A compact aerosol mobility imager (AMI) for instantaneous aerosol size distribution measurements, part I: Design and model evaluation

Aerosol Mobility Imager (AMI), a compact instrument for rapid measurements of aerosol size distribution, is presented. AMI employs a parallel plate mobility separator wherein the electric field strength varies along the width of the plate. Under the influence of the electric field, charged particles are spatially separated inside the separator according to their electrical mobility. A novel extraction growth cell (EGC) then extracts a portion of the flow carrying the spatially separated particles and converges the extracted flow into a narrow focusing nozzle slit, while simultaneously growing the particles by water condensation. Grown particles exiting the focusing slit are imaged to capture both the number and mobility dependent position of the particles, which allows for derivation of aerosol size distribution. The employment of the new EGC significantly reduces the instrument size, weight, and power consumption compared to the previously developed Fast Integrated Mobility Spectrometer (FIMS). The combination of a compact size and fast measurement speed is expected to make AMI an ideal instrument for deployments in many laboratory studies and field observations, including onboard platforms such as the unmanned aerial vehicle and tethered balloon system. In this first of a series of two papers, we present numerical simulations of particle trajectories and growth of spatially separated particles in AMI. The AMI performance characteristics, including transfer function, transmission efficiency, and mobility resolution, are derived from the simulated particle trajectories. The results indicate that AMI can match the 1 Hz time resolution of FIMS with a comparable dynamic size range of 8–400 nm in particle diameter, while maintaining a mobility resolution above 3. The experimental demonstration and characterization of AMI are presented in an accompanying paper.