The effects of aging processes on critical supersaturation ratios of ultrafine carbon aerosols

The condensation properties of polydisperse aged ultrafine carbon aerosols (particle diameter < 1 mu m) have been investigated by means of a variable supersaturation condensation nucleus counter. The critical supersaturation (S-o), as the point, where 50% of all particles have been activated and gre w to droplets was compared to the median dry particle diameter for pure car bon aerosols, benzol[a]pyrene-tagged carbon aerosols and external mixtures of the carbon particles with sodium chloride and sulphuric acid aerosols. A dditionally, ozone as oxidising gaseous compound was added in some of the e xperiments. Simple coagulation of pure and benzo[a]pyrene-tagged carbon par ticles resulted in only slightly lower values for S-c due to the increased median particle diameter. The formation of soluble functionalities on the p article surface, i.e. the coagulation with the soluble sodium chloride and sulphuric acid aerosols or the chemical decomposition of benzo[a]alpyrene i nto polar, hydrophilic products due to the reaction with ozone resulted in significant lower values for S-c for the modified carbon aerosol. The neces sary supersaturations for the increased hydrophilic particles dropped to at mospherically relevant values of 3% after 5 h reaction time (benzo[a]pyrene decomposition) and 15 h (coagulation with soluble particles), respectively . (C) 1999 Elsevier Science Ltd. All rights reserved.