J.D. Whyatt

Developing the Hull Acid Rain Model: Its Validation and Implications for Policy Makers.

The Hull Acid Rain Model (HARM) is extensively used in the UK to help in formulating strategies for reducing the emissions of acidifying pollutants. Here we describe the development of a 10 km version of HARM, which incorporates both software development and a new representation of the dry deposition of reduced N. Output from the new model for the UK is compared with S and N deposition data available from the UKs monitoring networks for 1992–1994 using linear regression. The model is able to reproduce the overall patterns of deposition for S, oxidised and reduced N. Although improved from earlier model versions, HARM still appears to underestimate dry deposition of reduced N. We describe the implications of the changes to HARM for assessing current policy commitments to reduce emissions of acidifying pollutants through changes in critical loads (CL) exceedance.

The Regional Distribution of Ozone Across the British Isles and its Response to Control Strategies.

The spatial pattern of summertime ozone concentrations across the British Isles has been revealed using a simple long-range transport model (ELMO). The model describes the chemical development of air parcels reaching an array of 3064 arrival points after 4 days of travel across Europe during typical summertime photochemical episodic conditions. Model results have been compared against observations and the model responses to VOC and NOx controls against established indicator ratios. The model has been used to assess the likely impacts of policy commitments under the Gothenburg Protocol to the United Nations Convention on Long-Range Transboundary Air Pollution and other policy instruments.

Spatially-varying surface roughness and ground-level air quality in an operational dispersion model

Urban form controls the overall aerodynamic roughness of a city, and hence plays a significant role in how air flow interacts with the urban landscape. This paper reports improved model performance resulting from the introduction of variable surface roughness in the operational air-quality model ADMS-Urban (v3.1). We then assess to what extent pollutant concentrations can be reduced solely through local reductions in roughness. The model results suggest that reducing surface roughness in a city centre can increase ground-level pollutant concentrations, both locally in the area of reduced roughness and downwind of that area. The unexpected simulation of increased ground-level pollutant concentrations implies that this type of modelling should be used with caution for urban planning and design studies looking at ventilation of pollution. We expect the results from this study to be relevant for all atmospheric dispersion models with urban-surface parameterisations based on roughness.

Assessment of uncertainties in a long range atmospheric transport model: Methodology, application and implications in a UK context

Acid deposition models are inherently simplified representations of real world behaviour and their performance is best evaluated by comparison with observations. National and international acid rain policy assessments handle observed and modelled deposition fields in different ways. Here, both the observed and modelled deposition fields are seen as uncertain and the Generalised Likelihood Uncertainty Estimation (GLUE) framework is used to choose acceptable sets of model input parameters that minimise the differences between them. These acceptable sets of model parameters are then used to estimate deposition budgets to the UK and to provide a probabilistic treatment of excess deposition over environmental quality standards (critical loads).

Assessing the Impacts of International Emissions Reduction Scenarios on the Acidification of Freshwaters in Great Britain with the First-Order Acidity Balance (FAB) Model and the Hull Acid Rain Model (HARM)

Critical loads models for acidity underpin international negotiations for the reduction of acid deposition through emissions controls. In Great Britain and Scandinavia, critical loads for freshwater ecosystems are calculated with the First-order Acidity Balance (FAB) model, which can provide a catchment based estimate of deposition reduction requirements of sulphur and nitrogen species in order to protect any aquatic target organism for which a critical chemical threshold is defined. The FAB model is applied to a national freshwaters database for Great Britain using three deposition scenarios generated with the Hull Acid Rain Model (HARM). Critical load exceedance and changes in three important chemical indicators (non-marine sulphate, nitrate and acid neutralising capacity) are assessed for 1990 baseline deposition levels, planned emissions reductions under existing international commitments (REF scenario), and a potential stringent emission reduction scenario under a multi-pollutant, multi-effect strategy (E10 scenario). Model outputs indicate that the number of sampled sites exceeding their critical load would be reduced by 60% and 73% respectively under the two future deposition scenarios. There is a clear need for a strategy to reduce both S and N deposition from 1990 levels if British freshwaters in sensitive areas are to be protected.

Evaluation and Inter-comparison of Acid Deposition Models for the UK

An evaluation has been made of a range of simple and complex atmospheric transport models, applied to estimate sulphur and nitrogen deposition in the UK in order to provide information to policy makers to support decisions on future model use. The models were evaluated by comparison with annually averaged measurements from the national monitoring networks. A number of statistical metrics were output to assess model performance and the models were compared graphically by plotting cross-country transects of concentrations in air.