The ozone source–receptor model – A tool for UK ozone policy
Introduction
The concentrations of ground-level ozone, a pollutant that affects human health, ecosystems and materials, widely exceed environmental quality standards across the UK and Europe (EEA, 2007, EEA, 2008, EEA, 2009, AQEG, 2009). Ozone is not emitted directly into the atmosphere, but is a secondary photochemical pollutant formed in the lower atmosphere from the sunlight-initiated oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxides (NOx). Elevated concentrations of ozone over the UK, such as those observed during July and August 2003, are especially generated when slow-moving or stagnant high pressure (anticyclonic) weather systems occurring in the spring or summer bring in photochemically reacting air masses from mainland Europe (see, for example, Jenkin et al., 2002a).
Under conditions characteristic of photochemical pollution episodes, the formation and transport of ozone can occur over hundreds of kilometres, with concentrations at a given location influenced by the history of the airmass over a period of up to several days. In addition to this, the increasing levels of ozone in the free troposphere on a global scale also influence regional scale photochemical processes by providing an increasing background ozone level upon which the regional and national scale formation is superimposed (LRTAP, 2007, AQEG, 2009). This effect now has to be considered when assessing whether proposed air quality standards for ozone are likely to be achieved.
The non-linear nature of ground-level ozone production requires the use of sophisticated chemical-transport models to understand the factors affecting its production and subsequent control. The UK Photochemical Trajectory (Derwent et al., 1998, Derwent et al., 2004, Derwent et al., 2007) and the ELMO (Metcalfe et al., 2002) models have previously been used to assess ozone in the UK and to assist the development of ozone control policies. The UK Photochemical Trajectory Model (UK PTM) generally simulates ozone formation for an idealised summertime photochemical pollution episode. Under these conditions, the model calculates peak or ‘worse case’ ozone concentrations. The ELMO model is a source–receptor model covering the UK, which also uses linear air mass trajectories. 4-day linear trajectories are calculated at 15° intervals around the compass rose for each 10 km × 10 km grid square. The ozone concentration for each receptor site is then derived by weighting the ozone concentrations on the individual trajectories by pollution roses derived from wind measurements. The peak ozone concentrations were then converted into metrics of direct policy interest (e.g., AOT40 (EC, 2008) and the number of days the maximum daily running 8-hour mean concentration exceeded 100 μg m−3), using relationships derived from ozone measurements.
A number of advanced Eulerian modelling systems are now available to the air pollution community, such as the US Community Multiscale Air Quality (Byun and Schere, 2006), the EMEP Unified (Simpson et al., 2003) and the CHIMERE (Bessagnet et al., 2008 and references therein) models. At the time of the development of the Ozone Source–receptor Model (OSRM), these models were either not available or required large computing resource and runtimes. The development of the OSRM followed from the requirements to include a more realistic description of the air mass history and to move towards modelling ozone in urban areas. The OSRM needs to be computationally efficient as one of its applications was to consider trajectories arriving at every hour for every day of the year to the ∼2950 receptor sites covering the UK on a 10 km × 10 km grid. A surface conversion module was developed to improve the performance of the model, especially for urban areas.
The OSRM can in principle be used to derive source–receptor relationships, i.e., estimates of the contribution of an emission source(s) to receptor locations on the model domain, although it has yet to be used in this mode. There is much interest in such relationships on the national, continental and global scales. The EMEP model (Simpson et al., 2003) has long been used to generate source–receptor matrices for use in IIASA’s integrated assessment modelling e.g., (Amann et al., 2005). There are a number of recent papers in which the impacts of ozone from different continental source regions have been calculated e.g., (Fiore et al., 2009, Wu et al., 2009, Zhang et al., 2009), as well as national studies, for example, to link O3 to NOx emissions in the USA (Tong and Mauzerall, 2008, Tong et al., 2009).
A range of ozone exposure metrics can be derived from the hourly concentrations to evaluate the impacts of ozone on human health, ecosystems and materials. The model was used to assess the effect on ozone over the UK of different policy measures (Hayman et al., 2006), which were developed for the UK Air Quality Strategy (Defra, 2006, Defra, 2007). In this paper, a description of the OSRM and the surface conversion module is given. Examples of the performance of the model against observations and other models are included together with an application of the OSRM to investigate the effect of different levels of NOx and VOC emission control.
Section snippets
The ozone source–receptor model
The OSRM is a Lagrangian model to describe photochemical ozone production in the UK. The OSRM is similar in concept to the UK Photochemical Trajectory Model (UK PTM) (Derwent et al., 1998, Derwent et al., 2004, Derwent et al., 2007) in that it simulates the chemical development of species in an air parcel moving along a trajectory and to the ELMO source–receptor model (Metcalfe et al., 2002) in that calculations can be undertaken to a 10 km × 10 km grid covering the UK. The OSRM has a number of
Results
In this section, the performance of the model is compared to observed concentrations, metrics and maps. The comparison is based on OSRM model runs that have been undertaken for each year from 1999 to 2005 using year-specific meteorology and emissions. The period included the photochemically-active year of 2003 when significant periods of elevated ozone concentrations occurred in the southern half of the UK during July and August and less photochemically-active years. Two types of model runs are
Application
The OSRM was used to assess the impact on ground-level ozone for future base case scenarios (2010, 2015 and 2020) and a number of additional measures developed for the Review of the UK Air Quality Strategy (Defra, 2006, Defra, 2007, Hayman et al., 2006). The model has also been used in the report prepared by the UK Department of Health on the health impacts of climate change (DOH, 2008) and for the Review of Transboundary Air Pollution (RoTAP, 2009).
UK NOx and VOC emissions in the “maximum
Discussion and summary
The Ozone Source–Receptor Model is a boundary layer Lagrangian chemical-transport model for photochemical ozone production. It has a number of notable differences and enhancements over the models previously used in the UK to assess ozone policy options (the UK Photochemical Trajectory (Derwent et al., 1998, Derwent et al., 2004, Derwent et al., 2007) and the ELMO (Metcalfe et al., 2002) models). The OSRM has a more realistic description of the air mass history (although there are versions of
Acknowledgements
This work was undertaken at AEA Energy and Environment prior to the departure of the corresponding author. We gratefully acknowledge the support for this work provided by the Department for Environment, Food and Rural Affairs (Defra) and the Devolved Administrations (the Scottish Executive, Welsh Assembly Government and the Department of the Environment for Northern Ireland) under contracts EPG 1/3/143 and 1/3/200. The corresponding author is grateful to the UK National Physical Laboratory’s
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Present address: Atmospheric Chemistry Services, 15 Ball Meadow, Okehampton, Devon, EX20 1FB, UK.
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Present address: SERCO TAS, 150 Harwell, Didcot, Oxfordshire OX11 0RA, UK.
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Present address: School of Science and Technology, Nottingham Trent University, Clifton Campus, Clifton Lane, Nottingham NG11 8NS, UK.