AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Hydroxyl Radical Mediated Aging of Oxidized Dodecanoic Acid Particles
JOSEPH KLEMS, W. Sean McGivern, National Institute of Standards and Technology
Abstract Number: 582 Working Group: Aerosol Chemistry
Abstract The hydroxyl (OH) radical initiated oxidation of organic species in the atmosphere represents a major source of secondary organic aerosol (SOA) and plays an important role in aerosol aging. In this work, the evolution of SOA generated from the oxidation of dodecanoic acid (DDA) was investigated as a function of OH radical exposure. In these experiments, OH radicals were generated in a quartz flow tube outfitted with 254 nm ultraviolet lamps and an adjustable shade via the photolysis of ozone in the presence of water. The OH radical exposure was varied by changing the length of the shade, and the resulting particles were collected on a filter and subjected to offline LC/MSn and GC/MS analyses. The use of a carboxylic acid precursor greatly facilitated the identification of a number of products, as they produced an abundance of ions in the negative mode mass spectrum. The major products were then quantified using chemical derivatization coupled to LC/MS2 experiments. At short OH exposures, structural isomers of oxo-DDA and hydroxy-DDA comprised the bulk of the particle mass, with some of the later undergoing cyclization to form alkyl-substituted lactones. Other SOA products detected included trace levels of DDA diketones, hydroxyketones and C6 - C10 oxo-acids, with each product demonstrating a variety of isomeric forms. Increasing the OH radical exposure decreased the relative concentration of the long chain products and substantially increased the concentration of the shorter chain products. This observation indicates that as the particles age, chain degradation becomes an increasingly important process, even for long chain organic molecules in a NOx free environment. The ability to track the aging of particle-phase products on the molecular level provides insights into the chemical processes that contribute to their formation not possible with other popular techniques.