A micro-fabricated double condenser method and apparatus for the measurement of number-size distribution of airborne nanoparticles is provided. The invention is an instrument which can measure particle size under 100 nanometers in-situ. The present invention includes features such as a small nanoparticle airborne particulate analyzer, with a voltage-stable and feedback-controlled instrument package. The invention features also include a micro-fabricated nanoparticle charging and sorting device (NCaS). The present invention provides a portable, lightweight, and efficacious particle-sizing instrument that is able to effectively count and size nanoparticles over a wide range of operating conditions.