The giant nucleus impactor (GNI)—a system for the impaction and automated optical sizing of giant aerosol particles with emphasis on sea salt. part i: Basic instrument and algorithms

Size distributions of giant aerosol particles (dry radius larger than 0.5 mm, sometimes referred to as coarse-mode aerosol particles) are not well characterized in the atmosphere. Measurements are problematic for these particles because they (i) occur in low concentrations, (ii) have difficulty in passing through air inlets, (iii) may be dry or deliquesced particles, and (iv) if sampled by impaction, typically require labor-intensive methods. In this study, a simple, high-volume impaction system called the Giant Nucleus Impactor (GNI), based on free-stream exposure of polycarbonate slides from aircraft, is described along with an automated optical microscope–based system for analysis of the impacted particles. The impaction slides are analyzed in a humidity-controlled chamber (typically 90% relative humidity) that ensures deliquescence of soluble (typi-cally sea salt) particles. A computer-controlled optical microscope with two digital cameras is used to acquire and analyze images of the aerosol particles. At relative humidities above deliquescence (74% RH for sea salt), such particles will form near-spherical cap solution drops on the polycarbonate slides. The sea-salt mass in each giant aerosol particle is then calculated using simple geometry and published water activity mea-surements. The system has a sample volume of about 10 L s^-1 at aircraft speeds of 105 m s^-1. For salt particles, the measurement range is from about 0.7 mm dry radius to at least 16 mm dry radius, with a size-bin resolution of 0.2 mm dry radius. The sizing accuracy was tested using polystyrene latex (PSL) beads of known size.