The glass slides are analyzed in a humidity-controlled box; this allows for particles to be analyzed under a wide range of relative humidity. An optical microscope with an attached digital camera is used to acquire images of the giant aerosol particles. A threshold-based system is used to determine the perimeter of giant aerosol particles. Salt particles will at high relative humidity form spherical cap solution drops; these appear as nearly circular particles on the image, and the salt mass in each giant aerosol particle is determined by assuming a NaCl composition. Irregular particles, presumably mineral dust and biological material, are sized using a 64-angle Fourier technique in order to simplify the shape classification.
For glass slides of 20 mm times 6.25 mm, the system has a sample volume of about 1 m3/s at aircraft speeds of 100 ms. For salt particles, the measurement range is from 2 micron dry radius to hundreds of micrometers, with a resolution of 0.5 micron dry radius. The lower detection limit is somewhat higher for irregular particles. The sizing accuracy was tested using glass beads and salt particles of known size.
Examples of size distributions are presented from two recent field experiments, one in a continental airmass south over the great lakes (AIRS-2) and one from a marine airmass (RICO). About 50000 particles were analyzed on each slide; this is about a factor 10-100 higher than most previous studies and this is a major improvement in establishing statistical significance for rare giant aerosol particles.
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