Direct reading instruments (DRIs) are popular devices for measuring aerosols because they provide rapid on-site measurement of particle size and/or concentration. However, the output of DRIs may drift over time requiring frequent manufacturer calibration. Given the possibility of drift, the output of DRIs should ideally be verified to ensure proper response before and after field use. Methods for verifying the output of DRIs particle size reading are available for use in laboratory and field. However, methods for verifying the DRIs concentration reading are complex and often use of stationary installations that are not suited for field work. The objective of this study was to develop a verification device that can be used in the field to verify the output of DRIs for measuring aerosol concentration. The new device uses a nebulizer that produces aerosols through vibrating mesh technology. This vibrating mesh nebulizer (VMN) uses only electrical input to generate aerosols and does not require compressed air. The verification device was able to produce stable output of aerosols at low concentrations (0.2 mg/m3 to 1.2 mg/m3). It was also possible to produce different concentration levels of aerosol by changing the electrical current to the VMN. The verification device was used to monitor and validate the output of a condensation particle counter and a photometer. Results showed that both instruments having valid output and did not require manufacture calibration. The verification device made it possible to monitor and verify the output of two DRIs. This was achieved by generating reproducible aerosol output with specific composition. This verification device presents a practical method to verify the concentration output of DRIs for measuring aerosols.