Improvement of an urban turbulence parametrization for meteorological operational forecast and air quality modeling

During the last century, urban pollution has increased with the growth of cities. Urban air quality has become a high priority as it is directly linked to concerns such as human exposure and health. The present work is dedicated to urban air quality modeling with focus on urban meteorology. The main goal is to improve meteorological and air quality simulations in urban areas. Based on measurements and numerical air quality simulations, Chapter 3 describes the meteorological situation and tests an emission inventory for an ozone pollution episode in Mexico City (2 March 1997), in order to determine the principal factors to be accounted in an air quality study. The Mexico City case shows the great influence of meteorological conditions and pollutant emissions on air quality. A thorough understanding of the phenomena governing the meteorological conditions, like small scale convergence, and an accurate emission inventory validated by VOC measurements in the city, are sensitive elements in an air quality study. In Chapter 4, the presence of an Urban Heat Island (UHI) is underlined over the city of Basel (Switzerland) with the BUBBLE measurements. Further on, the ability of aLMo (the operational weather prediction model of MeteoSwiss) to reproduce the effect of a city on the boundary layer atmospheric flow fields is investigated. Results show that aLMo is not able to reproduce the UHI and hence that its surface scheme based on the Monin-Obukhov Similarity Theory (MOST) is not adapted for the urban areas. Therefore, the Buildings Effects Parametrization (BEP) which has been developed by Martilli et al.(2002) especially for urban areas, is implemented in aLMo. Results show that alMo is now able to reproduce the main behavior of the urban boundary layer as the UHI and BEP has hence a real enhancement potential for aLMo. Chapter 5 shows the sensitivity of aLMo with respect to its vertical resolution. In order to limit the sensitivity of aLMo to the grid resolution, BEP is modified. The mesoscale model furnishes the upper boundary conditions for the inner calculation of BEP. That is, BEP recalculates independently the vertical profiles of wind, temperature and energy based on the surface fluxes of momentum, heat and turbulent kinetic energy. The results obtained show a decrease in the sensitivity to the resolution and a better agreement with the measurements. Furthermore, the modified version of BEP gives additional meteorological fields (temperature, wind and TKE) in the urban canopy. The computed temperature in the urban canopy shows a good agreement with measurements in a Basel street canyon. This work shows that it is not necessary to have a high resolution for taking into account modification of the atmospheric flow fields induced by a city.

Validation and application of an urban turbulence parameterisation scheme for mesoscale atmospheric models

Yves-Alain F. Roulet

Growing population, extensive use (and abuse) of the natural resources, increasing pollutants emissions in the atmosphere: these are a few obstacles (and not the least) one has to face with nowadays to ensure the sustainability of our planet in general, and of the air quality in particular. In the case of air pollution, the processes that govern the transport and the chemical transformation of pollutants are highly complex and non-linear. The use of numerical models for simulating meteorological fields, which in turn will determine the transport of pollutants in the atmosphere, is thus a very appropriate tool to describe and understand the air pollution problematic. This work focuses on the meteorological simulation, using a mesoscale model. The stress is particularly put on the parameterisation of urban induced effects on the meteorological fields above a city. A detailed urban parameterisation scheme has been implemented in a mesoscale model in a previous work. This new scheme takes into account the presence of a city in a more accurate way as the traditional method usually used in mesoscale models. In the first part of this work, the urban module was validated for a one-dimensional off-line simulation within and above a street canyon in the city of Basel/Switzerland. The simulation results were compared with measurements taken within and above the same street canyon during an intensive observation period in the frame of the BUBBLE project (Basel UrBan Boundary Layer Experiment). The comparison with the measurements and with a simulation using the traditional urban parameterisation showed that the detailed urban scheme improved the quality of the simulation of turbulent fluxes and meteorological parameters (wind, temperature) in the urban canopy. Further on, the mesoscale was applied for a three-dimensional simulation over the city of Basel and its surroundings. The results showed that the model is able to reproduce the wind pattern that prevailed during the simulated episode, and that the accuracy of the temperature simulation in the city is improved with the urban module. In the third part of this work, an air quality study was performed over the Mexico City basin. The mesoscale model simulated the meteorological conditions for the chosen episode (1 and 2 March 1997). The results were then passed to TAPOM, a Eulerian photochemical model that calculates the space and time distribution of air pollutants. The simulated concentrations over the basin showed good agreement with the observed values. The validated model could then be used to test some emissions reduction scenarios for Mexico City. The use of the detailed urban parameterisation scheme for meteorological fields and air pollutants concentration simulation improved the quality of the results in almost all the applied situations. Consequently, the full modelling tool presented and validated in this work can be used for air quality modelling studies over cities and their surroundings.

EPFL2004

Gas phase chemistry mechanisms for air quality modeling

Martin Junier

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.

EPFL2004

Understanding the origins and fate of air pollution in Bogota, Colombia

Erika Maria Zarate Torres

Bogota has more than 8 million inhabitants and is the 5th biggest urban agglomeration in Latin America. It has more than one million vehicles and a large number of small industries. High levels of air pollutants are thus detected. The purpose of this work consists in applying air quality modelling tools to the region of Bogota, aiming to acquire a deeper understanding of the factors that originate air pollution in the city, and of the way pollutants are dispersed and chemically transported. Furthermore, the knowledge gained is used to propose and evaluate air pollution abatement strategies. In the first part of this thesis, two versions of the traffic emission inventory are generated, one using standard CORINAIR traffic emission factors and the other using bulk real-world traffic emission factors. Both emission inventories are compared and evaluated with the help of numerical simulations. The emission inventory calculated using bulk real-world traffic emission factors generates simulated concentrations closer to the observed values. Thus, an innovative technique consisting in the combination of measurements and modelling to estimate and evaluate traffic emissions is proposed in this part of the study. In the second part, mesoscale meteorological and air quality models are applied to the city. The wind pattern developed over the complex topography of the region and the development of the plume of pollutants are simulated with success. In the third part of this work, the air quality model is used to study the plume of pollution in terms of the governing chemical regimes and the individual and combined effects of the main sources of emission. Traffic is the major contributor to the plume of pollutants in Bogota. Three feasible emission scenarios which are addressed to the mitigation of emissions from heavy traffic are evaluated with the model. Whereas reductions are attained for primary pollutants and aerosols (whose simulation is presented in the forth part of this work), levels of Ozone increase with these scenarios. The air quality model indicates that strategies directed to mitigate air pollution might have contradictory effects depending on the pollutant to be tackled. Air quality modelling proved to be a very useful tool for evaluating emission scenarios in advance and prioritizing actions to mitigate pollution in Bogota.

EPFL2007

Validation and application of an urban turbulence parameterisation scheme for mesoscale atmospheric models

Yves-Alain F. Roulet

EPFL2004

Understanding the origins and fate of air pollution in Bogota, Colombia

Erika Maria Zarate Torres

EPFL2007

Gas phase chemistry mechanisms for air quality modeling

Martin Junier

EPFL2004