Changes of fatty acid aerosol hygroscopicity induced by ozonolysis under humid conditions O. Vesna1,2, S. Sjogren3, E. Weingartner3, V. Samburova4, M. Kalberer4, H. W. Gäggeler1,2, and M. Ammann1 1Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Inst., Villigen, Switzerland 2Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland 3Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland 4Institute of Organic Chemistry, ETH Zürich, Zürich, Switzerland
Abstract. Unsaturated fatty acids are important constituents of the organic fraction
of atmospheric aerosols originating from biogenic or combustion sources.
Oxidative processing of these may change their interaction with water and
thus affect their effect on climate. The ozonolysis of oleic and arachidonic
acid aerosol particles was studied under humid conditions in a flow reactor
at ozone exposures close to atmospheric levels, at concentrations between
0.5 and 2 ppm. While oleic acid is a widely used proxy for such studies,
arachidonic acid represents polyunsaturated fatty acids, which may decompose
into hygroscopic products. The hygroscopic (diameter) growth factor at
93% relative humidity (RH) of the oxidized arachidonic particles
increased up to 1.09 with increasing RH during the ozonolysis. In contrast,
the growth factor of oleic acid was very low (1.03 at 93% RH) and was
almost invariant to the ozonolysis conditions, so that oleic acid is not a
good model to observe oxidation induced changes of hygroscopicity under
atmospheric conditions. We show for arachidonic acid particles that the
hygroscopic changes induced by humidity during ozonolysis are accompanied by
about a doubling of the ratio of carboxylic acid protons to aliphatic protons.
We suggest that, under humid conditions, the reaction of water with
the Criegee intermediates might open a pathway for the formation of smaller
acids that lead to more significant changes in hygroscopicity. Thus the
effect of water to provide a competing pathway during ozonolysis observed in
this study should be motivation to include water, which is ubiquitously
present in and around atmospheric particles, in future studies related to
aerosol particle aging.
Citation: Vesna, O., Sjogren, S., Weingartner, E., Samburova, V., Kalberer, M., Gäggeler, H. W., and Ammann, M.: Changes of fatty acid aerosol hygroscopicity induced by ozonolysis under humid conditions, Atmos. Chem. Phys., 8, 4683-4690, doi:10.5194/acp-8-4683-2008, 2008.