7. Answers and Discussion

Task 3.1. Simulation of ethene chamber experiments

A1. The decay of ethene is substantially under-predicted. All the concentration profiles are over-predicted. The ozone peak is over-predicted by about 30%. In fact the ozone has probably not yet peaked in the simulation.

A2. The inclusion of the measured dilution rate unsurprisingly improves the profiles of all species.

A3. The model gives an excellent prediction of the ethene decay. However, this is not surprising as both the high and low NO_x ethene experiments were used to tune the auxiliary mechanism used. Peak O₃ is over-predicted by 10%.

A4. The model run with constrained j(NO₂) (and hence constrained photolysis rates) gives similar results to that with calculated photolysis. However, due to the measured j(NO₂) being generally slightly higher than the calculated j(NO₂) the profiles are all slightly increased (except obviously for ethene decay) due to the slight increase photo reactivity of the system. Also the timing of most of the profiles is improved.

Task 4.1. Simulation of toluene chamber experiments

A1. MCMv3.1 model to measurement agreement for both the ozone concentration profile and NO oxidation rate is improved. The MCMv3.1 toluene scheme has an increased branching ratio for ring opening and a slightly decreased cresol yield. This (along with the increased photolysis rates of the unsaturated dicarbonyl products) leads to increased radicals, most notably at the beginning of the experiment (Bloss et al. 2005a). However, changes to the cresol mechanism decrease the rate of radical formation in the middle of the experiment. This leads to some O₃ reduction being achieved whilst increasing the radical budget during the early stages of the experiment. However, comparisons of the OH and toluene decay profiles show that there is still significant missing OH. In fact, calculations of the missing OH over the whole experiment show that the missing OH source is actually higher for MCMv3.1 than MCMv3 (Bloss et al. 2005a).

A2. Peak O₃ is decreased by 12 % compared to MCMv3.1 and now over predicts the measured peak by about 19 %. The model decay of toluene is faster than MCMv3.1 as a result of the increased OH giving a better representation of the observations but still under estimates the reactivity of the system as evidenced by the OH plot.

A3. MCMv3 gives an excellent representation of the loss of cresol but vastly over-predicts the amount of ozone. OH is well predicted during the early part of the experiment but is under predicted in the latter stages.

MCMv3.1. under-predicts the decay of cresol and hence indicates an under prediction of the oxidising capacity of the system which is show in the OH plot. However, the ozone profile is improved although we now under-predict ozone. As expected, the simulated NO₂ concentration has been reduced and NO to NO₂ conversion is now slower than the measurements suggest.

The differences seen here contribute to the differences seen in the toluene mechanism evaluation in the last section.

Task 5.1 Simulation of 1,3,5-trimethylbenzene experiment

The model results that should be obtained from this task are given in psi_tmb_results.xls.