Evaluation of MCM v3, using environmental chamber data P. G. Pinho 1, C.A. Pio 1 and M.E. Jenkin 2 1 Departamento de Ambiente e Ordenamento, Universidade de Aveiro, Aveiro, Portugal 2 Department of Environmental Science and Technology, Imperial College London, Silwood Park, Ascot, Berkshire SL5 7PY, UK

The main objective of the present study is the evaluation and refinement of degradation mechanisms included in MCMv3, using the large environmental chamber dataset of the Statewide Air Pollution Research Center (SAPRC) at the University of California Riverside.

Dataset of the Statewide Air Pollution Research Center (SAPRC) at the University of California Riverside. 76 chamber characterization runs: 481 single VOC runs involving 37 types of VOCs. 447 incremental reactivity experiments involving 87 types of VOCs or mineral spirits or solvent samples. 673 mixture runs involving various types of simple or complex mixtures or ambient ROG surrogates.

Environmental Chambers of SAPRC database

Initial evaluation, carried out using butane-NO X photo-oxidation, develops a base for the evaluation of degradation mechanism for a number of VOC represented in MCM v3. The auxiliary mechanism parameters were the same used by Carter in SAPRC-99 evaluation. Butane-NO X photo-oxidation experiments were used for initial assessment of the auxiliary mechanism parameters

Within this assessment, were considered the photooxidation experiments involving the major carbonyl products formed during butane degradation, namely: Butane-NO X photo-oxidation mechanism testing and refinement was carried out by an iterative procedure, considering butane photo-oxidation and each of the sub-systems in turn. 46 butane-NO X -air; 6 MEK-NO X -air; 11 CH 3 CHO-NO X -air; 24 HCHO-NO X -air.

The precursor decay rate and formation of carbonyl products and PAN were also used as criteria of model performance. The quantity D(O 3 -NO) was used as the main criterion of model performance because: -it is an indicator of the ability of the mechanism to simulate the chemical processes that cause O 3 formation, giving a useful measure, even when O 3 is suppressed by the presence of excess NO; - use of this measure allows a direct comparison with the SAPRC-99 published results.

HCHO-NO X experiments MCM v3 mechanism was found to under-predict D(O 3 -NO) in many (but not all) of the runs, principally for the CTC and DTC chambers. Inserted change: Photolysis parameters were updated in line the latest IUPAC recommendations. - cross sections based on the data of Meller and Moortgat (2000); - quantum yields based on data from Smith et al., (2002). The new cross sections are 5-10% higher than the values previously recommended by IUPAC (and adopted for the MCM by Jenkin et al., 1997).

MEK-NO X experiments MEK - MCM v3 mechanism over-predict D(O 3 -NO) in many of the chamber runs, particularly in the early stages of the experiment. Inserted change: Optimization of quantum yield. This led to a best fit value of In MCM v3, a wavelength- independent value of 0.34 is applied, based on Raber and Moortgat, (1996) is close to the extremity of the uncertainty limit of the determination of Raber and Moortgat, (1996) is also consistent with the value of 0.15 obtained by Carter, during evaluation and optimization of the SAPRC-99.

D(O 3 -NO) (ppm) vs. time (min) MEK (ppm) vs. time (min)

- MCM v3a give generally good fits to D(O 3 -NO) data in butane–NO X photo-oxidation. With most of the data being fit by the mechanism within +/- 30%, with no consistent biases. The scatter in the results was indicative of run-to-run variability. Butane-NO X -system - MCM v3a gives good fits to HCHO-NO X, CH 3 CHO-NO X and MEK-NO X experiments Initial evaluation, carried out using butane-NO X photo- oxidation, develops a base for the evaluation of degradation mechanism for a number of VOC represented in MCM v3.

Isoprene assessment photooxidation experiments involving the major carbonyl products formed during isoprene degradation were considered, namely: The available NO X photo-oxidation runs for these major carbonyl products were used to test and refine the corresponding subsets of the mechanism, prior to a re- evaluation of the full isoprene scheme. 9 isoprene-NO X -air, 8 MACR-NO X -air; 5 MVK-NO X -air.

D(O 3 -NO) (ppm) vs. time (min) Isoprene (ppm) vs. time (min)

MACR (ppm) vs. time (min) MVK (ppm) vs. time (min)

MACR-NO X experiments MCM v3 was found to over-predict significantly the observed D(O 3 -NO) in the initial stages of the experiments, but with the final values being slightly lower than those observed.

Inserted changes:  Change in OH yield from the ozonolysis of MACR MCM v (based on closest hydrocarbon analogues (Jenkin et al., 1997)) MCM v3a (based in Aschmann et al., 1996)  Reaction of O( 3 P) with MACR was incorporated into the mechanism.  Change in photolysis quantum yield MCM v (based in Raber and Moortgat 1996) MCM v3a (obtained by best fit)

D(O 3 -NO) (ppm) vs. time (min) MACR (ppm) vs. time (min)

MVK-NO X experiments MCM v3 was found to over-predict significantly the observed D(O 3 -NO) in the initial stages of the MVK-NO X experiments, but with the final values being slightly lower than those observed

Inserted changes:  Change in OH yield from the ozonolysis of MVK MCM v (based on closest hydrocarbon analogues (Jenkin et al., 1997)) MCM v3a (based in Aschmann et al., (1996) and Paulson et al., (1998))  Reaction of O( 3 P) with MVK was incorporated into the mechanism.  Change in photolysis quantum yield MCM v (based in Raber and Moortgat 1996) MCM v3a - quantum yield expression of Gierczak et al. (1997), recommended by IUPAC.

D(O 3 -NO) (ppm) vs. time (min) MVK (ppm) vs. time (min)

Isoprene-NO X experiments Inserted change: Reaction of O( 3 P) with isoprene was incorporated into the mechanism. Branching ratio values were obtained by optimization. O( 3 P) + CH 2 =C(CH 3 )CH=CH 2  CH 2 -O- C(CH 3 )CH=CH O( 3 P) + CH 2 =C(CH 3 )CH=CH 2 (+3O 2 )  CH 3 C(O)CH 2 O 2 + HCHO + CO + HO It was also clear that the value assigned to the branching ratio for radical production from the O(3P) reaction is influenced by assumptions concerning the formation of hydroxyalkenyl nitrates, since this also has an impact on the radical balance in the system.

Results: The influence of the reactions involving MACR and MVK was found to be very minor. The inclusion of the reaction of O( 3 P) with isoprene was the major factor responsible for the differences in the simulations of the parent isoprene system performed with MCM v3 and with the modified mechanism (MCM v3a).

D(O 3 -NO) (ppm) vs. time (min) Isoprene (ppm) vs. time (min)

MACR (ppm) vs. time (min) MVK (ppm) vs. time (min)

Current Evaluations Alkenes Ethene Propene 1-butene 1-hexene Monoterpenes  -pinene  -pinene

ETHENE Trend to under prediction. PROPENE Reasonable fit. 1BUTENE Over prediction. 1HEXENE Over prediction.  -PINENE Trend to over prediction.  -PINENE Reasonable fit in 4 of 6 runs.