines in FIG. 1 represent the bounds of the claim and form no part of the claimed design.