The Master Chemical Mechanism
The Master Chemical Mechanism has been updated from MCMv3 to MCMv3.1 in order to take into account improvements in the understanding of aromatic photo-oxidation. Newly available kinetic and product data from the literature have been used to update the MCM mechanisms of 4 aromatic systems: benzene, toluene, p-xylene and 1,3,5-trimethyl benzene. In particular, the degradation mechanisms for hydroxyarenes have been revised following the observation of high yields of ring-retaining products, and product studies of aromatic oxidation under low NOx conditions have provided information on the branching ratios of first generation products (Jenkin et al. 2003, Bloss et al. 2005a)
As part of the EU EXACT project (Effects of the oXidation of Aromatic Compounds in the Troposphere) two series of chamber experiments were carried out at EUPHORE and a comprehensive dataset on aromatic smog systems, under a variety of conditions, was obtained including a set of experiments focusing on subsets of the toluene system (see Bloss et al. 2005a and 2005b for more information).
The performance of the updated MCM aromatic mechanisms was evaluated against the EXACT dataset and where appropriate results from EXACT have been used to refine these mechanisms. The development work on mono-aromatics has been extended to update the degradation schemes of the other mono-aromatics with saturated alkyl side chains.
The following exercise looks at modelling some of the EXACT experiments using aromatic mechanisms extracted from MCMv3.1.
Explore the toluene mechanism on the MCM website using the search tool.
Q1. Try to construct a SMILES string to search for toluene (C7H8).
Q2. Below is a schematic representation of the toluene mechanism from MCMv3. Identify and fill in the similar branching ratios of OH + Toluene from MCMv3.1 on the other scheme below. NOTE: One of the channels represented in MCMv3 has been removed in MCMv3.1.
Q3. What are the co-products of glyoxal and methyl glyoxal in the peroxy bicyclic ring opening route in MCMv3.1? Give MCM names and structures.
Q4. What is the ratio of glyoxal to methyl glyoxal in this primary production route?
Open the model tolbox_mcm3.fac This model contains the toluene photo-oxidation mechanism as defined in MCMv3 (which is available from the website in the "archive" section).
Initialise the model to the initial values from the toluene photo-smog experiment carried out on the 27/09/2001 listed in Table 4. Start the model at the appropriate time and output every 5 minutes until the end of the experiment (57 times).
NOTE: When you constrain a VARIABLE species you will have to remove it from the VARIABLES declarations list and declare it as a PARAMETER.
Modify the output to calculate the following species: MRTOLUENE, MRO3, MRNO, MRNO2 and OH.
Run the model and compare the results with the experimental data contained in tol270901.xls by pasting your results into the "MCMv3" worksheet.
• ***If you have a EUROCHAMP account which allows you to access the DATABASE please download the data for TOLUENE, O3, NO, and NO2 for the experiment on the 27/09/01 and compare these to your model runs. If you do not have a suitable account then compare to the experimental data already in the spreadsheet.***
Q5. How well are the concentration profiles simulated?
Locate the toluene mechanism on the MCM website. Extract the mechanism using the “subset mechanism extractor”.
Paste the mechanism into the appropriate place in the model tolbox_mcm31.fac
Paste the species VARIABLES and RO2 summation list into the appropriate place in the model. (NOTE: Take care that the extracted summation list does not contain any Criegee Intermediates (radicals whose name will end in “OO”, “OOA” or “OOB” etc…)). Also, some of the inorganic species are repeated in the extracted species VARIABLES list. Remove these.
Table 4. Initial concentrations and other parameters for the EXACT toluene photo smog experiment carried out on the 27/09/2001.
|Start time (hh:mm)||10:08|
|End time (hh:mm)||15:00|
|Dilution Rate (s-1||1.58 × 10 -5|
As with the MCMv3 model, initialise the model to the initial values from the toluene photo-smog experiment carried out on the 27/09/2001 listed in Table 4 and constrain the model to the measured water, j(NO2) and temperature.
Run the model and compare the results with the experimental data contained in tol270901.xls by pasting your results into the "MCMv31" worksheet.
Q6. How do the new concentration profiles compare to the measurements and the MCMv3 results?
In general, MCMv3.1 shows improved ability to simulate some of the EXACT observations and represents our current understanding of aromatic degradation. However, significant discrepancies remain concerning ozone formation potential and oxidative capacity of aromatic hydrocarbon systems:
Ideas and strategies for resolving these issues have been suggested and additional laboratory and smog chamber experiments are required in order to investigate them further (see Bloss et al. 2005 for more details).
A number of possible mechanistic fixes that have been investigated include:
As mentioned above, one means of decreasing ozone production while increasing OH concentrations is to implement a regeneration of OH without conversion of NO to NO2 (and hence O3). One such OH regeneration route postulated is a H shift in the peroxy radical formed in the peroxy bicyclic route, which is shown in Figure 2 below:
|Figure 2. Schematic of MCMv3.1 peroxy bicyclic route for toluene oxidation and postulated OH regeneration pathway (Bloss et al. 2005b).|
Introduce this OH regeneration route into the MCMv3.1 toluene model.
Open tolboxmcm31.fac and add the following OH regeneration route to the end of the reaction listing in the EQUATIONS routine.
% KDEC*0.167 : TLBIPERO2 = GLYOX + C5DICARB +OH ;
% KDEC*0.167 : TLBIPERO2 = GLYOX + TLFUONE + OH ;
% KDEC*0.25 : TLBIPERO2 = MGLYOX + MALDIAL + OH ;
% KDEC*0.167 : TLBIPERO2 = GLYOX + C4MDIAL + OH ;
% KDEC*0.25 : TLBIPERO2 = MGLYOX + BZFUONE + OH ;
This scheme replaces the usual peroxy bicyclic ring opening route. Therefore, comment out the following reactions:
% KRO2NO*0.889 : TLBIPERO2 + NO = NO2 + TLBIPERO ;
% KRO2NO*0.111 : TLBIPERO2 + NO = TLBIPERNO3 ;
% KRO2NO3 : TLBIPERO2 + NO3 = NO2 + TLBIPERO ;
% KRO2HO2*0.820 : TLBIPERO2 + HO2 = TLBIPEROOH ;
% 8.80D-13*RO2*0.60 : TLBIPERO2 = TLBIPERO ;
% 8.80D-13*RO2*0.20 : TLBIPERO2 = TLBIPER2OH ;
% 8.80D-13*RO2*0.20 : TLBIPERO2 = TLOBIPEROH ;
Run the model and compare the results with the experimental data contained in tol270901.xls by pasting your results into the "OH regen" worksheet.
Q7 How do the new concentration profiles compare to the measurements, and the MCMv3 and MCMv3.1 model runs?
The OH regeneration step currently seems like the most favoured explanation of the under prediction of the radicals but over prediction of ozone from the updated aromatic degradation in MCMv3.1. High level ab-initio calculations on aromatic systems are being used to investigate the likelihood of such an OH regeneration route occurring under tropospheric conditions and a similar step has been added to the SAPRC mechanism which is the most used tropospheric chemistry mechanism incorporated into air quality models in the US.
The chemistry of hydroxyarenes was updated following new product studies indicating higher ring retaining products in aromatic systems than previously assumed (see Bloss et al. 2005a for more details).
As part of the EXACT campaign a photo-smog experiment on cresol (a subset of the toluene mechanism) was performed on the 04/10/2001.
Open tolbox_mcm31.fac and constrain it to the initial condition for the cresol experiment performed on the 04/10/2001 as listed in Table 5. NOTE: remove the initial concentration of TOLUENE or replace it with CRESOL.
Start the model at the appropriate time and output every 5 minutes until the end of the experiment.
Modify the output to calculate the following species: MRCRESOL, MRO3, MRNO, MRNO2 and OH.
Table 5. Initial concentrations and other parameters for the EXACT cresol photo smog experiment carried out on 04/10/2001.
|Start time (hh:mm)||11:06|
|End time (hh:mm)||15:06|
|Dilution Rate (s-1)||1.58 × 10-5|
Run the model and compare the results with the experimental data and a similar model run using the MCMv3 toluene mechanism contained in cresol041001.xls by pasting your results into the "MCMv31" worksheet.
Q8 How do the new concentration profiles compare to the measurements and the MCMv3 results?