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

Abstract

Atmospheric mineral aerosols contain CaCO3 as a reactive component. A novel method to produce CaCO3 aerosol was developed by spraying Ca(HCO3)2 solution, which was generated from a CaCO3 suspension and CO2. By aerosol mass spectrometry the freshly sprayed and dried aerosol was characterized to consist of pure Ca(HCO3)2 which under annealing in a tube furnace transformed into CaCO3. Transmission Electron Microscopy demonstrated that the particles produced were spherical. 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(HCO3)2 particles were somewhat more hygroscopic than CaCO3 particles. However, during humidification a restructuring took place and ∼2/3 of the Ca(HCO3)2 was transformed to CaCO3. The mixed Ca(HCO3)2/CaCO3(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 CaCO3(s) (κ=0.0016±0.0004). Mass spectrometric composition analysis, restructuring, and insolubility of the mixed particles suggested that solid Ca(HCO3)2(s) was observed. This would be in contrast to the current belief that Ca(HCO3)2(s) is thermodynamically instable. The CCN activity of Ca(HCO3)2(s) aerosol (κ≈0.15) is remarkably higher than that of CaCO3 aerosol (κ=0.0019±0.0007) and less than that of Ca(NO3)2. The noticeable but limited solubility of Ca(HCO3)2 of ≈0.01 mol/l explains limited hygroscopic growth and good CCN activity. Experiments in the Large Jülich Aerosol Chamber indicated that Ca(HCO3)2(s) could exist for several hours under dry atmospheric conditions. However, it was likely buried in a protective layer of CaCO3(s). We conclude that Ca(HCO3)2 may be formed in the atmosphere in cloud droplets of activated mineral dust by reaction of CaCO3 with CO2 and H2O. The presence of Ca(HCO3)2 and as a consequence an enhanced CCN activity may alter the influence of mineral aerosol on global climate.