A method to improve the sensitivity of optical particle counters for large particles is presented. The basic idea is to increase the concentration in the sample air by connecting a virtual impactor to the sample inlet of an optical particle counter. For particles with D p > 3 μm, the number concentration is increased by a factor of 20. The improvement in statistical accuracy is particularly useful in Alter testing and clean room measurements. The differences in the size distributions measured with and without the virtual impactor are used in estimating the relationship between the aerodynamic and optical sizes of different aerosols.