Novel method of generation of Ca(HCO3)2 and CaCO3 aerosols and first determination of hygroscopic and cloud condensation nuclei activation properties

D. F. Zhao, A. Buchholz, Th. F. Mentel, K.-P. Müller, J. Borchardt, A. Kiendler-Scharr, C. Spindler, R. Tillmann, A. Trimborn, T. Zhu, A. Wahner
2010 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
more » ... 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 ( (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.
doi:10.5194/acp-10-8601-2010 fatcat:d3h2rgks7va7pa2cyhnguqn6rq