Novel method of generation of Ca(HCO3)2 and CaCO3 aerosols and first determination of hygroscopic and cloud condensation nuclei activation properties 1State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China
2Forschungszentrum Jülich, Institut für Chemie und Dynamik der Geosphäre − 2: Troposphäre, 52425 Jülich, Germany
3Aerodyne Research Inc., 45 Manning Rd, Billerica, MA 01821, USA
Received: 10 Mar 2010 – Published in Atmos. Chem. Phys. Discuss.: 26 Mar 2010 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.
Revised: 12 Aug 2010 – Accepted: 16 Aug 2010 – Published: 14 Sep 2010
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.
Citation: Zhao, D. F., Buchholz, A., Mentel, Th. F., Müller, K.-P., Borchardt, J., Kiendler-Scharr, A., Spindler, C., Tillmann, R., Trimborn, A., Zhu, T., and Wahner, A.: Novel method of generation of Ca(HCO3)2 and CaCO3 aerosols and first determination of hygroscopic and cloud condensation nuclei activation properties, Atmos. Chem. Phys., 10, 8601-8616, doi:10.5194/acp-10-8601-2010, 2010.