A particle sampler has been developed, built, and tested. The sampler draws ambient air at approximately 300 L/min and then splits the sampled air into a particle-rich sample stream and a particle-depleted, reject stream. The particle-rich stream contains only 0.3% of the inlet air (i.e., 1-L/min), but 50-90% of the ambient particles in the size range of 2.3 μm to 8.4 μm. This 1-L/min sample stream contains the particles at a concentration of approximately 150-270 times that of the ambient air. For this reason, the sampler is called an aerosol concentrator. By concentrating the particles of interest, we substantially improve the response time and detection limit characteristics of any detector that may be used downstream of the sampler. The aerosol concentrator is a three-stage virtual impactor. The first stage is a scalper drawing nominally 330 L/min of air through a conventional single-nozzle virtual impactor. Particles larger than 10 microns are retained in the 30-L/min minor flow and rejected from the sampler. The remaining 300 L/min of air passes through a two-stage, concentrating virtual impactor (CVI) that splits the flow into a 1-L/min sample stream and a 299-L/min reject stream. The reject stream consists of 285 L/min from the first stage and 14 L/min from the second stage. A blower draws the 299-L/min reject stream and exhausts it through the nozzle of an ejector. The ejector contains a venturi-like tube that aspirates the 30-L/min reject stream from the scalper, making an overall exhaust stream of 329 L/min. Fifty to ninety percent of the particles in the size range of 2.3 microns to 8.4 microns originally in the 300-L/min stream are now contained in the 1 L/min sample stream. The sampler has no valves, and the particles in the 1 L/min sample stream do not encounter a blower, minimizing the losses of particles in the size range of interest. The emphasis on low losses improves the detection limit and speed of detection of the downstream instrumentation and also reduces the frequency of cleaning the sampler.