AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
The Importance of Relative Humidity and Particle Phase on the Oxidation of Unsaturated Compounds in Aerosols
PETER GALLIMORE, Francis Pope, Pattanun Achakulwisut, Jason Lee, Stephen Fuller, Vanesa Carrascon, James F. Davies, Alex Björkegren, David Spring, Markus Kalberer, University of Cambridge
Abstract Number: 486 Working Group: Aerosol Chemistry
Abstract Organic compounds constitute a major fraction of tropospheric aerosol. The atmospheric properties of aerosol particles are critically influenced by their chemical composition, which continually changes due to gas/particle partitioning, heterogeneous oxidation and reactions occurring within the particles.
This study illustrates the importance of relative humidity (RH) and particle phase on the ozonolysis of three unsaturated carboxylic acids used as model organic aerosols: oleic, maleic and arachidonic acids. It combines two experimental techniques: An Electrodynamic Balance (EDB) is used to levitate single particles and measure changes in mass, size and phase [1]. An aerosol flow reactor is coupled to ultra-high resolution mass spectrometry to identify in detail the compounds formed in the oxidised particles [2].
While the hygroscopicity of maleic acid and arachidonic acid increased significantly after aging at high RH, oleic acid aerosol shows minimal water uptake before and after oxidation. There is also a marked contrast in the secondary chemistry occurring in the particle phase. We interpret this in terms of the fate of the initial reactive intermediates formed during ozonolysis [3]. For oleic acid, their relative stability, combined with low aerosol water content, means they can act as building blocks for oligomer formation. For aqueous maleic acid aerosols, they are rapidly hydrolysed and acid-catalysed reactions between “stable” products form secondary esters and hydrates.
We also illustrate the complex interplay between ozonolysis and particle phase. Liquid arachidonic acid aerosols rapidly form an amorphous crust when oxidized at low RH. Oxidation of maleic acid dissolved in the surface water of solid aerosols promotes further water uptake and, through a positive feedback mechanism, complete dissolution of the crystalline core.