Cloud condensation nucleation activities of calcium carbonate and its atmospheric ageing products

Abstract

Aerosol particles can serve as cloud condensation nuclei (CCN) to form cloud droplets, and its composition is a main factor governing whether an aerosol particle is an effective CCN. Pure mineral dust particles are poor CCN; however, changes in chemical composition of mineral dust aerosol particles, due to heterogeneous reactions with reactive trace gases in the troposphere, can modify their CCN properties. In this study we investigated the CCN activities of CaCO₃ (as a surrogate for mineral dust) and its six atmospheric ageing products: Ca(NO₃)₂, CaCl₂, CaSO₄, Ca(CH₃SO₃)₂, Ca(HCOO)₂, and Ca(CH₃COO)₂. CaCO₃ has a very low CCN activity with a hygroscopicity parameter (κ) of 0.001–0.003. The CCN activities of its potential atmospheric ageing products are significantly higher. For example, we determined that Ca(NO₃)₂, CaCl₂ and Ca(HCOO)₂ have κ values of ∼0.50, similar to that of (NH₄)₂SO₄. Ca(CH₃COO)₂ has slightly lower CCN activity with a κ value of ∼0.40, and the κ value of CaSO₄ is around 0.02. We further show that exposure of CaCO₃ particles to N₂O₅ at 0% relative humidity (RH) significantly enhances their CCN activity, with κ values increasing to around 0.02–0.04. Within the experimental uncertainties, it appears that the variation in exposure to N₂O₅ from ∼550 to 15 000 ppbv s does not change the CCN activities of aged CaCO₃ particles. This observation indicates that the CaCO₃ surface may be already saturated at the shortest exposure. We also discussed the atmospheric implications of our study, and suggested that the rate of change in CCN activities of mineral dust particles in the troposphere is important to determine their roles in cloud formation.