Date of final exam: 06/02/2003

E-mail: veronica.gabusi@unibs.it

Tutor: Prof.ssa  G. Finzi, Università degli Studi di Brescia

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PHOTOCHEMICAL POLLUTION EXPOSURE: ASSESSMENT AND CONTROL MODELS   ___________________________________________________________________________________________________________

Advisor:

Prof.ssa  G. Finzi, Università degli Studi di Brescia

Summary of the thesis

Over the last decades the pollutants characterising the atmosphere chemical composition have been changing substantially and in the recent years one of the main concern has been the photochemical pollution. The term “photochemical pollution” refers to a complex set of chemical oxidants, as ozone (O3) and peroxyacetylnitrate (PAN), results of the interaction of high solar irradiation levels with the precursor substances of nitrogen oxides and volatile organic compounds. NOx are produced by combustion processes and are mainly emitted by vehicles and industries, while volatile organic compounds (VOC) are both anthropogenic and biogenic hydrocarbons. These prerogatives make photochemical smog a typical summer phenomenon, mainly occurring in domains nearby large urban areas.
Ozone is a highly reactive gas composed of three oxygen atoms; it is highly predominant and important in two different areas of the atmosphere: the stratosphere and the troposphere. In the stratosphere, miles above the earth surface, ozone provides a protective shield by filtering out dangerous ultraviolet radiation from the sun. Stratospheric O3 has being depleted by chlorofluorocarbons during last century and has been one of the focus of scientific research in recent years. On the other hand, tropospheric ozone is formed in the lower atmosphere and is a component of the ambient air to which people, agricultural crops, forests, and ecosystems are exposed every day. Consequently tropospheric O3 is a major environmental concern because of its adverse impacts on human health, materials and ecosystems. Human exposure to elevated O3 concentrations levels can give rise to inflammatory response and decrease in lung function. O3 exposure of ecosystems and agricultural crops results in visible foliar injury and in crop yield and seed production reductions. Therefore O3 formation, its transport and its effects are vital as well as complex, making so essential the investigation of the physical and chemical processes associated with it, in order to develop efficient control strategies.
The tropospheric ozone formation is consequent to a large number of photochemical non-linear reactions taking place in the atmosphere and depends on temperature, humidity and solar radiation as well on the primary emissions of nitrogen oxides and volatile organic compounds. This complexity in O3 formation makes quite difficult the design of groundlevel ozone abatement plans, so that photochemical models result crucial tools in evaluating and developing efficient strategies for the reduction of ozone precursor emissions. Therefore modelling systems can represent suitable tools in air quality management, allowing both to study photochemical pollution and to analyse and assess appropriate emission reduction strategies.
Since ozone is considered one of the most significant air pollutants with respect to its potential impact to human health and natural ecosystems both in terms of critical episodes and long-term exposure, then not only ozone peak concentrations have to be examined, but also ozone exposures on "seasonal" scale need to be quantified in order to assess the comprehensive effects of photochemical pollution. Consequently it is crucial to perform simulations on a seasonal basis also in order to quantify policies effects with respect to long-term air quality standards.
For this purpose, local monitored concentrations sites are surely of interest, but even more important is the knowledge of pollutant spatial distribution in the area. In fact, the spatial and temporal variation are crucial information to evaluate the impact of photochemical pollution on natural ecosystems.
In order to assess the long-term impact of ozone concentrations, a cumulative approach may be applied to estimate the number of exceedances of a threshold, defined with respect to the objective (either health, crops or ecosystems protection); in this work critical levels for ozone effects on vegetation are defined using the AOT40 index (i.e. the cumulative exposure over 40 ppb for daylight hours during a growing season), while for the health protection, according to the WHO guidelines, the main concern is on the peak concentrations, well represented by the AOT60.
The following study is part of a research performed in the frame of EUROTRAC-2 scientific project (European Project on Transport and Transformation of Environmentally Relevant Trace Constituents in the Troposphere over Europe) and in particular belongs to SATURN (Studying Atmospheric Pollution in Urban Areas) subproject. The final goal of the Italian operational unit coordinated by Brescia University is to set up a decision support system allowing the urban and regional environmental managers to examine alternative emission scenarios, in order to delineate proper photochemical pollutants abatement strategies.
The resulting integrated modelling system GAMES (Gas Aerosol Model for Evaluation System) has been designed and implemented. It consists of some chief modules: the transport and chemical model CALGRID, the meteorological model CALMET and the emission evaluation model POEM. The photochemical CALGRID model is Eulerian three-dimensional. It implements an accurate advection-diffusion scheme in terrain-following coordinates with vertical variable spacing; a resistance-based dry deposition algorithm takes into account pollutant properties, local meteorology and terrain features. The CALGRID chemical module deals only with gas phase species. Inputs needed are the meteorological fields as well as the emissions fields. Three-dimensional meteorological fields have been reconstructed by CALMET model, merging background field with measurements, introducing mesoscale features (mountainvalley breezes and local effects) revealed by ground-level measurements and estimating temperature fields as well as turbulence parameters. The emission model POEM has been specifically designed to produce present and alternative emission field estimates by means of an integrated top-down and bottom-up approach. POEM can be applied in particular starting from Italian CORINAIR emission inventory and integrates diffuse and main point sources estimates based on different activity sectors. The road transport, agriculture and biogenic emissions are estimated by means of a bottom-up approach, while the emissions due to other source categories are computed disaggregating the CORINAIR data set.
In this study the modelling system has been optimised in order to perform seasonal assessments. Since the long-term simulation gave rise to some critical issues, as the run time and the resources management, a time splitting procedure has been developed to allow the use of a PC computing platform.
The modelling systemhas been applied to a Northern Italy domain, including the whole Regione Lombardia, characterised by complex terrain, high urban and industrial emissions and a close road network. The models have been run for the summer period April-September 1996 to evaluate ozone long-term exposure indexes (AOT40 and AOT60).

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