Master Chemical Mechanism

The free-radical chemistry was studied using a zerodimensional box-model based upon the Master Chemical Mechanism (MCM). Two versions of the model were used, with different levels of chemical complexity, to explore the role of hydrocarbons upon free-radical budgets under very clean conditions. The detailed model was constrained to measurements of CO, CH4 and 17 NMHCs, while the simple model contained only the CO and CH4 oxidation mechanisms, together with inorganic chemistry. The OH and HO2 (HOx) concentrations predicted by the two models agreed to within 5-10%. 

This paper investigates the radical chemistry of the clean marine boundary layer in the Southern Ocean during the SOAPEX-2 (Southern Ocean Photochemistry Experiment 2) campaign using an observationally constrained box-model based on the Master Chemical Mechanism (Jenkin et al., 1997, 2003 Saunders et al., 2003). The primary aim of SOAPEX-2 was to study free radical chemistry in the remote marine boundary layer in the Southern Hemisphere. Sections 2 and 3 of this paper describe the SOAPEX-2 site and the measurements that were made during the campaign. Section 4 describes the models used and Sect. 5 presents the results. Finally, Sect. 6 contains the summary and the conclusions. 

The mechanisms for the NMHCs (except DMS) required to fully characterise OH chemistry were extracted from a recently updated version of the Master Chemical Mechanism (MCM 3.0, available at http //mcm.leeds.ac.uk/MCM/). The MCM treats the degradation of 125 volatile organic compounds (VOCs) and considers oxidation by OH, NO3, and O3, as well as the chemistry of the subsequent oxidation products. These steps continue until CO2 and H2O are formed as final products of the oxidation. The MCM has been constructed using chemical kinetics data (rate coefficients, branching ratios, reaction products, absorption cross sections and quantum yields) taken from several recent evaluations and reviews or estimated according to the MCM protocol (Jenkin et al., 1997, 2003 Saunders et al., 2003). The MCM is an explicit mechanism and, as such, does not suffer from the limitations of a lumped scheme or one containing surrogate species to represent the chemistry of many species. 

Two observationally constrained box-models, based on the Master Chemical Mechanism and with different levels of chemical complexity, have been used to study the HOx radical chemistry during the SOAPEX-2 campaign, which took place during the austral summer of 1999 (January-February) at the Cape Grim Baseline Air Pollution Station in northwestern Tasmania, Australia. The box-models were constrained to the measured values of long lived species and photolysis rates and physical parameters (NO, NO2, O3, HCHO, j(01D), j(N02), H2O and temperature). In addition the detailed model was constrained to the measured concentration of CO, CH4 and 17 NMHCs, while the simple model was additionally constrained only to CO and CH4. The models were updated to the latest available kinetic data and completed with a simple description of the heterogeneous uptake and dry deposition processes. 

Jenkin, M. E., Saunders, S. M., Wagner, V., and Pilling, M. J. Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part B) tropospheric degradation of aromatic volatile... 

Derwent, R. G., M. E. Jenkin, S. M. Saunders, and M. J. Pilling, Photochemical Ozone Creation Potentials for Organic Compounds in Northwest Europe Calculated with a Master Chemical Mechanism, Atmos. Environ., 32, 2429-2441 (1998). 

The mechanism of the hydroxyl radical-initiated oxidation of /i-pincnc in the presence of NO has been investigated using a discharge-flow system. Propagation of hydroxyl radicals was observed after the addition of O2 and NO, and the measured concentration profiles were compared with simulations based on both the master chemical mechanism and the regional atmospheric chemistry mechanism for /i-pinene oxidation.228... 

Derwent RG, Jenkin ME, Saunders SM, Pilling MJ (1998) Photochemical ozone creation potentials for organic cmnpounds in Northwest Europe calculated with a master chemical mechanism. Atmos Envirrai 32 2429—2441 POCP... 

The degradation schemes of four aromatic hydrocarbons benzene, toluene, /7-xylene and 1,3,5-trimethylbenzene, have been updated on the basis of new kinetic and mechanistic data from current literature and conference proceedings and are available as part of the latest version of the Master Chemical Mechanism (MCMv3.1) via the MCM website thttn //mcm.leeds.ac.uk/MCM). The performance of these schemes concerning ozone formation from tropospheric aromatic oxidation has been evaluated using detailed environmental chamber datasets from the two EU EXACT measurement campaigns at EUPHORE (EXACT I - September 2001 and EXACT II - My 2002 (Pilling et al, 2003)). 

Jenkin, M.E., S.M. Saunders, V. Wagner and M.J. Pilling Protocol for the development of the master chemical mechanism MCMv3 (Part B) Tropospheric degradation of aromatic volatile organic compounds, Atmospheric Chemistry and Physics Discussions, 2, pl905-1938 (2002). Atmospheric Chemistry and Physios, 3,181-193 (2003)... 

MCM The Master Chemical Mechanism website, http //chmhn9.leeds.ac.uk/MCM/. Undated. Last accessed 10/2004 (2004)... 

There are a large number of condensed chemical reaction mechanisms to describe and represent atmospheric chemical transformations involving air pollutants. On the contrary only a very limited number of mechanisms exist with a detailed description of chemical transformation processes for primary and secondary atmospheric species. The Master Chemical Mechanism (MCM) (Jenkin et al, 1997) and SAPRC (Carter and Liumann, 1991) are two well known examples. 

The MCM is a near-explicit chemical mechanism that describes the detailed degradation of a series of emitted VOC, and the resultant generation of ozone and other secondary pollutants, under conditions appropriate to the planetary boundary layer. Version 3 of the Master Chemical Mechanism (MCM v3) considers the oxidation of 125 VOC. The complete mechanism comprises 12,691 reactions of 4,351 organic species and 46 associated inorganic reactions, which were defined on the basis of the MCM scheme writing protocols (Jenkin et al, 1997, Saunders et al, 2003 Jenkin et al, 2003). Although MCM v3 has recently been superseded by v 3.1, the chemistry for non-aromatic VOC remains unchanged. 

This extensive data set showed good agreement with a Master Chemical Mechanism for isoprene oxidation. In addition, one of the predictions of the mechanism is that at low NOx levels, like those seen in Surinam, isoprene hydroperoxides (six isomers, e.g. HOCH2C(OOH)(CH3)CH=CH2) will accumulate. It was noted that correlations between isoprene and other VOCs (different times of day and altitude) were greatest with MlOl ", which could be indicative of isoprene hydroperoxides. This result is an example where PTR-MS analysis can detect previously unmeasured VOCs, although as mentioned above, verification of the identity of unknown positive ions requires complementary methods (e.g. GC-MS). 

The Master Chemical Mechanism (MCM - University of Leeds) is a near-explicit chemical mechanism describing the degradation of 135 VOCs in the troposphere. The organic component of the version MCMv3 contains in the region of 12600 reactions and 4500 chemical species. 

Fig. 8 Pie-chart showing the contributions to OH reactivity calculated using a zero-dimensional box model constrained to the Master Chemical Mechanism for comparison with observations of OH reactivity made during the OPS campaign in the Borneo rainforest...

Xia AG, Michelangeli DV, Makar PA (2008) Box model studies of the secondary organic aerosol formation under different HC/NO(x) conditions using the subset of the Master Chemical Mechanism for alpha-pinene oxidation. J Geophys Res Atmos 113 D10... 

Fig. 3.1 A schematic of the mechanism generation protocol employed in the Master Chemical Mechanism development for tropospheric VOC degradation. Reproduced from (Saunders et al. 2003a) under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License ...

Saunders, S.M., Jcmkin, M.E., Derwent, R.G., Pilling, M.J. Protocol for the development of the master chemical mechanism, MCM V3 tropospheric degradation of non-aromatic VOC. Atmos. Chem. Phys. 3, 161-180 (2003a)... 

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