A system for detecting and analyzing particles in an air stream includes an inlet, a particle concentrator and a particle discriminator having an air channel with a cross-sectional geometry that changes within at least one of the inlet, particle concentrator and particle discriminator. The system may have a sheath air stage including a port for providing sample air, at least one sheath air inlet port for providing sheath air, and a sheath air combining region. The system may include an airflow compression stage having a varying air channel that narrows as the air stream traverses the airflow compression stage to pre-concentrate particles within an interior region of the air stream. The system may include an airflow expansion stage having an air channel that widens to slow the airstream and particle velocities. A portion of the air channel height may be narrowed to allow a larger thermophoretic force to be generated.