Heterogeneous nucleation of a common atmospheric aerosol: Ammonium sulfate

Natural and anthropogenic aerosols have a direct impact on the earth's radi ative balance. Many of the common atmospheric aerosols contain hygroscopic salts and therefore their physical state and composition are sensitive to t he relative humidity. The phase of the aerosol particle will dramatically a ffect the relative rates of atmospheric reactions, and radiative properties such as scattering and optical activity. In order to understand these atmo spheric processes and provide accurate input for global modeling, it is imp erative that these phase transitions be well defined. It is thought that th e degree of metastability is influenced by the presence of foreign particul ate matter. These solid impurities are predicted to reduce the barrier of n ucleation by providing a surface upon which heterogeneous nucleation can be catalyzed. We report the results of CaCO3 and BaSO4 solid impurities on th e efflorescence of(NH4)(2)SO4-H2O micro-droplets. The results show that bot h solid impurities act as catalysts for heterogeneous nucleation by causing a decrease in the degree of metastability at the efflorescence point. BaSO 4 does not prove to be significantly more efficient than CaCO3 in spite of having the same crystalline structure and similar lattice parameters as in (NH4)(2)SO4.