The Weather Research and Forecasting model was applied to analyze variations in the planetary boundary layer (PBL) structure over Southeast England including central and suburban London. The parameterizations and predictive skills of two nonlocal mixing PBL schemes, YSU and ACM2, and two local mixing PBL schemes, MYJ and MYNN2, were evaluated over a variety of stability conditions, with model predictions at a 3 km grid spacing. The PBL height predictions, which are critical for scaling turbulence and diffusion in meteorological and air quality models, show significant intra‐scheme variance (> 20%), and the reasons are presented. ACM2 diagnoses the PBL height thermodynamically using the bulk Richardson number method, which leads to a good agreement with the lidar data for both unstable and stable conditions. The modeled vertical profiles in the PBL, such as wind speed, turbulent kinetic energy (TKE), and heat flux, exhibit large spreads across the PBL schemes. The TKE predicted by MYJ were found to be too small and show much less diurnal variation as compared with observations over London. MYNN2 produces better TKE predictions at low levels than MYJ, but its turbulent length scale increases with height in the upper part of the strongly convective PBL, where it should decrease. The local PBL schemes considerably underestimate the entrainment heat fluxes for convective cases. The nonlocal PBL schemes exhibit stronger mixing in the mean wind fields under convective conditions than the local PBL schemes and agree better with large‐eddy simulation (LES) studies.

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.

NO2 (40µg/m 3 ) to be achieved in 2005 (Air Quality Strategy) and 2010 (EU limit values) and the annual mean limit value for PM10 of 20µg/m 3 to be achieved by 2010. These levels will be achieved across much of the UK because of reductions in industrial emissions and the significant improvement in road transport emissions arising from clean fuels and the implementation of EURO standards for vehicle emissions. However concentrations of NO2 and PM10 remain a problem in large urban areas and more especially in London. This paper presents the application of the ADMS-Urban model (Carruthers et al. 1998) to determine likely areas of exceedence of the levels in London for the target years. METHODOLOGY The key components of the study are model set-up, validation, a sensitivity study, comparisons with other prediction techniques and calculation of contour maps for the base year (1999) and target years (2004, 2005, 2010).

Air pollution alerts broadcast directly to vulnerable individuals had a high level of acceptance, with 94 per cent of users surveyed reporting satisfaction or high satisfaction and 71 per cent reporting a change in their behaviour.

As with other characteristic types of complex fluid flows, turbulent flows over and between different types or flexible obstacles above resistive surfaces have many features in common. This is why such flows can be studied in a similar conceptual framework and with similar techniques of analysis, computation and measurement.

The ADMS-urban atmospheric dispersion modelling system has been applied to review of air quality in central London in 1996/1997 and assessment of future air quality against air quality objectives in 2005. Model performance is assessed by in situ validation against monitoring data. This case study illustrates how scientific uncertainty needs to be considered when using model output in such a policy context. Model precision, carefully defined, is ±10% with bias between 0 and +12% (model over-prediction) for annual mean nitrogen dioxide and respirable particulate (PM10) concentrations and for the 90th percentile of daily mean PM10. As expected, the model is less accurate for the maximum and 99.8th percentile of hourly mean nitrogen dioxide concentrations and for total NOx. We propose probabilistic mapping techniques should be used to formalise and clarify how uncertainty is translated into the definition of an Air Quality Management Area (AQMA) on a map. This also identifies the extent to which air quality objectives have been defined for which current dispersion model performance is inadequate. It is recommended that the capabilities of modelling alongside measurement need to be considered at an early stage in the formulation of future air quality management policy.

Turbulent fluctuations are important in dispersion problems where short timescale peak values can be critical. These situations include: a) releases of toxic, flammable or odourous substances where high short time scale impacts may be dangerous or cause a nuisance; b) estimating concentrations for regulatory purposes for comparison with limits based on a short timescale, such as the UK 15-minute or WHO 10-minute levels for SO2; c) assessing the uncertainties in dispersion model predictions and aiding model comparison with measured data. The ADMS 3 dispersion model includes a module to calculate turbulent fluctuations which is based on a "two particle dispersion" concept but has much in common with Gifford's meandering plume model. It has been used in critical applications where odour and exceedences of a regulatory value were the issues. In this paper, the fluctuations model is described and each of the three applications is illustrated.

The main features and functionality of the urban air quality version of ADMS (ADMS-Urban) have been presented at previous Harmonization Workshops. The system has now been set up and applied to air quality management, decision support and air quality forecasting in a number of major cities in Europe and beyond, including London. Key recent developments of the system include a regional chemistry model, links to real time and forecasted meteorological data and an advanced emission database (EMIT) for calculating current and projected road traffic emissions from different diurnal traffic flow profiles and vehicle mixes. In this study we consider a detailed analysis of the impact of various road traffic measures, such as high maintenance, low emission zones, increased bus use, reduced traffic flow etc, applied to the Birmingham metropolitan area. We show the impact on compliance or otherwise with the UK National Air Quality Strategy objectives and the proposed EU air quality daughter objectives. We also present high-resolution I maps of calculations for London of predictions based on projected vehicle emission changes for the years 2004 and 2005. Both these sets of results are being used by policy makers in local authorities in the preparation of their air quality management plans.

ADMS-Urban has been used in approximately half of the pilot studies of air pollution recently carried out in the UK. The pilot areas varied in spatial extent and in the nature of the sources, the balance between traffic, industrial and domestic sources, and the pollutants which are important. The studies used local emission inventories, and outputs included time-series of predicted concentrations which could be compared with measured data at monitoring sites, as well as contour concentration plots across the study area.

Invasion percolation is a computing technique used for solving the transport problem for flow systems dominated by capillary and gravity forces. This work introduces a new technique that attempts to extend the percolation algorithms to flow systems dominated by viscous forces, while retaining the ability to honour capillary and gravity effects.

This paper shows results of validation of the Atmospheric Dispersion Modelling System (ADMS) against the Indianapolis dataset, comparing arcwise maxima and analysing the results using the BOOT statistical package. There is good agreement between the quality 3 daytime observed and predicted concentrations. At night-time calculated concentrations depend critically on the value of the surface roughness and minimum Monin-Obukhov length.

This paper describes the use of urban emission inventory data and an urban scale dispersion model (ADMS-Urban) to calculate concentrations of NOx and NO2 in London. The dispersion model used in this study is a second generation Gaussian dispersion model which is characterised by the use of boundary layer similarity profiles to parameterise the variation of turbulence with height within the boundary layer. The model has an integral chemistry model which is used to predict concentrations of nitrogen dioxide and ozone. The paper examines the performance of the integrated modelling system to predict concentrations from emission sources in Greater London. Predictions have been compared with observed data at four locations, two locations in Central London and two in East London. Predicted concentrations for a summer and winter period have been calculated and modelled and measured times series data have been compared. Statistical analyses have been carried out to assist in the comparison of model predictions with monitored data. Although no absolute significance can be attached to the numerical values of these measures, taken cumulatively, some conclusions regarding the emissions inventory data and the models’ performance can be made. The applicability of the model of atmospheric chemistry used in this study to calculate the concentrations of nitrogen dioxide is appraised and a comparison has been made with concentrations of nitrogen dioxide predicted from oxides of nitrogen using empirical relationships.

Some results are presented for recent studies to develop new approaches for (i) validating atmospheric dispersion models using crosswind concentration transects, and (ii) summarising and comparing the predictions of models across a range of efflux and meteorological conditions.

In this paper, we propose a set of tests for the hydrocarbon trap and leakage phenomena and discuss the assumptions implicit in these models. Our aim is to: (a) resolve and expose some of the potential modelling errors and mis-conceptions which can result from incorrect numerical representation of these physical phenomena; and (b) to set in place a sound basis for representing more complex effects which are difficult to validate.