Effect of chemical mixing state on the hygroscopicity and cloud nucleation properties of calcium mineral dust particles 1Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, 92093, USA
2Department of Atmospheric Science, Colorado State University, Fort Collins, CO, 80523, USA
3Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, 92093, USA
*currently at: Department of Atmospheric Science, Colorado State University, Fort Collins, CO, 80523, USA
Received: 11 Dec 2008 – Published in Atmos. Chem. Phys. Discuss.: 28 Jan 2009 – Published: 20 May 2009Abstract. 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 sulphate 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 sulphate, 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 sulphate 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 sulphate 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., 9, 3303-3316, doi:10.5194/acp-9-3303-2009, 2009.