Present-day numerical air quality models are considered essential tools for predicting
future air pollutant concentrations and depositions, contributing to the development
of new effective strategies for the control and the reduction of pollutant emissions.
They simulate concentrations and depositions of pollutants on a wide range of scales
(global, national, urban scale) and they are used for identifying critical areas,
integrating measurements and achieving a deeper scientific understanding of the
physical and chemical processes involving air pollutants in the atmosphere.
The use of comprehensive air quality models started in the late 1970s and since then
their development has increased rapidly, hand in hand with the rapid increase in
computational resources. Today more and more complex and computationally
expensive numerical models are available to the scientific community. One of these
tools is the Community Multi-Scale Air Quality System (CMAQ), developed in the
1990s by the US Environmental Protection Agency (EPA) and currently widely
applied across the world for air pollution studies. This work focuses on the
application of CMAQ to the United Kingdom, for estimating concentrations and
depositions of acidifying pollutants (NOX, NHX, SOX) on a national scale.
The work is divided into seven chapters, the first one describing the main issues
related to the emission and dispersion in the atmosphere of acidifying species. It also
includes a brief overview of the main international policies signed in the last thirty
years in order to reduce the problem of acidification in Europe, as well as a brief
description of some models mentioned in this thesis.
The second one describes the main features of CMAQ and addresses some issues
such as the use of a nesting process for achieving temporally and spatially resolved
boundary concentrations, and the implementation of the model on parallel machines,
essential for reducing the simulation computing time. It also describes how this study
is part of a wider context, which includes the application of CMAQ in the United
Kingdom by other users with different scientific purposes (aerosols processes, air
quality in the urban area of London, contribution of UK power stations to
concentrations and depositions etc.).
The third part of the thesis focuses on the application and evaluation over the United
Kingdom of the 5th Generation Mesoscale Model MM5, used for providing 3D
meteorological input fields to CMAQ. This study was performed assuming that an
accurate representation of depositions and concentrations of chemical species cannot
be achieved without a good estimate of the meteorological parameters involved in
most of the atmospheric processes (transport, photochemistry, aerosol processes,
cloud processes etc.).
The fourth part of the thesis describes the preliminary implementation of the Sparse
Matrix Operational Kernel Emission System (SMOKE) in the United Kingdom. The
processor provides input emissions to CMAQ. The use of SMOKE is usually avoided
in CMAQ applications of outside America, and CMAQ input emission files are
prepared by the application of other software. The reason is that the model requires
radical changes for being applied outside Northern and Central America. Some of
these changes have been made in this study such as the adaptation of the European
emission inventory EMEP and the UK National Inventory NAEI to the modelling
system for point and area sources, the introduction of new European emission
temporal profiles in substitution of the American ones and the introduction of new
geographical references for the spatial allocation of emissions.
In the fifth chapter the results of CMAQ application over the UK are discussed. The
study focuses on NOX, SO2, NH3 and +
4 NH . Maps of concentration are presented and
modelled data are compared to measurements from two different air quality networks
in the UK. An analysis of the performance of CMAQ over the UK is also performed.
In the final chapter an annual inter-comparison between CMAQ and the Lagrangian
transport model FRAME is carried out. Maps of annual wet deposition fluxes of
NHX, NOY and SOX for year 1999 are presented. The results of both models are
compared to one another and they are also compared to values from the UK official
data set CBED.
Finally, the last chapter suggests the work that has to be done in the future with
CMAQ and it summarizes the conclusions