AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Identification and Characterization of Visible Absorption Components in Methylglyoxal-Ammonium Sulfate Mixtures
W. SEAN MCGIVERN, Thomas C. Allison, James Radney, Christopher Zangmeister, National Institute of Standards and Technology
Abstract Number: 600 Working Group: Aerosol Chemistry
Abstract Methylglyoxal (MG) is formed in large quantities from the atmospheric oxidation of numerous volatile organic compounds, particularly aromatic species. MG in aqueous solution shows only a small visible absorption and thus absorbs little incoming solar radiation as a droplet. However, reaction of MG and ammonium salts is known to cause the visible absorption to increase markedly, leading to the conversion of a near-colorless droplet to a partial absorber. Chromatographic analysis of the products from the reaction of MG and ammonium sulfate has revealed that, although the overall composition is complex, most of the visible absorption is caused by a small number of compounds. Fraction collection of the eluate from an HPLC analysis of the reaction mixture revealed only a single "brown" fraction that showed any visible absorption. MS-MS analysis of this absorbing fraction demonstrates that the visibly absorbing products primarily consist of imino compounds of aldol-like condensation reactions. The presence of the nitrogen in the pi-conjugated system of the imino-substituted products redshifts the ultraviolet absorption spectrum relative to the pure aldol condensate such that visible absorption can occur in the the long-wavelength tail. Density functional theory has been used to calculate how the presence of the imino and carbonyl moieties on these product molecules affects the near-UV absorption spectra and individual pKa values.