A supplemental filter for use in combination with a conventional annular air filter of an internal combustion engine. The supplemental filter in accordance with the present invention is for use in trapping hydrocarbon emissions. The supplemental filter includes a flexible cover which snugly envelops the conventional annular filter. Further, the supplemental filter includes means to effect sealing the flexible cover against opposite axial ends of the conventional filter. The supplemental filter further includes means, carried by the flexible cover, for distributing carbon media over a significant portion of the flexible cover.