Research Program Area: Atmospheric Processes
During this experimental program, we have used the facilities and expertise available at the Air Pollution Research Center, University of California, Riverside, to investigate the atmospheric chemistry of selected aromatic hydrocarbons found in California's atmosphere. Experiments were carried out in large volume (5800 to ~7500 liter) chambers with analysis of reactants and products by gas chromatography (with flame ionization and mass spectrometric detection) and in situ Fourier transform infrared spectroscopy. The gas chromatographic analyses included the use of Solid Phase MicroExtraction (SPME) fibers coated with derivatizing agent for on-fiber derivatization of carbonyl-containing compounds, with subsequent gas chromatographic (GC) analyses of the carbonyl-containing compounds as their oximes. This technique was especially useful for the identification and quantification of 1,2-dicarbonyls and unsaturated 1,4-dicarbonyls, some of which are not commercially available and most of which and do not elute from gas chromatographic columns without prior derivatization. We showed that the OH radical-initiated reaction of 3-methyl-2-butenal in the presence of NO is a good in situ source of gaseous glyoxal which can be routinely used for calibration of the SPME fiber sampling for the quantitative analysis of glyoxal from other reaction systems (and this reaction is now being used by other research groups for that purpose). We have observed the formation of a series of 1,2-dicarbonyls and unsaturated 1,4-dicarbonyls from the OH radical-initiated reactions of toluene, o-, m- and p-xylene and 1,2,3-, 1,2,4- and 1,3,5,- trimethylbenzene. The observation of these products as their di-oximes shows that they are indeed present in their dicarbonyl form and not as isomeric furanones. The second major task involved investigation of the dependence of the formation yields of selected products of the OH radical-initiated reactions of toluene, naphthalene and biphenyl as a function of the NOx concentration; specifically, formation of glyoxal from naphthalene and formation of 3-nitrotoluene, 1- and 2-nitronpahthalene and 3-nitrobiphenyl from toluene, naphthalene and biphenyl, respectively. We measured the formation yields of glyoxal from the reaction of OH radicals with naphthalene as a function of the initial NOx concentration, using the photolysis of methyl nitrite in air to generate OH radicals in the presence of NOx and the dark reaction of O3 with 2-methyl-2-butene to generate OH radicals in the absence of NOx. We showed that glyoxal is a first-generation product, with no obvious evidence for a change in the glyoxal formation yield with initial NOx concentration over the range < 0.1-5 ppmv. A more direct investigation of the effect of NOx on product formation was initiated by studying the formation of 3-nitrotoluene, 1- and 2-nitronaphthalene and 3-nitrobiphenyl from the OH radical-initiated reactions of toluene, naphthalene and biphenyl, respectively. Analytical methods and procedures were developed to analyze for these nitro-aromatics at the low concentrations expected to be formed from these reactions, and the procedures developed were used to study formation of 3-nitrotoluene from the toluene reaction. Our results are in excellent agreement with laboratory kinetic data concerning the functional form of the dependence of the 3-nitrotoluene formation yield on NOx concentration over the range 0.02-10 ppmv, showing that we have a quantitative understanding of the reactions involved in 3-nitrotoluene formation. The data obtained in this Contract will prove important for including into chemical mechanisms for modeling photochemical air pollution and the formation of nitroaromatics in the atmosphere. Additional work is needed to complete investigation of the formation of nitro-PAHs (and specifically 1- and 2-nitronaphthalene and 3-nitrobiphenyl) under atmospheric conditions and to compare these laboratory predictions with ambient atmospheric measurements of nitroaromatics and their parent aromatic hydrocarbons.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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