Studies of Atmospheric Pollutants in Urban Districts

Abstract

To study pollution near a motorway intersection, concentrations of oxides of nitrogen, carbon monoxide and total hydrocarbons were monitored, and calculated by a generally applicable programme. Road-side dilution was measured. After calibration by flow of exponential dilution, unattended instruments recorded pollutant concentrations in analogue form. Zeroes and calibrations were measured during visits and automatically. Intersection traffic-flows, and simultaneous weather readings from a nearby airport, were obtained. The data were stored and manipulated by computer - principles for processing environmental data are suggested. Data, identified by gas, observation type, site, time and date, were calibrated, zero corrected and averaged by an appropriate programme. Similarly, programmes calculated hourly traffic-flows from instantaneous readings of cumulative counters; hourly flows were interpolated from non-hourly counts, to minimise abstraction. For simple usage, input was flexible and accepted missing values; default options included an assumed zero or calibration. Another programme calculated gaseous pollutant concentrations for any elevated, curved or straight roads from dilution of traffic emissions. Ordnance Survey map references defined road geometries and axes were rotated downwind. Point-source plumes, from elements stepped along each road, were integrated. Statistical comparison of calculated with measured concentrations showed a high correlation; the regression coefficients were not unity because of uncertain emissions estimates, errors of measurement and approximations of calculation. Concentrations at a more distant site were noticeably lower than at the intersection. A sampling technique was developed to measure concentrations at two separate locations simultaneously using a single analyser: dilution curves calculated by the programme for the motorway and the intersection were compatible with concentration gradients measured in the field. Concentrations of pollutants at the intersection were related to the traffic-based model, but exact agreement between calculation and field measurement is still hard to achieve, because many variables are involved