Publication

Gas phase chemistry mechanisms for air quality modeling

Martin Junier
2004
EPFL thesis
Abstract

During the last few decades, air pollution has become one of the major environmental and public health issue in every important cities over the world. Photochemical pollutants, like ozone which play a central role in today's air pollution problems, are formed in the atmosphere by reaction of two emitted precursors: Volatile Organic Carbons (VOCs) and NOx (NO + NO2). Ozone is a highly non linear process because its formation is driven by complex chain reactions. The decrease of ozone concentration produced by a reduction of its emitted precursors is therefore unpredictable, unless calculated by numerical photochemical models. The simulation of photochemical air pollution requires detailed chemical mechanisms and a lot computer resources. The number of chemical species within the chemical mechanism has to be confined to the strict minimum in order to minimise the CPU time. A solution is to lump the immense number of VOC species involved in atmospheric pollution in a convenient smaller number of mechanism species, keeping enough details to generate accurate results in reasonable calculations times. The calculation of all kinetic data of a lumped mechanism is a tremendous work unless carried out by a generation programme. CHEMATA, presented in this work, is a chemical mechanisms generation programme designed to create lumped and explicit tropospheric gas phase chemical mechanisms. Based on the widely used mechanism RACM, CHEMATA generated an extended mechanism to test the carbonyl species parameterisation of RACM and two smaller mechanisms to compare two lumping methods (the reduced mechanism and the small mechanism). The new mechanisms have been implemented in a bOx model and in the 3D eulerian air quality model TAPOM, also presented in this work. TAPOM have been run with the four chemical mechanisms on three simulations domains (Mexico City, Milan and Bogota) presenting different emissions strengths and meteorological conditions. The comparisons between the different mechanisms, in a bOx or 3D model and with or without emission reductions lead to the following conclusions: The comparison between the extended mechanism and RACM shows that the treatment of the carbonyl species in RACM does not induce notable errors in mesoscale modelling. The use of the extended mechanism should be kept for special simulations when enhanced precision in VOCs is required or for time periods longer than 2 or 3 days. The reduced mechanism is the best compromise between CPU time and accuracy. When calculating photochemical pollution, or emission reduction scenarios, this mechanism can save a lot of time. The small mechanism presents a clear tendency to produce more "VOC sensitive" results, which can lead to severe ozone overestimations. It should only be used for qualitative simulation when CPU time is a critical issue.

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Related concepts (35)
Air pollution
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. It is also the contamination of indoor or outdoor surrounding either by chemical activities, physical or biological agents that alters the natural features of the atmosphere. There are many different types of air pollutants, such as gases (including ammonia, carbon monoxide, sulfur dioxide, nitrous oxides, methane and chlorofluorocarbons), particulates (both organic and inorganic), and biological molecules.
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs. A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage of an overall chemical reaction. The detailed steps of a reaction are not observable in most cases. The conjectured mechanism is chosen because it is thermodynamically feasible and has experimental support in isolated intermediates (see next section) or other quantitative and qualitative characteristics of the reaction.
Ground-level ozone
Ground-level ozone (O3), also known as surface-level ozone and tropospheric ozone, is a trace gas in the troposphere (the lowest level of the Earth's atmosphere), with an average concentration of 20–30 parts per billion by volume (ppbv), with close to 100 ppbv in polluted areas. Ozone is also an important constituent of the stratosphere, where the ozone layer (2 to 8 parts per million ozone) exists which is located between 10 and 50 kilometers above the Earth's surface.
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