Cloud Condensation Nuclei Activity of CaCO<sub>3</sub> Particles with Oleic Acid and Malonic Acid Coatings
Abstract. Condensation of carboxylic acids on mineral particles will lead to coatings, and impact on the particles' potential to act as cloud condensation nuclei (CCN). To determine how the CCN activity of mineral particles is impacted by carboxylic acid coatings, the CCN activity of CaCO3 particles and CaCO3 particles with oleic acid and malonic acid coatings were compared in this study. The results revealed that small amounts of oleic acid coating (volume fraction (vf) ≤ 4.1 %) decreased the CCN activity of CaCO3 particles, while more oleic acid coating (vf ≥ 14.8 %) increased the CCN activity of CaCO3 particles. This phenomenon has not been reported before. On the other hand, malonic acid coating (vf = 0.4–42 %) increased the CCN activity of CaCO3 particles regardless of the amount of the coating. The CCN activity of CaCO3 particles with malonic acid coating increased with the amount of malonic acid coating. Even smallest amounts of malonic acid coating (vf = 0.4 %) significantly enhanced the CCN activity of CaCO3 particles from κ = 0.0028 ± 0.0001 to κ = 0.0123 ± 0.0005. This supports that a small amount of water-soluble organic acid coating may significantly enhance the CCN activity of mineral particles. The presence of about 50 % relative humidity during the coating process with malonic acid additionally increased the CCN activity of the coated CaCO3 particles, probably because more CaCO3 reacts with malonic acid at higher relative humidity.