Novel method of generation of Ca(HCO3)2 and CaCO3 aerosols and first determination of hygroscopic and cloud condensation nuclei activation properties
Atmospheric Chemistry and Physics
Atmospheric mineral aerosols contain CaCO 3 as a reactive component. A novel method to produce CaCO 3 aerosol was developed by spraying Ca(HCO 3 ) 2 solution, which was generated from a CaCO 3 suspension and CO 2 . By aerosol mass spectrometry the freshly sprayed and dried aerosol was characterized to consist of pure Ca(HCO 3 ) 2 which under annealing in a tube furnace transformed into CaCO 3 . Transmission Electron Microscopy demonstrated that the particles produced were spherical. The method
... erical. The method was able to generate aerosol of sufficient concentration and proper size for the study of physiochemical properties and investigations of heterogeneous reactions of mineral aerosol. The dried Ca(HCO 3 ) 2 particles were somewhat more hygroscopic than CaCO 3 particles. However, during humidification a restructuring took place and ∼2/3 of the Ca(HCO 3 ) 2 was transformed to CaCO 3 . The mixed Ca(HCO 3 ) 2 /CaCO 3 (s) particles were insoluble with a growth factor of 1.03 at 95% (hygroscopicity parameter κ=0.011 ± 0.007) relative humidity. This compares to a corresponding growth factor of 1.01 for CaCO 3 (s) (κ=0.0016 ± 0.0004). Mass spectrometric composition analysis, restructuring, and insolubility of the mixed particles suggested that solid Ca(HCO 3 ) 2 (s) was observed. This would be in contrast to the current belief that Ca(HCO 3 ) 2 (s) is thermodynamically instable. The CCN activity of Ca(HCO 3 ) 2 (s) aerosol Correspondence to: Th. F. Mentel (firstname.lastname@example.org) (κ ≈ 0.15) is remarkably higher than that of CaCO 3 aerosol (κ=0.0019±0.0007) and less than that of Ca(NO 3 ) 2 . The noticeable but limited solubility of Ca(HCO 3 ) 2 of ≈ 0.01 mol/l explains limited hygroscopic growth and good CCN activity. Experiments in the Large Jülich Aerosol Chamber indicated that Ca(HCO 3 ) 2 (s) could exist for several hours under dry atmospheric conditions. However, it was likely buried in a protective layer of CaCO 3 (s). We conclude that Ca(HCO 3 ) 2 may be formed in the atmosphere in cloud droplets of activated mineral dust by reaction of CaCO 3 with CO 2 and H 2 O. The presence of Ca(HCO 3 ) 2 and as a consequence an enhanced CCN activity may alter the influence of mineral aerosol on global climate. Published by Copernicus Publications on behalf of the European Geosciences Union. Experimental Preparation of Ca(HCO 3 ) 2 solution Ca(HCO 3 ) 2 solutions were prepared at room temperature (∼22 • C) by bubbling CO 2 (Praxair Industriegase GmbH&Co. KG, purity 99.995%) at a gauge pressure of Atmos.