A rectangular slit jet impactor has been designed and fabricated for use as an airborne collector of supergiant aerosol particles. The width of the slit is variable, allowing the impactor size cutoff to be set so that only particles greater than a determined size are collected. The jet convergence angle was made to be 53° 13′ to ensure that the width of the slit is always the same as the distance from jet mouth to the impaction substrate. Experimental calibration of the impactor indicates that this configuration gives excellent impaction characteristics.