A method for sizing submicrometer particles in air collected on Formvar films and imaged by scanning electron microscope

Abstract

Abstract. A method was developed to systematically investigate individual aerosol particles collected onto a polyvinyl formal (Formvar)-coated copper grid with scanning electron microscopy. At very mild conditions with a low accelerating voltage of 2 kV and Gentle Beam mode aerosol particles down to 20 nm in diameter can be observed. Subsequent processing of the images with digital image analysis provides size resolved and morphological information (elongation, circularity) on the aerosol particle population. Polystyrene nanospheres in the expected size range of the ambient aerosol particles (20–900 nm in diameter) were used to confirm the accuracy of sizing and determination of morphological parameters. The relative standard deviation of the diameters of the spheres was better than ±10% for sizes larger than 40 nm and ±18% for 21 nm particles compared to the manufacturer's certificate. Atmospheric particles were collected during an icebreaker expedition to the high Arctic (north of 80°) in the summer of 2008. Two samples collected during two different meteorological regimes were analyzed. Their size distributions were compared with simultaneously collected size distributions from a Twin Differential Mobility Particle Sizer, which confirmed that a representative fraction of the aerosol particles was imaged under the electron microscope. The size distributions obtained by scanning electron microscopy showed good agreement with the Twin Differential Mobility Sizer in the Aitken mode, whereas in the accumulation mode the size determination was critically dependent on the contrast of the aerosol with the Formvar-coated copper grid. The morphological properties (elongation, circularity) changed with the number of days the air masses spent over the pack-ice area north of 80° before the aerosol particles were collected at the position of the icebreaker and are thus an appropriate measure to characterize transformation processes of ambient aerosol particles.