ESTGV - DA - Teses de doutoramento (após aprovadas pelo júri)
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- Chamber validation of the explicit master chemical mechanism (MCM) and application to air pollution studiesPublication . Pinho, Paulo Gabriel Fernandes de; Pio, Casimiro Adrião; Jenkin, MichaelABSTRACT: The present dissertation consisted essentially in the evaluation of several schemes included in Master Chemical Mechanism and in its capability of predicting the transformation processes of pollutants in the atmospheric environment. This was achieved by comparison of model simulations with environmental chamber data, and also by incorporation of the chemical mechanism into a tropospheric model, allowing the comparison of model predictions with atmospheric observational data. Butane, ethene, propene, 1-butene, 1-hexene, isoprene, α−pinene, β−pinene, formaldehyde, acetaldehyde, methacrolein, acetone, methylethyl ketone, and methylvinyl ketone degradation mechanisms included in version 3.1 of Master Chemical Mechanism (MCM v3.1) were evaluated using environmental chamber datasets from the Statewide Air Pollution Research Center at the University of California. Photo-oxidation of butane, and its degradation products methylethyl ketone, acetaldehyde and formaldehyde, was made in conjunction with an initial evaluation of the chamber-dependent auxiliary mechanisms. The MCM v3.1 mechanism for butane was found to provide an acceptable reaction framework for describing the NOX-photo-oxidation experiments on the above systems, although a number of parameter modifications and refinements were identified which resulted in an improved performance. These generally relate to the magnitude of sources of free radicals from carbonyl photolysis processes. The simulations for ethene and propene systems were found to be sensitive to the branching ratios assigned to molecular and free radical forming pathways of the O(3P) reactions. With this constraint, the MCM v3.1 mechanisms for ethene and propene generally performed well. Evaluation of the MCM v3.1 1-butene and 1-hexene degradation mechanisms was found to be inconclusive. Photo-oxidation of Isoprene was made simultaneously with its degradation products, methacrolein and methylvinyl ketone. The MCM v3.1 mechanism for isoprene was found to provide an acceptable reaction framework for describing the NOX-photo-oxidation experiments, although a number of parameter modifications and refinements were identified which resulted on improved performance. These all relate to the magnitude of sources of free radicals from organic chemical process, such as carbonyl photolysis rates and the yields of radicals from the reactions of O3 with unsaturated oxygenates, and specific recommendations are made for refinements. In addition to this, it was necessary to include a representation of the reactions of O(3P) with isoprene, methacrolein and methylvinyl ketone. α-Pinene and β-pinene degradation mechanism included in MCM v3.1 has been evaluated and refined. α-pinene mechanism was found to overestimate D(O3-NO) specially for low VOC/NOx ratios. In addition, the simulated decay of α-pinene was in advance and formation of the product HCHO is underpredicted relative to the observations. A number of parameter modifications and refinements were identified which resulted in an improved performance. These all relate to the magnitude of sources of free radicals from organic chemical process. The reaction of O(3P) with α-pinene was incorporated, although the effect of its inclusion was found to be small. The main refinements proposed to α-pinene degradation mechanism are the increasing of nitrate yields from the initially formed RO2 and decreasing the OH yield from the reaction of O3. β-pinene mechanism was evaluated against the chamber data. However, an improvement of it was not obtained for all the evaluated runs. As a result of chamber evaluation of MCM, it is possible to make a number of suggestions and recommendations for future work, in relation to gaps and uncertainties in the kinetic and mechanistic database, and the availability of chamber data: confidence in chamber evaluations requires the uncertainty to be reduced on the kinetics of the reaction of OH with NO2; Chamber evaluation of VOC systems requires confirmatory information on their reactions with O(3P); further information is required to quantify fully free radical formation from reactions of ozone with alkenes under atmospheric conditions. The MCM v3.1 was incorporated into a Photochemical, one layer, Trajectory box Model (PTM) using a Lagrangian approach. The PTM was used to evaluate and intercompare the original and adapted versions of MCM v3.1, (resulting from refinements provided by chamber evaluation), at normal environmental conditions, in the polluted boundary layer atmosphere. The PTM was applied to a real situation in the Portuguese west coast, during an ozone summer episode, by following the air mass transported by sea breeze, from the coast line (Aveiro) to a location, approximately 65 kilometres inland (Covelo). The PTM was applied during the period of June 29 and 30 (2001), using meteorological and air quality data, obtained from a field campaign. The ozone concentrations obtained by application of PTM, using as chemical mechanism MCM v3.1 are a good approximation to the measured values in Sangalhos and Covelo. The refinements in MCM v3.1 resulting from chamber evaluation show only a very small change when applied to these field conditions. Sensitivity tests show that temperature has a large effect in calculated ozone concentrations. The field evaluation of MCM v3.1 inserted in the photochemical trajectory model indicate that there are too many model uncertainties to allow a quantitative analysis for the several species represented on explicit mechanisms.