GROWTH AND STRUCTURAL-CHANGE OF COMBUSTION AEROSOLS AT HIGH RELATIVE-HUMIDITY

Condensational growth and structural changes of combustion aerosol particles were investigated by means of a tandem differential mobility analyzer. Sodium chloride particles, carbon particles, acid freshly emitted combustion particles with diameters between 50 and 100 nm were investigated. A dry, highly monodisperse aerosol was exposed to high relative humidity (RH), and the electrical mobility diameter was measured as a function of RH. A growth factor was determined by comparing the dry particle mobility diameter with the diameter of the humidified particles. NaCl particles showed the expected behavior with a rapid increase of the growth factor at the deliquescence humidity. Carbon particles shrank to about 85% of their original diameter when exposed to RH = 90%. This shrinking was attributed to capillary condensation in small angle cavities of aggregates. Capillary forces induced on any asymmetric part of the particles cause them to become a more compact structure. Particles emitted from a four-stroke spark ignition engine showed a different behavior: Smaller particles (dry diameter d(o) = 51.5 nm) did not change in size up to RH = 95% and then started to grow. Larger aggregates (d(o) = 108 nm) first shrank to approximately 99% of their original size and then started to grow at RH greater than or equal to 95%, That means that the combustion particles were at least partly covered with material able to reduce the water pressure at the particle surface, In addition, these particles were more compact compared to carbon particles, resulting in less restructuring. With increasing aging time in a dark bag, the growth factor of the combustion particles increased.