Modelling of the photooxidation of toluene: conceptual ideas for validating detailed mechanisms

Abstract

Abstract. Toluene photooxidation is chosen as an example to examine how simulations of smog-chamber experiments can be used to unravel shortcomings in detailed mechanisms and to provide information on complex reaction systems that will be crucial for the design of future validation experiments. The mechanism used in this study is extracted from the Master Chemical Mechanism Version 3 (MCM v3) and has been updated with new modules for cresol and γ -dicarbonyl chemistry. Model simulations are carried out for a toluene-NO x experiment undertaken at the European Photoreactor (EUPHORE). The comparison of the simulation with the experimental data reveals two fundamental shortcomings in the mechanism: OH production is too low by about 44%, and the ozone concentration at the end of the experiment is over-predicted by 55%. The radical budget was analysed to identify the key intermediates governing the radical transformation in the toluene system. Ring-opening products, particularly conjugated γ -dicarbonyls, were identified as dominant radical sources in the early stages of the experiment. The analysis of the time evolution of radical production points to a missing OH source that peaks when the system reaches highest reactivity. First generation products are also of major importance for the ozone production in the system. The analysis of the radical budget suggests two options to explain the concurrent under-prediction of OH and overprediction of ozone in the model: (1) missing oxidation processes that produce or regenerate OH without or with little NO to NO 2 conversion or (2) NO 3 chemistry that sequesters reactive nitrogen oxides into stable nitrogen compounds and at the same time produces peroxy radicals. Sensitivity analyCorrespondence to: M. J. Pilling (M.J.Pilling@chem.leeds.ac.uk sis was employed to identify significant contributors to ozone production and it is shown how this technique, in combination with ozone isopleth plots, can be used for the design of validation experiments.

Characterization and testing of a new environmental chamber

“…For example, values of 3.0 in a 8 m 3 chamber and of 1.4 in a 90 m 3 chamber have been reported (Platt et al, 2013;Carter et al, 2005). In our chamber the surface-to-volume ratio is 2.0, which is equal to the surface-to-volume ratio of several other 27-30 m 3 chambers (Wang et al, 2014;Paulsen et al, 2005;Cocker et al, 2001).…”

“…Atmospheric transformation processes have been studied extensively and for decades in environmental chambers with different designs (e.g., Wang et al, 2014;Platt et al, 2013;Carter et al, 2005;Paulsen et al, 2005;Cocker et al, 2001). The most used chamber materials are glass, stainless steel, and Teflon.…”

The production and persistence of ΣRONO<sub>2</sub> in the Mexico City plume

“…In addition to uncertainty in the yield of nitrates from the initial oxidation of aromatic compounds, there is a high degree of uncertainty in the yields of ring-opening vs. ring-retaining products (Hamilton et al, 2003;Jenkin et al, 2003;Wagner et al, 2003;Wyche et al, 2009). The products of ringopening pathways could themselves have high nitrate yields and significant nitrate formation could thus result from oxidation of the first-generation products of aromatic oxidation.…”

“…We should also note that the nighttime reaction of NO 3 with alkenes is known to give rise to ANs production without associated O 3 production (Warneke et al, 2004). The nitrate formation rate from NO 3 -initiated oxidation is typically much higher than for OH-initiated oxidation and this reaction could represent a significant source of ANs.…”

Atmospheric photochemistry of aromatic hydrocarbons: OH budgets during SAPHIR chamber experiments

“…This mismatch between simulated OH concentrations and those indirectly inferred from the decay rates of aromatic reactants was attributed to potentially missing OH production processes (Wagner et al, 2003). Our straightforward experimental investigation of the radical balance however suggests that so far unaccounted OH production reactions are not likely to be a reason for this OH underprediction.…”