An air filtration system including a pre-filtration unit having a filter section and a spacer section attached to the filter section and extending downstream from the filter section, a pre-filter attachment device having a pre-filter securing section and a first plurality of coupling members, and a filtration unit. The filter section includes a pre-filter. The spacer section includes an at least partially open end located on a downstream side of the spacer section to expose a downstream face of the pre-filter. The filtration unit includes a filter frame, an upper section thereof including an upstream-facing surface having a plurality of openings, a filter secured inside the lower section of filter frame, and a second plurality of coupling members located on the filter frame and coupleable with the first plurality of coupling members of the pre-filter attachment device to removeably secure the pre-filtration unit to the filtration unit.