D.R. Middleton

Analysis and interpretation of air quality data from an urban roadside location in Central London over the period from July 1991 to July 1992

Abstract Air quality data from a one year study at an urban roadside location in Central London are reported for a large number of both organic and inorganic air pollutants. The location is 5 m from the kerbside of a moderately busy road, Exhibition Road, enclosed on each side by buildings. The pollutants measured included NO, NO2, O3, SO2, CO and CO2. together with 28 non-methane hydrocarbons, including measurements of the air toxics: benzene and 1,3-butadiene. The interpretation of the air quality data used simple statistical techniques, including principal components analysis, and air pollution models of the Gaussian plume and box model variety. A comprehensive validation of the published emission inventory estimates for London was attempted using the observed air quality data. Separate contributions from the local traffic on Exhibition Road, from the general traffic in Central London, natural gas leakage and fuel combustion have been identified. The agreement between the emission factors required to explain the observed concentrations and those in the literature was excellent for a number of air pollutants and these should be of value in environmental impact assessments. Attention was also directed to an intense wintertime pollution episode which occurred during the study period, when the concentrations of all pollutants were considerably higher than usual. Elevated nitrogen dioxide concentrations were a significant feature of this episode and the box model was used to ascertain its likely production mechanism. A relationship between the hourly mean nitrogen dioxide and NOx concentrations was defined which should have general applicability in air quality policy assessments.

NOMOGRAMS FOR CALCULATING POLLUTION WITHIN STREET CANYONS

Abstract The Environment Act 1995 has introduced the notion of local air quality management which requires that air quality in towns be reviewed and assessed. There is a need to identify those streets that are worst affected by vehicular pollutants. Such worst cases are likely to be narrow congested streets with tall buildings on each side. A nomogram presented here allows rapid screening of pollution in congested street canyons. The strong dependence on wind direction is reduced to the two extremes, namely wind along and wind across the canyon. Then canyon concentrations are estimated according to street geometry and traffic flow. The nomogram is designed for use by local authorities, is quick and easy to use, and paper or computer versions are available. It is suggested that detailed monitoring or modelling may only be required when simple screening methods predict high air pollution.

Investigating the Surface Energy Balance in Urban Areas – Recent Advances and Future Needs

Recent advances in understanding of the surface energy balance of urban areas, based on both experimental investigations andnumerical models, are reviewed. Particular attention is directedto the outcome of a COST-715 Expert Meeting held in April 2000,as well as experiments initiated by that action. In addition, recentcomplete parameterisations of urban effects in meso-scalemodels are reviewed. Given that neither the surface energybalance, nor its components, normally are directly measuredat meteorological stations, nor are there guidelines for theset-up of representative meteorological stations in urbanareas, this paper also provides recommendations to closethese gaps.

Sensitivity of nitrogen dioxide concentrations to oxides of nitrogen controls in the United Kingdom

Abstract There is a possibility of further controls on emissions to the atmosphere of nitrogen oxides to meet air quality objectives in the UK. Data in the National Air Quality Archive were used to calculate the likely sensitivity of hourly concentrations of nitrogen dioxide in ambient urban air to changes in the total oxides of nitrogen. Since the role of atmospheric chemical reactions is to make the response non-linearly dependent on the emissions control, we seek to establish the magnitude and sign of the effects that this non-linearity might cause. We develop a quantitative approach to analysing the non-linearity in the data. Polynomial curve fits have been developed for the empirical ratio NO 2 xa0:xa0NO x (the ‘yield’). They describe nitrogen dioxide concentrations using total oxides of nitrogen. The new functions have the important feature of increased yield in winter episodes. Simpler functions tend to omit this feature of the yields at the highest hourly concentrations. Based on this study, the hourly nitrogen dioxide objective in the UK may require emissions control of no more than ≈50% on total oxides of nitrogen at the most polluted sites: other sites require less or even no control.

Evaluation of dispersion model parameters by dual Doppler lidars over West London, England

Urban dispersion models have parameters for the turbulence and mixing height. We identify the parameters in meteorological pre-processors, which are, in principle, measurable by pulsed Doppler lidar. This technique measures radial velocity from the Doppler shifted wavelengths of backscattered laser pulses. First results obtained from single and dual lidar operations are described. Using two scanning lidars increases the range of parameters that can be sensed remotely. Scanned Dual Doppler lidar provides a novel remote sensing method for studying the rural-urban interface. Results are presented from a recent field experiment at an airfield site in West London. Results are compared with data from the Met Office operational dispersion model, NAME, a model using numerical weather forecast data. We show three methods for the urban mixing height. The paper concludes by considering the potential merits of dual-lidar remote sensing of urban dispersion model parameters.

Meteorological data from numerical weather prediction or observations for dispersion modelling

Synoptic observations of wind speed, direction, cloud cover, precipitation and temperature are inputs to regulatory dispersion modelling. Numerical Weather Prediction (NWP) data generated by national meteorological services are easily formatted for dispersion models. Changing from observations to NWP data raises concerns amongst regulators and model users – how might impact assessments differ if the source of meteorological data is changed. It is important to identify factors influencing the acceptability of NWP data. This paper compares numerical and surface synoptic meteorological data at locations across the UK. Further work is needed to harmonise the regulatory use of meteorological data.

Dispersion model input parameters from numerical weather prediction or synoptic observations

Regulatory dispersion modelling uses hourly synoptic observations (OBS) from a nearby station. Alternatively, hourly Numerical Weather Prediction (NWP) data are useable in air quality management. In parallel runs, OBS data and NWP (two grid scales, ~60 km and ~12 km) were input to the meteorological pre-processor of the Met Office NAME III dispersion model. Frequency distribution results for dispersion parameters including wind speed, friction velocity, heat flux, Monin Obukhov length and Boundary Layer Depth (BLD) have been analysed because they influence dispersion calculations. New stability maps compared dispersion climatology from NWP with pre-processor results using OBS data.