Atmos. Chem. Phys. Discuss., 9, 2609-2644, 2009
www.atmos-chem-phys-discuss.net/9/2609/2009/
doi:10.5194/acpd-9-2609-2009
© Author(s) 2009. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Effect of chemical mixing state on the hygroscopicity and cloud nucleation properties of calcium mineral dust particles
R. C. Sullivan1,*, M. J. K. Moore1, M. D. Petters2, S. M. Kreidenweis3, G. C. Roberts3, and K. A. Prather1,3
1Dept. of Chemistry and Biochemistry, Univ. of California, San Diego, La Jolla, CA, 92093, USA
2Dept. of Atmospheric Science, Colorado State Univ., Fort Collins, CO, 80523, USA
3Scripps Institution of Oceanography, Univ. of California, San Diego, La Jolla, CA, 92093, USA
*present address: Dept. of Atmospheric Science, Colorado State Univ., Fort Collins, CO, 80523, USA

Abstract. Atmospheric mineral dust particles can alter cloud properties and thus climate by acting as cloud condensation nuclei (CCN) that form cloud droplets. The CCN activation properties of various calcium mineral dust particles were studied experimentally to investigate the consequences of field observations showing the segregation of sulfate from nitrate and chloride between individual aged Asian dust particles, and the enrichment of oxalic acid in Asian dust. Each mineral's observed apparent hygroscopicity was primarily controlled by its solubility, which determines the degree to which the mineral's intrinsic hygroscopicity can be expressed. The significant increase in hygroscopicity caused by mixing soluble hygroscopic material with insoluble mineral particles is also presented. Insoluble minerals including calcium carbonate, representing fresh unprocessed dust, and calcium sulfate, representing atmospherically processed dust, had similarly small apparent hygroscopicities. Their activation is accurately described by a deliquescence limit following the Kelvin effect and corresponded to an apparent single-hygroscopicity parameter, κ, of ~0.001. Soluble calcium chloride and calcium nitrate, representing atmospherically processed mineral dust particles, were much more hygroscopic, activating similar to ammonium sulfate with κ~0.5. Calcium oxalate monohydrate (κ=0.05) was significantly less CCN-active than oxalic acid (κ=0.3), but not as inactive as its low solubility would predict. These results indicate that the common assumption that all mineral dust particles become more hygroscopic and CCN-active after atmospheric processing should be revisited. Calcium sulfate and calcium oxalate are two realistic proxies for aged mineral dust that remain non-hygroscopic. The dust's apparent hygroscopicity will be controlled by its chemical mixing state, which is determined by its mineralogy and the chemical reaction pathways it experiences during transport.

Citation: Sullivan, R. C., Moore, M. J. K., Petters, M. D., Kreidenweis, S. M., Roberts, G. C., and Prather, K. A.: Effect of chemical mixing state on the hygroscopicity and cloud nucleation properties of calcium mineral dust particles, Atmos. Chem. Phys. Discuss., 9, 2609-2644, doi:10.5194/acpd-9-2609-2009, 2009.
 
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