ROG surrogate

The ROG surrogate used in the ambient surrogate - NOx experiments consisted of a simplified mixture of designed to represent the major elasses of hydrocarbons and aldehydes measured in ambient urban atmospheres, with one eompound used to represent each model speeies used in condensed lumped-molecule meehanism. The eight representative compounds used were n-butane, n-octane, ethene, propene, trans-2-butene, toluene, m-xylene, and formaldehyde. (See Carter et al., 1995b, for a diseussion of the derivation of this surrogate). 

Prediction of Ozone formation in Ambient ROG Surrogate - NOx Experiments... 

Figure 1. Matrix of initial ROG and NOx levels in the ambient ROG surrogate - NO experiments carried out in the UCR EPA chamber. The initial ROG and NOx levels used in the base case experiments for the incremental reactivity studies of the coatings VOC emissions are also shown.

The various characterization experiments were used to derive the chamber characterization parameters and evaluate the ehamber eharaeterization model as discussed above. The single organic - NOx experiments were earried out to demonstrate the utility of the ehamber to test the mechanisms for these eompounds, for whieh data are available in other ehambers, and to obtain well-charaeterized meehanism evaluation data at lower NOx levels than previously available. The formaldehyde -i- CO - NOx experiments were carried out beeause they provided the most ehemieally simple system that model calculations indicated was insensitive to chamber effeets, to provide a test for both the basic mechanism and the light eharaeterization assigmnents. The aromatie - CO - NOx experiments were carried out because aromatic - NOx experiments were predieted to be very sensitive to the addition of CO, because it enhances the effects of radicals formed in the aromatic system on ozone formation. The ambient surrogate - NOx experiments were earried out to test the ability of the mechanism to simulate ozone formation under simulated ambient eonditions at various ROG and NOx levels. 

For the single VOC - NOx or VOC - CO - NOx experiments, the model is able to simulate the A([03]-[N0]) to within 25% or better in most cases, which is better than the 30% seen in previous mechanism evaluations with the older chamber data (Carter and Lurmann, 1990, 1991 Gery et al., 1989, Carter, 2000). However, there are indications of non-negligible biases in model simulations of certain classes of experiments. The cleaner conditions and the relatively lower magnitude of the chamber effects may make the nm-to-run scatter in the model performance may be less than in the simulations of the previous data, and this tends to make relatively small biases in the model performance more evident. There are, for example, definite biases in the model to under predict O3 formation and NO oxidation in the surrogate - NOx experiments carried out at lower ROG/NOx ratios. These cases are discussed further elsewhere (Carter, 2004b). 

Figure 6. Concentration-time plots of selected compounds in the lowest NOx ambient ROG - NOx surrogate experiment in the initial evaluation experiments (NOx 1 ppb, ROG 300 ppbC.

Figure 3. Plots of die bias for the SAPRC-99 mechanism to vmder predict NO oxidation and O3 formation against normalized ROG/NOx ratios for ambient surrogate -NOx experiments in three environmental chambers.

Figure 4. Plots of the mechanism under prediction bias for model simulations UCR EPA ambient surrogate - NOx experiments against normalized ROG/NOx ratios for the SAPRC-99 and Carbon Bond mechanisms. The SAPRC-99 calculations show effects of varying the MONO offgasing chamber effect parameters and using a lower OH + NO2 rate constant.

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