We demonstrate an integrated optofluidic system for monitoring the particle mass concentration and the Sauter mean diameter of polydisperse flowing suspensions of water and standardized test dust. For optimum integration, a planar emitter-receiver unit is developed and fabricated on Si technology. A vertical cavity surface emitting laser at 850 nm serves as light source, and monolithically integrated segmented photodiodes detect both the attenuation of the primary light beam and the scattered light. The optical system is integrated into a planar transparent polymethylmethacrylate substrate by micromilling of an optical freeform surface and electron beam evaporation of reflective layers following the concept of planar integrated free-space optical systems. We are able to detect 1 mg/L of the standards ISO 12103-A2 (fine test dust), -A3 (medium test dust), and -A4 (coarse test dust) and to determine the Sauter mean diameter of the particle size distribution.