title: Novel approach to identifying supersaturated metastable ambient aerosol particles.
abstract: Atomic force microscopy (AFM) was used to determine the shape of fine and ultrafine ambient aerosol particles with sizes between 25 and 700 nm after soft landing on a solid substrate. The particles were collected in summer during daytime at a relative humidity around 50%. To avoid kinetically induced deformation, as previously observed using high-velocity sampling in impactors, the particles were collected on pore filters at very low face velocities (on the order of 10 cm/s). The shape of the collected particles was quantified in terms of their height and apparent diameter. The amount of broadening introduced by the pyramidal shape of the nonideally sharp AFM tips was calibrated using Latex reference spheres with a range of diameters. The height-to-diameter ratios, H/D, of the collected aerosol particles could be extracted from the measured data. Specified in terms of volume-equivalent (dry) diameters, Dv, the size selected frequency distributions of the H/Dv-ratios were found to be bimodal. A small mode centered at H/Dv = 1.0 +/- 0.1 is attributed to nonhygroscopic particles that retained their shape after deposition on the substrate. The large mode, with a peak at H/Dv = 0.65 +/- 0.05, reflects soft particles which were strongly deformed due to vertical collapse after deposition. The pronounced deformation suggests that these particles had previously experienced deliquescence and, when collected at a comparatively low humidity, were in a metastable, supersaturated aqueous state. After landing and indoor sample storage the water evaporated, resulting in minimum H/Dv-ratios as low as 0.45. The dried metastable fraction amounted to 81 +/- 12% in the size range 150 < Dv < 700 nm, and 79 +/- 10% for 50 < or = Dv < or = 150 nm, but only 26 +/- 10% for Dv < 50 nm. Comparison with recently reported data suggests that the observed metastable fraction is the same as the hygroscopic fraction identified by other means. The interpretation is further substantiated by a comparison of the size distributions of collected and airborne particles.
authors.affiliation: GSF National Research Center for Environment and Health, Institute of Radiation Protection, 85758 Neuherberg, Germany. wittmaack@gsf.de
authors.forename: Klaus
authors.initials: K
authors.lastname: Wittmaack
authors.forename: Markus
authors.initials: M
authors.lastname: Strigl
mesh_terms: D000336:Aerosols; D006813:Humidity; D018625:Microscopy, Atomic Force; D010316:Particle Size; D014867:Water / Q000032:analysis
chemical_list: D000336:Aerosols; D014867:Water