GNI—An aircraft-based system for the impaction and automated optical sizing of giant aerosol particles with emphasis on sea salt

Size distributions of giant aerosol particles (dry radius larger than 0.5 μm) are not well characterized in the atmosphere. Measurement is problematic for these particles that occur in low concentrations due to (i) difficulties in passing through air inlets, (ii) problems in discriminating between dry and deliquesced particles, (iii) labor intensive analysis methods, etc. In this study we present a simple, high-volume impaction system based on free-stream exposure of polycarbonate slides from aircraft, and we describe an automated optical microscope-based system for analysis of the impacted particles. The system is called the Giant Nuclei Impactor or GNI for short. This system provides a desperately needed increase in capability for sampling sea-salt aerosol particles. The impaction slides are analyzed in a humidity-controlled box (typically 90% relative humidity) that allows for deliquescence of sea salt particles. A computer controlled optical microscope with two digital cameras is used to acquire and analyze images of the aerosol particles. Salt particles will form near-spherical cap solution drops at high relative humidity. The salt mass in each giant aerosol particle is then calculated using simple geometry and Kohler theory by assuming a NaCl composition. The system has a sample volume of 10 L s-1 at aircraft speeds of 130 m s-1. For salt particles, the measurement range is from about 0.5 μm dry radius to tens of micrometers, with a resolution of 0.2 μm dry radius. The sizing accuracy was confirmed using glass beads of known size. A comprehensive uncertainty analysis is performed for the GNI manual impaction and automatic optical microscope system for sizing giant aerosol particles, with particular emphasis on sea-salt particles. The uncertainty analysis includes (i) sizing accuracy, (ii) concentration accuracy, (iii) particle break-up, and (iv) impaction effects leading to coalescence between particles on the slide. The latter is investigated using stochastic Monte-Carlo modeling. Size spectra from the 2008 VOCALS aircraft experiment off the coast of northern Chile are used to examine the horizontal and vertical variability of giant sea-salt particles.