In-situ atomic force microscopy investigation of aerosols exposed to different humidities

In-situ atomic force microscopy (AFM) studies were performed on aerosol sam ples showing the potential of a topochemical approach for gaining informati on on chemical and physical aerosol propel-ties. The behavior of single sub -micron particles has been investigated with respect to changing humidity i n the surrounding atmosphere. Volume calculations allowed monitoring of the se changes on a quantitative basis. As expected these in-situ experiments s howed the restructuring of particles with highly agglomerated chain-like st ructures induced by condensation and evaporation on a nanometer scale. The particle volumes decreased as the branched chain-like structure changed int o a more regular clump-like structure. The degree of restructuring was clea rly depending on the chemical surface properties as could be proven for soo t-like test aerosol particles. The collapse of the chain-like structure on a nanometer scale was found to be significantly more pronounced for soot pa rticles previously exposed to ozone. Furthermore, in-situ studies were perf ormed on ammonium sulfate test aerosol. Though a distinct deliquescence poi nt typical for salts could not be detected, neither in the topography nor i n the phase image, ammonium sulfate test aerosol particles seemed to partia lly dissolve in humid atmosphere and hence to decrease in volume. Thus, the volume decrease induced by purging with humid nitrogen and subsequent dryi ng which was also observed for a considerable fraction of urban aerosol, co uld be interpreted in terms of composition and surface properties consideri ng the geometrical structure (i.e. state of agglomeration) of the particles .