Atmospheric Chemistry and Physics Discussions
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2007
10.5194/acpd-7-5341-2007
http://www.atmos-chem-phys-discuss.net/7/5341/2007/
http://www.atmos-chem-phys-discuss.net/7/5341/2007/acpd-7-5341-2007.html
http://www.atmos-chem-phys-discuss.net/7/5341/2007/acpd-7-5341-2007.pdf
5341
5364
2007-04-19
Modelling the cloud condensation nucleus activity of organic acids
Z. Varga
G. Kiss
kissgy@almos.uni-pannon.hu
H.-C. Hansson
Department of Earth and Environmental Sciences at University of Pannonia, Egyetem u. 10, 8200 Veszprém, Hungary
Air Chemistry Group of the Hungarian Academy of Sciences, University of Pannonia, P.O. Box 158, 8201 Veszprém, Hungary
Institute of Applied Environmental Research, Stockholm University, Stockholm, 10691, Sweden
In this study vapour pressure osmometry was used to determine water activity
in solutions of organic acids. The surface tension of the solutions was also
monitored in parallel and then Köhler curves were calculated for nine
organic acids (oxalic, malonic, succinic, glutaric, adipic acid, maleic
acid, malic acid, citric acid and pinonic acid). Surface tension depression
is negligible for most of the organic acids in dilute (≤1 w/w%)
solutions. Therefore, these compounds affect the supersaturation only in the
beginning phase of droplet formation but not necessarily at the critical
size. An exception is cis-pinonic acid which remarkably depress surface
tension also in dilute (0.1 w/w%) solution and hence at the critical
point. The surface tension of organic acid solutions is influenced by the
solubility of the compound, the length of the carbon chain and also by the
polar functional groups present in the molecule. Similarly to surface
tension solubility plays an important role also in water activity: compounds
with higher solubility (e.g. malonic, maleic, and glutaric acid) reduce
water activity significantly in the early phase of droplet formation while
less soluble acids (e.g. succinic and adipic acid) are saturated in small
droplets and the solution starts diluting only in bigger droplets. As a
consequence, compounds with lower solubility have a minor effect on water
activity in the early phase of droplet formation. To deduce the total effect
Köhler curves were calculated and critical supersaturations were
determined for the organic acids using measured surface tension and water
activity. It was found that critical supersaturation grew with growing
carbon number. Oxalic acid had the lowest critical supersaturation in the
size range studied and it was comparable to the activation of ammonium
sulfate. The <i>S<sub>c</sub></i> values obtained in this study were compared to data
from CCNC measurements. In most cases good agreement was found.
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