Photolysis mechanism - butenedial

Figure 2 shows a model/measurement comparison from a butenedial photolysis experiment in the absence of NOx. The loss of butenedial is well predicted by MCMvS.l. However, the HO2 concentration is over-estimated by MCMv3.1 by almost an order of magnitude during the early part of the experiment. The time-dependent behaviour is also not well reproduced by the simulation as in the experiment an initial fast increase in concentration is followed by a slower linear increase until the chamber closes, while the simulation shows a fast rise followed by a fall in the HO2 concentration even while the photolysis continues. The photolysis mechanism for butenedial in the absence of NOx as implemented in MCMvS.l is shown schematically in Figure 4. This indicates fliat two HO2 radicals should be formed for each molecule of maleic anhydride and glyoxal produced, and while both these product concentrations are over-estimated this is not sufficient to account for the large over-prediction ofH02.

Figure 4. Schematic representation of MCMv3.1 butenedial photolysis mechanism in the absenee ofNOx.

Thuener et al. (2003) propose an alternative butenedial photolysis mechanism based on produet experiments. However, this mechanism also requires two HO2 radicals per maleic anhydride moleeule formed. One must conclude that a different HO2 formation pathway is operating and/or a significant loss process for HO2 is not included in the mechanism. 

Concerning the molecular products of butenedial photolysis (Figure 3), the yield of 2(5H)-furanone is well predicted by the simulation, glyoxal and maleic anhydride are overpredicted while the CO yield in the simulation is much lower than observed experimentally. In MCMv3.1 glyoxal and CO are formed as co-products (Figure 4), but this is not consistent with the different yields of these products observed experimentally. Thuener et al. (2003) include a different source of CO in their proposed mechanism, i.e. direct formation fi om photolysis with a yield of 20%. A possible co-product for direct CO production is acrolein formed by an H-shift and C-C cleavage. The acrolein concentration was below the detection limit of the measurement technique, and its maximum yield was estimated to be 10%. No other direct photolysis products were observed and it was not possible to positively determine the mechanism and co-products for CO formation. 

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See also in sourсe #XX -- [ ]

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