10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Methylthreonic, Methylerythronic, and Methyltartaric acids: Highly Oxygenated Markers for Isoprene Secondary Organic Aerosol Aging
MOHAMMED JAOUI, Rafal Szmigielski, Nestorowicz Klara, A. Kolodziejczyk, K.J. Rudziński, W. Danikiewicz, Michael Lewandowski, Tad Kleindienst, EPA/ORD/NERL
Abstract Number: 96 Working Group: Carbonaceous Aerosol
Abstract We investigate the formation of new highly oxygenated molecules (HOMs) associated with secondary organic aerosol (SOA) originated from isoprene photooxidation and ozonolysis under a wide range of conditions. SOA was generated by the oxidation of isoprene/air mixtures in the presence of NOx, H2O2, methyl nitrite, or ozone. The effect of relative humidity, acidity, and OH radical scavengers (ozonolysis experiments) on the formation of these HOMs was investigated. Using a detailed analysis of mass spectra obtained from GC-MS of silylated derivatives in EI and CI modes, and UPLC/ESI/QTOF HRMS, and collision-induced dissociation in positive and negative modes, we identified not only typical isoprene products, but also six HOMs: 2-methylerythronic acid (2MeTrA), 3-methylerythronic acid (3MeTrA), 2-methylthreonic acid (2MTrA), 3-methylthreonic acid (3MTrA), L-methyltartaric acid (L-MTA), and D-methyltartaric acid (D-MTA). We confirmed their structure by synthesizing them in our laboratory. Isoprene HOMs with O/C ratio of 1 and 1.2 were not detected in the presence of OH scavenger during ozonolysis, suggesting OH radicals are important in isoprene HOMs formation. The presence of these HOMs in the gas and particle phase simultaneously provides evidence of their gas/particle partitioning. The occurrence of these HOMs in ambient aerosol collected during the past 15 years in several locations in USA and Europe provide direct molecular evidence of isoprene aging processes in ambient aerosol. For example, the average concentrations of 2MeTrA + 2MTrA, 3MeTrA + 3MTrA, and L-MTA + D-MTA in Duke Forest samples were 0.42, 0.11, and 3.49 ng m-3. We also present results from time-resolved experiment of gas and particle products, where 4-hydroxy-3-methyl-but-2-enal, and 4-hydroxy-2-methyl-but-2-enal are proposed as possible precursors to HOM formation.
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